Private: Main Body

Chapter 2 – The Universe and the Solar System

R. Adam Dastrup, MA, and GISP

The Whirlpool Galaxy is a spiral galaxy about 23 million light-years from Earth. Its interactions with the yellowish dwarf galaxy NGC 5195 are of interest to astronomers because the galaxies are near enough to Earth to be well-studied. Decades ago astronomers could not tell if these two galaxies were passing each other, but radio astronomy has supplied astronomers with essential data outlining their interactions. Using this data, astronomers have simulated the interaction. NGC 5195 came from behind and then passed through the main disk of M51 about 500 to 600 million years ago. The dwarf galaxy crossed the disk again between 50 and 100 million years ago and is now slightly behind M51. These interactions appear to have intensified the spiral arms that are the dominant characteristic of the Whirlpool Galaxy.

The Whirlpool Galaxy is a classic spiral galaxy located in the Canes Venatici constellation.

Astronomers can learn about objects unimaginably far away from Earth using telescopes that sense all wavelengths on the electromagnetic spectrum. Imagine what Galileo would do if he could see the images and data astronomers have available to them now. The study of the universe is called cosmology and cosmologists study the structure and changes in the present universe. The universe contains all of the star systems, galaxies, gas, and dust, plus all the matter and energy that exists now, that existed in the past, and that will exist in the future. The universe includes all of space and time known in existence.

2.1 Expanding Universe

What did the ancient Greeks recognize as the universe? In their model, the universe contained Earth at the center, the Sun, the Moon, five planets, and a sphere to which all the stars were attached. This idea held for many centuries until Galileo’s telescope helped allow people to realize that Earth is not the center of the universe. They also found out that there are many more stars than were visible to the naked eye. All of those stars were in the Milky Way Galaxy.

In the early 20th century, an astronomer named Edwin Hubble discovered that what scientists called the Andromeda Nebula was over 2 million light-years away, many times farther than the farthest distances that had ever been measured. Hubble realized that many of the objects that astronomers called nebulas were not clouds of gas, but were collections of millions or billions of stars that we now call galaxies.

A typical spiral galaxy in the constellation Coma Berenices, is about 55,000 light-years in diameter and approximately 60 million light-years away from Earth.

Hubble showed that the universe was much larger than our galaxy. Today, we know that the universe contains about a hundred billion galaxies, about the same number of galaxies as there are stars in the Milky Way Galaxy. After discovering that there are galaxies beyond the Milky Way, Edwin Hubble went on to measure the distance to hundreds of other galaxies. His data would eventually show how the universe is changing, and would even yield clues as to how the universe formed. Today we now know that the universe in nearly 14 billion years old.


If we look at a star through a prism, we will see a spectrum or a range of colors through the rainbow. The spectrum will have specific dark bands where elements in the star absorb light of specific energies. By examining the arrangement of these dark absorption lines, astronomers can determine the composition of elements that make up a distant star. In fact, the element helium was first discovered in our Sun, not on Earth, by analyzing the absorption lines in the spectrum of the Sun.

While studying the spectrum of light from distant galaxies, astronomers noticed something strange. The dark lines in the spectrum were in the patterns they expected, but they were shifted toward the red end of the spectrum, as shown in Figure below. This shift of absorption bands toward the red end of the spectrum is known as redshift.

Redshift occurs when the light source is moving away from the observer or when the space between the observer and the source is stretched. What does it mean that stars and galaxies are redshifted? When astronomers see redshift in the light from a galaxy, they know that the galaxy is moving away from Earth. What astronomers are noticing is that all the galaxies have a redshift, strongly indicating that all galaxies are moving away from each other causing the Universe to expand.

Redshift can occur with other types of waves too, called the  Doppler Effect. An analogy to redshift is the noise a siren makes as it passes. As it passes by, the ambulance will seem to lower the pitch of its siren. This is because the sound waves shift towards a lower pitch when the ambulance speeds away. Though redshift involves light instead of sound, a similar principle operates in both situations.


The Expanding Universe

Edwin Hubble combined his measurements of the distances to galaxies with other astronomers’ measurements of redshift. From this data, he noticed a relationship, called Hubble’s Law, that states that the farther away a galaxy is, the faster it is moving away from us. What this leads to is the hypothesis that the universe is expanding.

The figure here by NASA shows a simplified diagram of the expansion of the universe. If we look closely at the diagram, the formation of the universe and the energy is quite high. Over the course of the 13.7 billion years, the energy begins to cool enough to create trillions of stars and over time develop into galaxies. Over time, the galaxies continue to cool and expand farther apart from each other.

2.2 Formation of the Universe

Before Hubble, most astronomers thought that the universe did not change. However, if the universe is expanding, what does that say about where it was in the past? If the universe is expanding, the next logical thought is that in the past it had to have been smaller.

The Big Bang Theory

The Big Bang theory is the most widely accepted cosmological explanation of how the universe formed. According to the Big Bang theory, the universe began about 13.7 billion years ago. Everything that is now in the universe was squeezed into a very small volume of hot, chaotic mass. An enormous explosion, a big bang – caused the universe to start expanding rapidly. All the matter and energy in the universe and even space itself came out of this explosion. What came before the Big Bang? There is no way for scientists to know since there is no remaining evidence.

A representation of the evolution of the universe over 13.77 billion years. The far left depicts the earliest moment we can now probe, when a period of “inflation” produced a burst of exponential growth in the universe. For the next several billion years, the expansion of the universe gradually slowed down as the matter in the universe pulled on itself via gravity. More recently, the expansion has begun to speed up again as the repulsive effects of dark energy have come to dominate the expansion of the universe.

After the Big Bang

In the first few moments after the Big Bang, the universe was unimaginably hot and dense. As the universe expanded, it became less dense and began to cool. After only a few seconds, protons, neutrons, and electrons formed. After a few minutes, those subatomic particles came together to create hydrogen. Energy in the universe was significant enough to initiate nuclear fusion, and hydrogen nuclei were fused into helium nuclei. The first neutral atoms that included electrons did not form until about 380,000 years later.

The matter in the early universe was not smoothly distributed across space. Dense clumps of matter held close together by gravity were spread around. Eventually, these clumps formed countless trillions of stars, billions of galaxies, and other structures that now form most of the visible mass of the universe. If we look at an image of galaxies at the far edge of what we can see, we are looking at great distances. However, we are also looking across a different type of distance. Because it takes so long for light from so far away to reach us, we are also looking back in time.

Dark Matter and Dark Energy

The Big Bang theory is still the best scientific model we have for explaining the formation of the universe and many lines of evidence support it. However, recent discoveries continue to shake up our understanding of the universe. Astronomers and other scientists are now wrestling with some unanswered questions about what the universe is made of and why it is expanding. A lot of what cosmologists do is create mathematical models and computer simulations to account for these unknown phenomena, such as dark energy and dark matter.

The things we observe in space are objects that emit electromagnetic radiation. However, scientists think that matter that emits light makes up only a small part of the matter in the universe. The rest of the matter, about 80 percent, is dark matter. Dark matter emits no electromagnetic radiation so we cannot observe it directly. However, astronomers know that dark matter exists because its gravity affects the motion of objects around it. When astronomers measure how spiral galaxies rotate, they find that the outside edges of a galaxy rotate at the same speed as parts closer to the center. This can only be explained if there is a lot more matter in the galaxy than they can see.

Gravitational lensing occurs when light is bent from a very distant bright source around a super-massive object. To explain strong gravitational lensing, more matter than is observed must be present. With so little to go on, astronomers do not know much about the nature of dark matter. One possibility is that it could just be ordinary matter that does not emit radiation in objects such as black holes, neutron stars, and brown dwarfs, objects more massive than Jupiter but smaller than the smallest stars. However, astronomers cannot find enough of these types of objects, which they have named MACHOS (massive astrophysical compact halo object), to account for all the dark matter, so they are thought to be only a small part of the total.

Another possibility is that the dark matter is thought to be much different from the ordinary matter we see. Some appear to be particles that have gravity, but don’t otherwise appear to interact with other particles. Scientists call these theoretical particles WIMPs, which stands for Weakly Interactive Massive Particles. Most scientists who study dark matter think that the dark matter in the universe is a combination of massive astrophysical compact halo object (MACHOS) and some exotic matter such as weakly-interacting massive particles (WIMPs). Researching dark matter is an active area of scientific research, and astronomers’ knowledge about dark matter is changing rapidly.

Astronomers who study the expansion of the universe are interested in knowing the rate of that expansion. Is the rate fast enough to overcome the attractive pull of gravity? If yes, then the universe will expand forever, although the expansion will slow down over time.  If no, then the universe would someday start to contract, and eventually get squeezed together in a big crunch, the opposite of the Big Bang.

Recently astronomers have made a discovery that answers that question: the rate at which the universe is expanding is increasing. In other words, the universe is expanding faster now than ever before, and in the future, it will expand even faster. So now astronomers think that the universe will keep expanding forever. However, it also proposes a problematic new question: What is causing the expansion of the universe to accelerate? One possible hypothesis involves a new, hypothetical form of energy called dark energy. Some scientists think that dark energy makes up as much as 72 percent of the total energy content of the universe.

Star Systems

Stars are the most widely recognized astronomical objects, and represent the most fundamental building blocks of galaxies. The age, distribution, and composition of the stars in a galaxy trace the history, dynamics, and evolution of that galaxy. Moreover, stars are responsible for the manufacture and distribution of heavy elements such as carbon, nitrogen, and oxygen, and their characteristics are intimately tied to the characteristics of the planetary systems that may coalesce about them. Consequently, the study of the birth, life, and death of stars is central to the field of astronomy.” (NASA)

Although constellations have stars that usually only appear to be close together, stars may be found in the same portion of space. Stars that are grouped tightly together are called star systems. Larger groups of hundreds or thousands of stars are called star clusters. The image shown here is a famous star cluster known as Pleiades, which can be seen with the naked autumn sky.

The Pleiades, an open cluster consisting of approximately 3,000 stars at a distance of 400 light-years from Earth in the constellation of Taurus. It is also known as “The Seven Sisters.”

Although the star humans know best is a single star, many stars—in fact, more than half of the bright stars in our galaxy—are star systems. A system of two stars orbiting each other is a binary star. A system with more than two stars orbiting each other is a multiple star system. The stars in a binary or multiple star system are often so close together that they appear as only through a telescope can the pair be distinguished.

Star clusters are divided into two main types, open clusters, and globular clusters. Open clusters are groups of up to a few thousand stars that are loosely held together by gravity. Pleiades is an open cluster that is also called the Seven Sisters. Open clusters tend to be blue and often contain glowing gas and dust and are made of young stars that formed from the same nebula. The stars may eventually be pulled apart by gravitational attraction to other objects.

Globular clusters are groups of tens to hundreds of thousands of stars held tightly together by gravity. Globular clusters have a definite, spherical shape and contain mostly reddish stars. The stars are closer together, closer to the center of the cluster. Globular clusters do not have much dust in them — the dust has already formed into stars.

This stellar swarm is one of the densest of the 147 known globular star clusters in the Milky Way galaxy. Located about 28,000 light-years from Earth, it contains hundreds of thousands of stars, all held together by their mutual gravitational attraction. Globular clusters are particularly useful for studying stellar evolution, since all of the stars in the cluster have the same age (about 12 billion years), but cover a range of stellar masses. Every star visible in this image is either more highly evolved than, or in a few rare cases more massive than, our own Sun. Especially obvious are the bright red giants, which are stars similar to the Sun in mass that are nearing the ends of their lives.

To view more amazing images taken by the Hubble telescope, click here. If you would like to download free eBooks of Hubble imagery, click here.

2.3 Types of Galaxies

Galaxies are the most prominent groups of stars and can contain anywhere from a few million stars to many billions of stars. Every star that is visible in the night sky is part of the Milky Way Galaxy. To the naked eye the closest major galaxy, the Andromeda Galaxy, looks like only a dim, fuzzy spot but that fuzzy spot contains one trillion stars.

Spiral and Elliptical Galaxies

Spiral galaxies spin, so they appear as a rotating disk of stars and dust, with a bulge in the middle, like the Sombrero Galaxy. Several arms spiral outward in the Pinwheel Galaxy and are appropriately called spiral arms. Spiral galaxies have lots of gas and dust and lots of young stars.

Barred spiral galaxy was photographed by the Hubble telescope. In the core of the larger spiral structure, the nucleus shows its own extraordinary and distinct “grand-design” spiral structure that is about 3,300 light-years long.

Other galaxies are egg-shaped and called an elliptical galaxy. The smallest elliptical galaxies are as small as some globular clusters. Giant elliptical galaxies, on the other hand, can contain over a trillion stars. Elliptical galaxies are reddish to yellowish because they contain mostly old stars. Most elliptical galaxies contain very little gas and dust because they had already formed. However, some elliptical galaxies contain lots of dust. Why might some elliptical galaxies contain dust?

Irregular and Dwarf Galaxies

Galaxies that are not elliptical galaxies or spiral galaxies are irregular galaxies. Most irregular galaxies were once spiral or elliptical galaxies that were then deformed either by gravitational attraction to a more massive galaxy or by a collision with another galaxy.

Dwarf galaxies are small galaxies containing only a few million to a few billion stars. Dwarf galaxies are the most common type in the universe. However, because they are relatively small and dim, we do not see as many dwarf galaxies from Earth. Most dwarf galaxies are irregular in shape. However, there are also dwarf elliptical galaxies and dwarf spiral galaxies

Look back at the picture of the spiral galaxy, Andromeda. Next to our closest galaxy neighbor are two dwarf elliptical galaxies that are companions to the Andromeda Galaxy. One is a bright sphere to the left of center, and the other is a long ellipse below and to the right of center. Dwarf galaxies are often found near larger galaxies. They sometimes collide with and merge into their larger neighbors.

This image of an irregular galaxy is about 7.5 million light-years away and 70 000 light-years across.

2.4 Milky Way Galaxy

On a dark, clear night, a milky band of light will stretch across the sky. This band is the disk of a galaxy, the Milky Way Galaxy is our galaxy and is made up of millions of stars along with a lot of gas and dust. Although it is difficult to know what the shape of the Milky Way Galaxy is because we are inside of it, astronomers have identified it as a typical spiral galaxy containing about 100 billion to 400 billion stars.

Like other spiral galaxies, our galaxy has a disk, a central bulge, and spiral arms. The disk is about 100,000 light-years across and 3,000 light-years thick. Most of the Galaxy’s gas, dust, young stars, and open clusters are in the disk. Scientists know that the Milky Way is a spiral galaxy because of the shape of the galaxy as from Earth’s perspective, the velocities of stars and gas in the galaxy show a rotational motion, and the gases, color, and dust are typical of spiral galaxies.

The central bulge is about 12,000 to 16,000 light-years wide and 6,000 to 10,000 light-years thick. The central bulge contains mostly older stars and globular clusters. Some recent evidence suggests the bulge might not be spherical, but is instead shaped like a bar. The bar might be as long as 27,000 light-years long. The disk and bulge are surrounded by a faint, spherical halo, which also includes old stars and globular clusters. Astronomers have discovered that there is a gigantic black hole at the center of the galaxy.

Artist’s conception of the spiral structure of the Milky Way with two major stellar arms.

The Milky Way Galaxy is a significant place. Our solar system, including the Sun, Earth, and all the other planets, is within one of the spiral arms in the disk of the Milky Way Galaxy. Most of the stars we see in the sky are relatively nearby stars that are also in this spiral arm. Earth is about 26,000 light-years from the center of the galaxy, a little more than halfway out from the center of the galaxy to the edge.

Just as Earth orbits the Sun, the Sun and solar system orbit the center of the Milky Way Galaxy. One orbit of the solar system takes about 225 to 250 million years. The solar system has orbited 20 to 25 times since it formed 4.6 billion years ago. Astronomers have recently found that at the center of the Milky Way, and most other galaxies, is a supermassive black hole, though a black hole cannot be seen.

2.5 Nuclear Fusion within Stars

The Sun is Earth’s primary source of energy, yet the planet only receives a small portion of its energy, and the Sun is just an ordinary star. Many stars produce much more energy than the Sun. The energy source for all stars is nuclear fusion.

Stars are made mostly of hydrogen and helium, which are packed so densely in a star that in the star’s center the pressure is enormous enough to initiate nuclear fusion reactions. In a nuclear fusion reaction, the nuclei of two atoms combine to create a new atom. Most commonly, in the core of a star, two hydrogen atoms fuse to become a helium atom. Although nuclear fusion reactions require much energy to get started, once they are going they produce enormous amounts of energy.

In a star, the energy from fusion reactions in the core pushes outward to balance the inward pull of the star’s gravity. This energy moves outward through the layers of the star until it finally reaches the star’s outer surface. The outer layer of the star glows brightly, sending the energy out into space as electromagnetic radiation, including visible light, heat, ultraviolet light, and radio waves.

In particle accelerators, subatomic particles are propelled until they have attained almost the same amount of energy as found in the core of a star. When these particles collide head-on, new particles are created. This process simulates the nuclear fusion that takes place in the cores of stars. The process also mimics the conditions that allowed for the first helium atom to be produced from the collision of two hydrogen atoms in the first few minutes of the universe.

2.6 Star Classification

Think about how the color of a piece of metal changes with temperature. A coil of an electric stove will start out black but with added heat will start to glow a dull red. With more heat, the coil turns a brighter red, then orange. At extremely high temperatures the coil will turn yellow-white, or even blue-white. A star’s color is also determined by the temperature of the star’s surface. Relatively cool stars are red, warmer stars are orange or yellow, and extremely hot stars are blue or blue-white.

Color is the most common way to classify stars. The class of a star is given by a letter, where each letter corresponds to a color and temperature range. Note that these letters do not match the color names; they are left over from an older system that is no longer used. For most stars, the surface temperature is also related to size. Bigger, bluish-white stars produce more energy and have hotter surfaces than smaller, yellow stars that produce less energy. , so their surfaces are hotter than smaller stars. These stars tend toward bluish white.

Stars have a life cycle that is expressed similarly to the life cycle of a living creature: they are born, grow, change over time, and eventually die. Most stars change in size, color, and class at least once in their lifetime. What astronomers know about the life cycles of stars is because of data gathered from visual, radio, and X-ray telescopes. To learn more about star formation from the European Space Agency (ESA), click here.

The Main Sequence

For most of a star’s life, nuclear fusion in the core produces helium from hydrogen, a stage called a main sequence star. This term comes from the Hertzsprung-Russell diagram shown below. For stars on the main sequence, the temperature is directly related to brightness. A star is on the main sequence as long as it can balance the inward force of gravity with the outward force of nuclear fusion in its core. The more massive a star, the more it must burn hydrogen fuel to prevent internal gravitational collapse. Because they burn more fuel, massive stars have higher temperatures, but run out of hydrogen sooner than smaller stars.

Our Sun, a yellow star, has been a main sequence star for about 5 billion years and will continue on the main sequence for about 5 billion more years. Very large stars may be on the main sequence for only 10 million years. Very small stars may last tens to hundreds of billions of years.

Hertzsprung-Russell Diagram identifying many well known stars in the Milky Way galaxy.

Red Giants and White Dwarfs

As a star begins to use up its hydrogen, it fuses helium atoms together into heavier atoms such as carbon. A blue giant star has exhausted its hydrogen fuel and is a transitional phase. When the light elements are mostly used up the star can no longer resist gravity, and it starts to collapse inward. The outer layers of the star grow outward and cool. The larger, cooler star turns red and so is called a red giant.

The current size of the Sun (now in the main sequence) compared to its estimated maximum size during its red-giant phase in the future

Eventually, a red giant burns up all of the helium in its core. What happens next depends on how massive the star is. A typical star, such as the Sun, stops fusion completely. Gravitational collapse shrinks the star’s core to a white, glowing object about the size of Earth, called a white dwarf, which will ultimately fade out.

Supergiants and Supernovas

A star that runs out of helium will end its life much more dramatically. When very massive stars leave the main sequence, they become red supergiants. Unlike a red giant, when all the helium in a red supergiant is gone, fusion continues. Lighter atoms fuse into heavier atoms up to iron atoms. Creating elements heavier than iron through fusion uses more energy than it produces, so stars do not ordinarily form any heavier elements. When there are no more elements for the star to fuse, the core succumbs to gravity and collapses, creating an explosion called a supernova.

A supernova explosion contains so much energy that atoms can fuse together to produce heavier elements such as gold, silver, and uranium. A supernova can shine as brightly as an entire galaxy for a short time. Nuclear fusion in stars created all elements with an atomic number greater than that of lithium.

Neutron Stars and Black Holes

After a supernova explosion, the leftover material in the core is incredibly dense. If the core is less than about four times the mass of the Sun, the star becomes a neutron star. A neutron star is made almost entirely of neutrons, relatively large particles that have no electrical charge.

If the core remaining after a supernova is more than about five times the mass of the Sun, the core collapses into a black hole. Black holes are so dense that not even light can escape their gravity. With no light, a black hole cannot be observed directly. However, a black hole can be identified by the effect that it has on objects around it, and by radiation that leaks out around its edges.

2.7 Measuring Distant Stars

Parallax is an apparent shift in position that takes place when the position of the observer changes. To see an example of parallax, try holding your finger about 1 foot (30 cm) in front of your eyes. Now, while focusing on your finger, close one eye and then the other. Alternate back and forth between eyes, and pay attention to how your finger appears to move. The shift in the position of your finger is an example of parallax. Now try moving your finger closer to your eyes, and repeat the experiment. Do you notice any difference? The closer your finger is to your eyes, the greater the position changes because of parallax.

Astronomers use this same principle to measure the distance to stars. Instead of a finger, they focus on a star, and instead of switching back and forth between eyes, they switch between the biggest possible differences in observing position. To do this, an astronomer first looks at the star from one position and notes where the star is relative to more distant stars. Now, where will the astronomer go to make an observation the greatest possible distance from the first observation? In six months, after Earth moves from one side of its orbit around the Sun to the other side, the astronomer looks at the star again. This time parallax causes the star to appear in a different position relative to more distant stars. From the size of this shift, astronomers can calculate the distance to the star.

NASA’s Hubble Extends Stellar Tape Measure 10 Times Farther Into Space Illustration of spatial scanning By applying a technique called spatial scanning to an age-old method for gauging distances called astronomical parallax, scientists now can use NASA’s Hubble Space Telescope to make precision distance measurements 10 times farther into our galaxy than previously possible.

Using NASA’s Hubble Space Telescope, astronomers now can precisely measure the distance of stars up to 10,000 light-years away; 10 times farther than previously possible. Astronomers have developed yet another novel way to use the 24-year-old space telescope by employing a technique called spatial scanning, which dramatically improves Hubble’s accuracy for making angular measurements. The technique, when applied to the age-old method for gauging distances called astronomical parallax, extends Hubble’s tape measure 10 times farther into space. “This new capability is expected to yield new insight into the nature of dark energy, a mysterious component of space that is pushing the universe apart at an ever-faster rate,” said Noble laureate Adam Riess of the Space Telescope Science Institute (STScI) in Baltimore, Md.

Parallax, a trigonometric technique, is the most reliable method for making astronomical distance measurements, and a practice long employed by land surveyors here on Earth. The diameter of Earth’s orbit is the base of a triangle and the star is the apex where the triangle’s sides meet. The lengths of the sides are calculated by accurately measuring the three angles of the resulting triangle. Astronomical parallax works reliably well for stars within a few hundred light-years of Earth.

For example, measurements of the distance to Alpha Centauri, the star system closest to our sun, vary only by one arc second. This variance in distance is equal to the apparent width of a dime seen from two miles away. Stars farther out have much smaller angles of apparent back-and-forth motion that are extremely difficult to measure.

Astronomers have pushed to extend the parallax yardstick ever deeper into our galaxy by measuring smaller angles more accurately. This new long-range precision was proven when scientists successfully used Hubble to measure the distance of a special class of bright stars called Cepheid variables, approximately 7,500 light-years away in the northern constellation Auriga. The technique worked so well, they are now using Hubble to measure the distances of other far-flung Cepheids. Such measurements will be used to provide a firmer footing for the so-called cosmic “distance ladder.” This ladder’s “bottom rung” is built on measurements to Cepheid variable stars that, because of their known brightness, have been used for more than a century to gauge the size of the observable universe. They are the first step in calibrating far more distant extra-galactic milepost markers such as Type Ia supernovae.

Riess and the Johns Hopkins University in Baltimore, Md., in collaboration with Stefano Casertano of STScI, developed a technique to use Hubble to make measurements as small as five-billionths of a degree. To make a distance measurement, two exposures of the target Cepheid star were taken six months apart, when Earth was on opposite sides of the sun. A very subtle shift in the star’s position was measured to an accuracy of 1/1,000 the width of a single image pixel in Hubble’s Wide Field Camera 3, which has 16.8 megapixels total. A third exposure was taken after another six months to allow for the team to subtract the effects of the subtle space motion of stars, with additional exposures used to remove other sources of error.

Riess shares the 2011 Nobel Prize in Physics with another team for his leadership in the 1998 discovery the expansion rate of the universe is accelerating — a phenomenon widely attributed to a mysterious, unexplained dark energy filling the universe. This new high-precision distance measurement technique is enabling Riess to gauge just how much the universe is stretching. His goal is to refine estimates of the universe’s expansion rate to the point where dark energy can be better characterized.

2.8 Models of the Solar System

Humans’ view of the solar system has evolved as technology, and scientific knowledge has increased. The ancient Greeks identified five of the planets, and for many centuries they were the only planets known. Since then, scientists have discovered two more planets, many other solar-system objects and even planets found outside our solar system.

Geocentric Model

The ancient Greeks believed that Earth was at the center of the universe. This view is called the geocentric model of the universe. Geocentric means “Earth-centered.” In the geocentric model, the sky, or heavens, are a set of spheres layered on top of one another. Each object in the sky is attached to a sphere and moves around Earth as that sphere rotates. From Earth outward, these spheres contain the Moon, Mercury, Venus, the Sun, Mars, Jupiter, and Saturn. An outer sphere holds all the stars. Since the planets appear to move much faster than the stars, the Greeks placed them closer to Earth.

The geocentric model worked well by explaining why all the stars appear to rotate around Earth once per day. The model also explained why the planets move differently from the stars and each other. One problem with the geocentric model is that some planets seem to move backward, in retrograde, instead of in their usual forward motion around Earth.

Around 150 A.D. the astronomer Ptolemy resolved this problem by using a system of circles to describe the motion of planets. In Ptolemy’s system, a planet moves in a small circle, called an epicycle. This circle moves around Earth in a larger circle, called a deferent. Ptolemy’s version of the geocentric model worked so well that it remained the accepted model of the universe for more than a thousand years.

Heliocentric Model

Ptolemy’s geocentric model worked but it was not only complicated, it occasionally made errors in predicting the movement of planets. At the beginning of the 16th century A.D., Nicolaus Copernicus proposed that Earth and all the other planets orbit the Sun. With the Sun at the center, this model is called the heliocentric model or “sun-centered” model of the universe. Copernicus’ model explained the motion of the planets, as well as Ptolemy’s model, did, but it did not require complicated additions like epicycles and deferents.

Although Copernicus’ model worked more simply than Ptolemy’s, it still did not perfectly describe the motion of the planets because, like Ptolemy, Copernicus thought planets moved in perfect circles. Not long after Copernicus, Johannes Kepler refined the heliocentric model so that the planets moved around the Sun in ellipses (ovals), not circles. Kepler’s model matched observations perfectly.

Because people were so used to thinking of Earth at the center of the universe, the heliocentric model was not widely accepted at first. However, when Galileo Galilei first turned a telescope to the heavens in 1610, he made several striking discoveries. Galileo discovered that the planet Jupiter has moons orbiting around it. This provided the first evidence that objects could orbit something besides Earth. Galileo also discovered that Venus has phases like the Moon, which provides direct evidence that Venus orbits the Sun.

Galileo’s discoveries caused many more people to accept the heliocentric model of the universe, although Galileo himself was found guilty of heresy for his ideas. The shift from an Earth-centered view to a Sun-centered view of the universe is referred to as the Copernican Revolution.

Watch this animation of the Ptolemaic and Copernican models of the solar system. Ptolemy made the best model he could with the assumption that Earth was the center of the universe, but by letting that assumption go, Copernicus came up with a much simpler model. Before people would accept that Copernicus was right, they needed to accept that the Sun was the center of the solar system.

Today we know that just as Earth orbits the Sun, the Sun and solar system orbit the center of the Milky Way galaxy. The center of the Milky way may likely be a massive black hole. One orbit of the solar system takes about 225 to 250 million years. It is believed that the solar system has orbited 20 to 25 times since it formed 4.6 billion years ago.

2.9 The Modern Solar System

Today, we know that our solar system is just one tiny part of the universe as a whole. Neither Earth nor the Sun is at the center of the universe. However, the heliocentric model accurately describes the solar system. In our modern view of the solar system, the Sun is at the center, with the planets moving in elliptical orbits around the Sun. The planets do not emit their light, but instead, reflect light from the Sun. Esri has created an excellenct story map called the Solar System Atlas.

NASA has created a great website called Solar System Exploration and National Geographic has created a great resource called Solar System. Both websites are splendid sources to introduce yourself to our solar system.


Since the early 1990s, astronomers have discovered other solar systems, with planets orbiting stars other than our own Sun, called extrasolar planets or simply, exoplanets. Some extrasolar planets have been directly observed, but most have been discovered by indirect methods. One technique involves detecting the very slight motion of a star periodically moving toward and away from us along our line-of-sight, known as a star’s radial velocity. This periodic motion can be attributed to the gravitational pull of a planet or, sometimes, another star orbiting the star.

A planet may also be identified by measuring a star’s brightness over time. A temporary, periodic decrease in light emitted from a star can occur when a planet crosses in front of the star it is orbiting, called a transit, momentarily blocking out some of the starlight. So far, more than 3,600 extrasolar planets have been identified, and the rate of discovery is increasing rapidly.

Planets and Their Motions

Since the time of Copernicus, Kepler, and Galileo, we have learned a lot more about our solar system. Astronomers have discovered two more planets (Uranus and Neptune), four dwarf planets (Ceres, Pluto, Makemake, Haumea, and Eris), more than 150 moons, and many, many asteroids and other small objects.

Although the Sun is just an average star compared to other stars, it is by far the most massive object in the solar system. The Sun is more than 500 times the mass of everything else in the solar system combined. The table below gives data on the sizes of the Sun and planets relative to Earth.

Object Mass (Relative to Earth) Diameter of Planet (Relative to Earth)
Sun 333,000 Earth’s mass 109.2 Earth’s diameter
Mercury 0.06 Earth’s mass 0.39 Earth’s diameter
Venus 0.82 Earth’s mass 0.95 Earth’s diameter
Earth 1.00 Earth’s mass 1.00 Earth’s diameter
Mars 0.11 Earth’s mass 0.53 Earth’s diameter
Jupiter 317.8 Earth’s mass 11.21 Earth’s diameter
Saturn 95.2 Earth’s mass 9.41 Earth’s diameter
Uranus 14.6 Earth’s mass 3.98 Earth’s diameter
Neptune 17.2 Earth’s mass 3.81 Earth’s diameter

Size and Shape of Planetary Orbits

The figure below shows the relative sizes of the orbits of the major planets within our solar system. In general, the farther away from the Sun, the higher the distance from one planet’s orbit to the next. The orbits of the planets are not circular but slightly elliptical with the Sun located at one of the foci.

While studying the solar system, Johannes Kepler discovered the relationship between the time it takes a planet to make one complete orbit around the Sun, its “orbital period,” and the distance from the Sun to the planet. If the orbital period of a planet is known, then it is possible to determine the planet’s distance from the Sun. This is how astronomers without modern telescopes could determine the distances to other planets within the solar system.

Distances in the solar system are often measured in astronomical units (AU). One astronomical unit is defined as the distance from Earth to the Sun. 1 AU equals about 150 million km, or 93 million miles. The table below shows the distances to the planets (the average radius of orbits) in AU. The table also shows how long it takes each planet to spin on its axis (the length of a day) and how long it takes each planet to complete an orbit (the length of a year); in particular, notice how slowly Venus rotates relative to Earth.

Planet Average Distance from Sun Length of Day Length of Year
Mercury 0.39 Astronomical Units (AU) 56.84 (In Earth Days) 0.24 (In Earth Years)
Venus 0.72 243.02 0.62
Earth 1.00 1.00 1.00
Mars 1.52 1.03 1.88
Jupiter 5.2 0.41 11.86
Saturn 9.54 0.43 29.46
Uranus 19.22 0.72 84.01
Neptune 30.06 0.67 164.8

The Role of Gravity

Isaac Newton was one of the first scientists to explore gravity. He understood that the Moon circles the Earth because a force is pulling the Moon toward Earth’s center. Without that force, the Moon would continue moving in a straight line off into space. Newton also came to understand that the same force that keeps the Moon in its orbit is the same force that causes objects on Earth to fall to the ground.

Newton defined the Universal Law of Gravitation, which states that a force of attraction, called gravity, exists between all objects in the universe. The strength of the gravitational force depends on how much mass the objects have and how far apart they are from each other. The greater the objects’ mass, the higher the force of attraction; also, the greater the distance between the objects, the smaller the force of attraction.

The distance between the Sun and each of its planets is substantial, but the Sun and each of the planets are also very large. Gravity keeps each planet orbiting the Sun because the star and its planets are enormous objects. The force of gravity also holds moons in orbit around planets. There are two additional key features of the solar system that helps us understand how it formed: 1) All the planets lie in nearly the same plane, or flat disk-like region, 2) All the planets orbit in the same direction around the Sun.

2.10 Formation of the Solar System

pay attention to after 6:10 minutes

A Giant Nebula

The most widely accepted explanation of how the solar system formed is called the nebular hypothesis. According to this hypothesis, the Sun and the planets of our solar system formed about 4.6 billion years ago from the collapse of a giant cloud of gas and dust, called a nebula.

The nebula was drawn together by gravity, which released gravitational potential energy. As small particles of dust and gas smashed together to create larger ones, they released kinetic energy. As the nebula collapsed, the gravity at the center increased, and the cloud started to spin because of its angular momentum. As it collapsed further, the spinning got faster, much as an ice skater spins faster when he pulls his arms to his sides during a spin.

Much of the cloud’s mass migrated to its center, but the rest of the material flattened out in an enormous disk. The disk contained hydrogen and helium, along with heavier elements and even simple organic molecules.

Formation of the Sun and Planets

As gravity pulled matter into the center of the disk, the density and pressure at the center became intense. When the pressure in the center of the disk was high enough, nuclear fusion within our star began, and the blazing star stopped the disk from collapsing further.

Meanwhile, the outer parts of the disk were cooling off. Matter condensed from the cloud and small pieces of dust started clumping together to create ever bigger clumps of matter. Larger clumps, called planetesimals, attracted smaller clumps with their gravity. Gravity at the center of the disk attracted more massive particles, such as rock and metal and lighter particles remained further out in the disk. Eventually, the planetesimals formed protoplanets, which grew to become the planets and moons that we find in our solar system today.

Because of the gravitational sorting of material with the inner planets, Mercury, Venus, Earth, and Mars, dense rock and metal formed. The outer planets, Jupiter, Saturn, Uranus and Neptune, condensed farther from the Sun from lighter materials such as hydrogen, helium, water, ammonia, and methane. Out by Jupiter and beyond, where it is frigid, these materials formed solid particles.

The nebular hypothesis was designed to explain some of the essential features of the solar system:

  • The orbits of the planets lie in nearly the same plane with the Sun at the center
  • The planets revolve in the same direction
  • The planets mostly rotate in the same direction
  • The axes of rotation of the planets are mostly nearly perpendicular to the orbital plane
  • The oldest moon rocks are 4.5 billion years

The two videos below, from the European Space Agency (ESA), discusses the Sun, planets, and other bodies in the Solar System and how they formed. The first part of the video explores the evolution of our view of the solar system starting with the early Greeks who reasoned that since some points of light, which they called planets, moved faster than the stars, they must be closer.

2.11 The Sun

Consider Earth, the Moon, and all the other planets and satellites in the solar system. The mass of all of those objects together accounts for only 0.2 percent of the total mass of the solar system. The remaining 99.8 percent of the solar system’s mass is within the Sun. The Sun is the center of the solar system and the most massive object in the solar system. This nearby star provides light and heat and supports almost all life on Earth.

The Sun is a sphere, composed almost entirely of the elements hydrogen and helium. The Sun is not solid or typical gas. Most atoms in the Sun exist as plasma, the fourth state of matter made up of superheated gas with a positive electrical charge.

Internal Structure

Because the Sun is not solid, it does not have a defined outer boundary. It does, however, have a definite internal structure with identifiable layers (Figure below). From inward to outward they are:

The Sun’s central core is plasma with a temperature of around 27 million degrees Celsius. At such high temperatures, hydrogen combines to form helium by nuclear fusion, a process that releases vast amounts of energy. This energy moves outward, towards the outer layers of the Sun.

The radiative zone, just outside the core, has a temperature of about 7 million degrees Celsius. The energy released in the core travels extremely slowly through the radiative zone. A particle of light, called a photon, travels only a few millimeters before it hits another particle. The photon is absorbed and then released again. A photon may take as long as 50 million years to travel all the way through the radiative zone.

In the convection zone, hot material from near the radiative zone rises, cools at the Sun’s surface, and then plunges back downward to the radiative zone. Convective movement helps to create solar flares and sunspots.

The Outer Layers

The next three layers make up the Sun’s atmosphere. Since there are no solid layers to any part of the Sun, these boundaries are fuzzy and indistinct.

The photosphere is the visible surface of the Sun, the region that emits sunlight. The photosphere is relatively cool, only about 6,700 degrees Celsius. The photosphere has several different colors; oranges, yellows, and reds, giving it a grainy appearance.

The chromosphere is a thin zone, about 2,000 km thick, that glows red as energy heats it from the photosphere. Temperatures in the chromosphere range from about 4,000-10,000 degrees Celsius. Jets of gas fire up through the chromosphere at speeds up to 72,000 km per hour, reaching heights as high as 10,000 km.

The corona is the outermost plasma layer and is called the Sun’s halo or crown. The corona’s temperature of 2 to 5 million°C is much hotter than the photosphere.

Diagram of the Sun.

Surface Features

The Sun’s surface features are quite visible, but only with specialized equipment. For example, sunspots are only visible with special light-filtering lenses.

The most noticeable surface feature of the Sun are cooler, darker areas known as sunspots. Sunspots are located where loops of the Sun’s magnetic field break through the surface and disrupt the smooth transfer of heat from lower layers of the Sun, making them cooler and darker and marked by intense magnetic activity. Sunspots usually occur in pairs. When a loop of the Sun’s magnetic field breaks through the surface, a sunspot is created where the loop comes out and where it goes back in again.

There are other types of interruptions of the Sun’s magnetic energy. If a loop of the sun’s magnetic field snaps and breaks, it creates solar flares, which are violent explosions that release vast amounts of energy. A strong solar flare can turn into a coronal mass ejection.

On August 31, 2012 a long prominence/filament of solar material that had been hovering in the Sun’s atmosphere, the corona, erupted out into space at 4:36 p.m. EDT. Seen here from the Solar Dynamics Observatory, the flare caused an aurora on Earth on September 3.

A solar flare or coronal mass ejection release streams of highly energetic particles that make up the solar wind. The solar wind can be dangerous to spacecraft and astronauts because it sends out massive amounts of radiation that can harm the human body. Solar flares have knocked out entire power grids and disturbed radio, satellite, and cell phone communications.

Another highly visible feature on the Sun is solar prominences. If plasma flows along a loop of the Sun’s magnetic field from sunspot to sunspot, it forms a glowing arch that reaches thousands of kilometers into the Sun’s atmosphere. Prominences can last for a day to several months. Prominences are also visible during a total solar eclipse.

2.12 The Inner Planets

On Earth, scientists can collect and analyze the chemistry of samples, do radiometric dating to determine their ages, and look at satellite images to see large-scale features. Rovers have landed on Mars and sent back enormous amounts of information, but much of the rest of what is known about the inner planets are from satellite images.

The inner planets, or terrestrial planets, are the four planets closest to the Sun: Mercury, Venus, Earth, and Mars. Unlike the outer planets, which have many satellites, Mercury and Venus do not have moons, Earth has one, and Mars has two. Of course, the inner planets have shorter orbits around the Sun, and they all spin more slowly. Geologically, the inner planets are all made of cooled igneous rock with iron cores, and all have been geologically active, at least early in their history. None of the inner planets has rings.

This diagram shows the approximate relative sizes of the terrestrial planets, from left to right: Mercury, Venus, Earth and Mars. Distances are not to scale. A terrestrial planet is a planet that is primarily composed of silicate rocks. The term is derived from the Latin word for Earth, “Terra”, so an alternate definition would be that these are planets which are, in some notable fashion, “Earth-like”.

Terrestrial planets are substantially different from gas giants, which might not have solid surfaces and are composed mostly of some combination of hydrogen, helium, and water existing in various physical states. Terrestrial planets all have roughly the same structure: a central metallic core, mostly iron, with a surrounding silicate mantle. Terrestrial planets have canyons, craters, mountains, volcanoes and secondary atmospheres.


The smallest planet, Mercury, is the planet closest to the Sun. Because of Mercury’s proximity to the Sun, it is difficult to observe from Earth, even with a telescope. However, the Mariner 10 spacecraft visited Mercury from 1974 to 1975. The MESSENGER spacecraft, which stands for Mercury Surface, Space Environment, Geochemistry, and Ranging, has been studying Mercury in detail since 2005. The craft is currently in orbit around the planet, where it is creating detailed maps. MESSENGER stands for Mercury Surface, Space Environment, Geochemistry, and Ranging.

The surface of Mercury is covered with craters. Ancient impact craters mean that for billions of years Mercury has not changed much geologically. Also, with minimal atmosphere, the processes of weathering and erosion do not wear down structures on the planet. Mercury is one of the solar system’s densest planets. It is relatively large, liquid core, made mostly of melted iron, takes up about 42% of the planet’s volume.

Mercury is named for the Roman messenger god, who could run extremely quickly, just as the planet moves very quickly in its orbit around the Sun. A year on Mercury, the length of time it takes to orbit the Sun is just 88 Earth days.

Despite its very short years, Mercury has very long days. A day is defined as the time it takes a planet to turn on its axis. Mercury rotates slowly on its axis, turning precisely three times for every two times it orbits the Sun. Therefore, each day on Mercury is 57 Earth days long. In other words, on Mercury, a year is only a Mercury day and a half long!

Mercury is close to the Sun so that it can get very hot. However, Mercury has virtually no atmosphere, no water to insulate the surface, and it rotates very slowly. For these reasons, temperatures on the surface of Mercury vary widely. In direct sunlight, the surface can be as hot as 427°C (801°F). On the dark side, or in the shadows inside craters, the surface can be as cold as -183°C (-297°F)! Although most of Mercury is extremely dry, scientists think there may be a small amount of water in the form of ice at the poles of Mercury, in areas that never receive direct sunlight.


Named after the Roman goddess of love, Venus is the only planet named after a female. Venus’ thick clouds reflect sunlight well, so Venus is very bright making it the brightest object in the sky besides the Sun and the Moon. Because the orbit of Venus is inside Earth’s orbit, Venus always appears close to the Sun. When Venus rises just before the Sun rises, the bright object is called the morning star. When it sets just after the Sun sets, it is the evening star.

Of the planets, Venus is most similar to Earth in size and density. Venus is also our nearest neighbor. The planet’s interior structure is similar to Earth’s with a large iron core and a silicate mantle. However, the resemblance between the two inner planets ends there.

Venus rotates in a direction opposite the other planets and opposite to the direction it orbits the Sun. This rotation is extremely slow, only one turn every 243 days. This is longer than a year on Venus where it only takes Venus 224 days to orbit the Sun.

Size comparison with Earth.

Venus is covered by a thick layer of clouds. Venus’ clouds are not made of water vapor like Earth’s clouds. Clouds on Venus are made mostly of carbon dioxide with a bit of sulfur dioxide and corrosive sulfuric acid. Because carbon dioxide is a greenhouse gas, the atmosphere traps heat from the Sun and creates a powerful greenhouse effect. Even though Venus is further from the Sun than Mercury, the greenhouse effect makes Venus the hottest planet. Temperatures at the surface reach 465°C (860°F). That is hot enough to melt lead.

The atmosphere of Venus is so thick that the atmospheric pressure on the planet’s surface is 90 times greater than the atmospheric pressure on Earth’s surface. The dense atmosphere obscures the surface of Venus, even from spacecraft orbiting the planet.

Since spacecraft cannot see through the thick atmosphere, radar is used to map Venus’ surface. Many features found on the surface are similar to Earth and yet are very different. The figure on the left shows a topographical map of Venus produced by the Magellan probe using radar.

Orbiting spacecraft have used radar to reveal mountains, valleys, and canyons. Most of the surface has large areas of volcanoes surrounded by plains of lava. In fact, Venus has many more volcanoes than any other planet in the solar system, and some of those volcanoes are very large. Most of the volcanoes are no longer active, but scientists have found evidence that there is some active volcanism.

Venus also has very few impact craters compared with Mercury and the Moon. What is the significance of this? Earth has fewer impact craters than Mercury and the Moon too. Is this for the same reason that Venus has fewer impact craters?

It is difficult for scientists to figure out the geological history of Venus. The environment is too harsh for a rover to go there. It is even more difficult for students to figure out the geological history of a distant planet based on the information given here. Still, we can piece together a few things.

On Earth, volcanism is generated because the planet’s interior is hot. Much of the volcanic activity is caused by plate tectonic activity. However, on Venus, there is no evidence of plate boundaries, and volcanic features do not line up the way they do at plate boundaries.

Because the density of impact craters can be used to determine how old a planet’s surface is, the small number of impact craters means that Venus’ surface is still relatively young. Scientists think that there is frequent, planet-wide resurfacing of Venus with volcanism taking place in many locations. The cause is the heat that builds up below the surface that has no escape until finally it destroys the crust and results in volcanoes.


Earth has vast oceans of liquid water, large masses of exposed land, and a dynamic atmosphere with clouds of water vapor. Earth also has ice covering its polar regions. Earth’s average surface temperature is 14°C (57°F). Water is a liquid at this temperature, but the planet also has water in its other two states, solid and gas. The oceans and the atmosphere help keep Earth’s surface temperatures reasonably steady.

Earth is the only planet known to have known life. The presence of liquid water, the ability of the atmosphere to filter out harmful radiation, and many other features make the planet uniquely suited to harbor life. Life and Earth now affect each other; for example, the evolution of plants allowed oxygen to enter the atmosphere in large enough quantities for animals to evolve. Although life has not been found elsewhere in the solar system, other planets or satellites may harbor primitive life forms. Life may also be found elsewhere in the universe.

NASA created these two images to exhibit high-resolution global composites of Moderate Resolution Imaging Spectroradiometer (MODIS) data.

The heat that remained from the planet’s accretion, gravitational compression and radioactive decay allowed the Earth to melt, probably more than once. As it subsequently cooled, gravity pulled metal into the center to create the core. Heavier rocks formed the mantle, and lighter rocks formed the crust.

Earth’s crust is divided into tectonic plates, which move around on the surface because of the convecting mantle below. Movement of the plates causes other geological activity, such as earthquakes, volcanoes, and the formation of mountains. The locations of these features are mostly related to current or former plate boundaries. Earth is the only planet known to have plate tectonics.

Earth rotates on its axis once per day, by definition. Earth orbits the Sun once every 365.24 days, which is defined as a year. Earth has one large moon, which orbits Earth once every 29.5 days, a period known as a month.

Earth’s moon is the only large moon orbiting a terrestrial planet in the solar system. The Moon is covered with craters; it also has vast plains of lava. The considerable number of craters suggests that Moon’s surface is ancient. There is evidence that the Moon formed when a large object, perhaps as large as the planet Mars, struck Earth in the distant past.


Mars is the fourth planet from the Sun and the first planet beyond Earth’s orbit. Mars is quite different from Earth and yet more similar than any other planet. Mars is smaller, colder, drier, and appears to have no life, but volcanoes are common to both planets and Mars has many.

Mars is easy to observe, so the planet has been studied more thoroughly than any other extraterrestrial planet in our solar system. Space probes, rovers, and orbiting satellites have all yielded information to planetary geologists. Although no humans have ever set foot on Mars, both NASA and the European Space Agency have set goals of sending people to Mars sometime between 2030 and 2040.

Viewed from Earth, Mars is reddish. The ancient Greeks and Romans named the planet after the god of war. However, the surface is not red from blood but significant amounts of iron oxide in the soil. The Martian atmosphere is very thin relative to Earth’s and has much lower atmospheric pressure. Although the atmosphere is made up mostly of carbon dioxide, the planet has only a weak greenhouse effect, so temperatures are only slightly higher than if the planet had no atmosphere.

Mars has mountains, canyons, and other features similar to Earth and some of these surface features are amazing for their size. Olympus Mons is a shield volcano, similar to the volcanoes that make up the Hawaiian Islands. However, Olympus Mons is also the largest mountain in the solar system. Mars also has the largest canyon in the solar system, Valles Marineris.

Global mosaic of 102 Viking 1 Orbiter images of Mars taken on orbit 1,334, 22 February 1980. The images are projected into point perspective, representing what a viewer would see from a spacecraft at an altitude of 2,500 km. At center is Valles Marineris, over 3000 km long and up to 8 km deep.

Mars has more impact craters than Earth, though fewer than the Moon. Discovery Channel created a video comparing geologic features and tectonics on Mars and Earth. Water cannot stay in liquid form on Mars because the atmospheric pressure is too low. However, there is much water in the form of ice and even prominent ice caps. Scientists also think that there is a lot of water ice present just under the Martian surface. This ice can melt when volcanoes erupt, and water can flow across the surface temporarily.

Scientists think that water once flowed over the Martian surface because there are surface features that look like water-eroded canyons. The presence of water on Mars, even though it is now frozen as ice, suggests that it might have been possible for life to exist on Mars in the past.

Mars has two tiny moons that are irregular rocky bodies. Phobos and Deimos are named after characters in Greek mythology; the two sons of Ares, who followed their father into war. Ares is equivalent to the Roman god Mars.

2.13 The Outer Planets

The four planets farthest from the Sun are the outer planets. These planets are much larger than the inner planets and are made primarily of gases and liquids, so they are also called gas giants or Jovian planets.

The gas giants are made up primarily of hydrogen and helium, the same elements that make up most of the Sun. Astronomers think that hydrogen and helium gases comprised much of the solar system when it first formed. Since the inner planets did not have enough mass to hold on to these light gases, their hydrogen and helium floated away into space. The Sun and the massive outer planets had enough gravity to keep hydrogen and helium from drifting away.

All of the outer planets have numerous moons. They all also have planetary rings, composed of dust and other small particles that encircle the planet in a thin plane.

The Solar System’s four giant planets against the Sun, to scale


Because Jupiter is so massive, it reflects much sunlight and appears bright in the night sky; only the Moon and Venus are brighter. This brightness is all the more impressive because Jupiter is quite far from the Earth, 5.20 AUs away. It takes Jupiter about 12 Earth years to orbit once around the Sun.

Jupiter is named for the king of the gods in Roman mythology. The planet is enormous, the most massive object in the solar system besides the Sun. Although Jupiter is over 1,300 times Earth’s volume, it has only 318 times the mass of Earth. Like the other gas giants, it is less dense than Earth. Hypothetically, astronauts trying to land a spaceship on the surface of Jupiter would find that there is no solid surface at all. Jupiter is made mostly of hydrogen, with some helium, and small amounts of other elements.

See swirling cloud formations in the northern area of Jupiter’s north temperate belt in this new view taken by NASA’s Juno spacecraft.

The upper layer of Jupiter’s atmosphere contains clouds of ammonia (NH3) in bands of different colors. These bands rotate around the planet, but also swirl around in violent storms. The Great Red Spot is an enormous, oval-shaped storm found south of Jupiter’s equator. This storm is more than three times as wide as the entire Earth. Clouds in the storm rotate in a counterclockwise direction, making one complete turn every six days or so. The Great Red Spot has been on Jupiter for at least 300 years since astronomers could first see the storm through telescopes.

It is a hurricane twice the size of the Earth. It has been raging at least as long as telescopes could see it, and shows no signs of slowing. It is Jupiter’s Great Red Spot, the largest swirling storm system in the Solar System. Like most astronomical phenomena, the Great Red Spot was neither predicted nor immediately understood after its discovery. Still today, details of how and why the Great Red Spot changes its shape, size, and color remain mysterious.

Jupiter has 63 moons orbiting it that we are aware of so far. Four are big enough and bright enough to be seen from Earth, using no more than a pair of binoculars. These four moons, Io, Europa, Ganymede, and Callisto, were first discovered by Galileo in 1610 and are referred to as the Galilean moons. The Galilean moons are more significant than the dwarf planets Pluto, Ceres, and Eris. Ganymede is not only the biggest moon in the solar system, but it is also even larger than the planet Mercury.

Scientists are particularly interested in Europa because it may be a place to find extraterrestrial life. Although the surface of Europa is a smooth layer of ice, there is evidence that there is an ocean of liquid water underneath. Europa also has a continual source of energy; it is heated as it is stretched and squashed by tidal forces from Jupiter. Numerous missions have been planned to explore Europa, including plans to drill through the ice and send a probe into the ocean. However, no such mission has yet been attempted.

In 1979, two spacecraft, Voyager 1 and Voyager 2, visited Jupiter and its moons. Photos from the Voyager missions showed that Jupiter has a faint ring system. This ring system is very faint, so it is difficult to observe from Earth. Recently, NASA launched the Juno satellite to study Jupiter. The images below came from the Juno satellite.


Saturn is famous for its beautiful rings. Although all the gas giants have rings, only Saturn’s can be easily seen from Earth. In Roman mythology, Saturn was the father of Jupiter.

Saturn’s mass is about 95 times the mass of Earth, and its volume is 755 times Earth’s volume, making it the second largest planet in the solar system. Saturn is also the least dense planet in the solar system. It is less dense than water meaning that Saturn would float on water. Saturn orbits the Sun once about every 30 Earth years.

Like Jupiter, Saturn is made mostly of hydrogen and helium gases in the outer layers and liquids at greater depths. The upper atmosphere has clouds in bands of different colors. These rotate rapidly around the planet, but there seems to be less turbulence and fewer storms on Saturn than on Jupiter. One interesting phenomenon that has been observed in the storms on Saturn is the presence of thunder and lightning. The planet likely has a small rocky and metallic core.

In 1610 Galileo first observed Saturn’s rings with his telescope, but he thought they might be two large moons, one on either side of the planet. In 1659, the Dutch astronomer Christian Huygens realized that the features were rings. Saturn’s rings circle the planet’s equator and appear tilted because Saturn itself is tilted about 27 degrees. The rings do not touch the planet.

The Voyager 1 and 2 spacecraft in 1980 and 1981 sent back detailed pictures of Saturn, its rings, and some of its moons. Saturn’s rings are made of particles of water and ice, with some dust and rocks. There are several gaps in the rings that scientists think have originated because 1) the material was cleared out by the gravitational pull within the rings or, 2) by the gravitational forces of Saturn and of moons outside the rings. The rings were likely formed by the breakup of one of Saturn’s moons or from a material that never accreted into the planet when Saturn formed initially.

Most of Saturn’s moons are very small, and only seven are large enough for gravity to have made them spherical. Only Titan is larger than Earth’s Moon at about 1.5 times its size. Titan is even more massive than the planet Mercury. Scientists are interested in Titan because its atmosphere is similar to what Earth’s was like before life developed. Nitrogen is dominant, and methane is the second most abundant gas. Titan may have a layer of liquid water and ammonia under a layer of surface ice. Lakes of liquid methane (CH4) and ethane (C2H6) are found on Titan’s surface. Although conditions are similar enough to those of early Earth for scientists to speculate that extremely primitive life may exist on Titan, the extreme cold and lack of carbon dioxide make it unlikely.


Uranus (YOOR-uh-nuhs) is named after the Greek god of the sky. From Earth, Uranus is so faint that it was unnoticed by ancient observers. William Herschel first discovered the planet in 1781.

Although Uranus is very large, it is incredibly far away, about 2.8 billion km (1.8 billion mi) from the Sun. Light from the Sun takes about 2 hours and 40 minutes to reach Uranus, and the planet orbits the Sun once about every 84 Earth years. Uranus has a mass about 14 times the mass of Earth, but it is much less dense than Earth. Gravity at the surface of Uranus is weaker than on Earth’s surface, so if a human were at the top of the clouds on Uranus, they would weigh about 10 percent less than what they would weigh on Earth.

Like Jupiter and Saturn, Uranus is composed mainly of hydrogen and helium, with an outer gas layer that gives way to liquid on the inside. Uranus has a higher percentage of icy materials, such as water, ammonia (NH3), and methane (CH4), than Jupiter and Saturn. When sunlight reflects off Uranus, clouds of methane filter out red light, giving the planet a blue-green color. There are bands of clouds in the atmosphere of Uranus, but they are hard to see in normal light, so the planet looks like a plain blue ball.

Most of the planets in the solar system rotate on their axes in the same direction that they move around the Sun. Uranus, though, is tilted on its side, so its axis is almost parallel to its orbit. In other words, it rotates like a top that was turned so that it was spinning parallel to the floor. Scientists think that Uranus was probably knocked over by a collision with another planet-sized object billions of years ago.

Uranus has a faint system of rings. The rings circle the planet’s equator, but because Uranus is tilted on its side, the rings are almost perpendicular to the planet’s orbit. Uranus also has 27 known moons, and all but a few of them are named after characters from the plays of William Shakespeare. The five biggest moons are Miranda, Ariel, Umbriel, Titania, and Oberon.


Neptune is the only major planet that can’t be seen from Earth without a telescope. Scientists predicted the existence of Neptune before it was discovered because Uranus did not always appear exactly where it should appear. They knew that the gravitational pull of another planet beyond Uranus must be affecting Uranus’ orbit.

Neptune was discovered in 1846, in the position that had been predicted, and it was named Neptune for the Roman god of the sea because of its bluish color. In many respects, Neptune is similar to Uranus. Neptune has slightly more mass than Uranus, but it is slightly smaller in size. Neptune is much farther from the Sun at nearly 4.5 billion km (2.8 billion mi) than Uranus. The planet’s slow orbit means that it takes 165 Earth years to go once around the Sun.

Neptune’s blue color is mostly because of frozen methane (CH4). When Voyager 2 visited Neptune in 1986, there was a sizeable dark-blue spot that scientists named the Great Dark Spot, south of the equator. When the Hubble Space Telescope took pictures of Neptune in 1994, the Great Dark Spot had disappeared, but another dark spot had appeared north of the equator. Astronomers think that both of these spots represent gaps in the methane clouds on Neptune.

The changing appearance of Neptune is caused by its turbulent atmosphere. The winds on Neptune are stronger than on any other planet in the solar system, reaching speeds of 1,100 km/h (700 mi/h), close to the speed of sound. This extreme weather surprised astronomers since the planet receives little energy from the Sun to power weather systems. Neptune is also one of the coldest places in the solar system. Temperatures at the top of the clouds are about -218 degrees C (-360 degrees F). Neptune has faint rings of ice and dust that may change or disappear in reasonably short time frames.

Neptune has 13 known moons. Triton is the only one of them that has enough mass to be spherical. Triton orbits in the direction opposite to the orbit of Neptune. Scientists think Triton did not form around Neptune, but instead was captured by Neptune’s gravity as it passed by.

2.14 Other Objects in the Solar System

When the solar system formed, most of the matter ended up in the Sun. Material spinning in a disk around the Sun clumped together into larger and larger pieces to form the eight planets. However, some of the smaller pieces of matter never joined one of these larger bodies and are still out there in space.


Asteroids are very small, rocky bodies that orbit the Sun. “Asteroid” means “star-like,” and in a telescope, asteroids look like points of light, just like stars. Asteroids are irregularly shaped because they do not have enough gravity to become round. They are also too small to maintain an atmosphere, and without internal heat, they are not geologically active. Collisions with other bodies may break up the asteroid or create craters on its surface.

Asteroid impacts have had dramatic impacts on the shaping of the planets, including Earth. Early impacts caused the planets to grow as they cleared their portions of space. An impact with an asteroid about the size of Mars caused fragments of Earth to fly into space and ultimately create the Moon. Asteroid impacts are linked to mass extinctions throughout Earth history.

The Asteroid Belt

Hundreds of thousands of asteroids have been discovered in our solar system. They are still being discovered at a rate of about 5,000 new asteroids per month. The majority of the asteroids are found in between the orbits of Mars and Jupiter, in a region called the asteroid belt. Although there are many thousands of asteroids in the asteroid belt, their total mass adds up to only about 4% of Earth’s moon.

Scientists think that the bodies in the asteroid belt formed during the formation of the solar system. The asteroids might have come together to make a single planet, but they were pulled apart by the intense gravity of Jupiter.

Near-Earth Asteroids

More than 4,500 asteroids cross Earth’s orbit; they are near-Earth asteroids. Between 500 and 1,000 of these are over 1 km in diameter. Any object whose orbit crosses Earth’s can collide with Earth, and many asteroids do. On average, each year a rock about 5–10 m in diameter hits Earth. Since past asteroid impacts have been implicated in mass extinctions, astronomers are always on the lookout for new asteroids and follow the known near-Earth asteroids closely so that they can predict a possible collision as early as possible.

Scientists are interested in asteroids because they are representatives of the earliest solar system. Eventually, asteroids could be mined for rare minerals or construction projects in space. A few missions have studied asteroids directly. NASA’s DAWN mission orbited asteroid Vesta from July 2011 to September 2012 and is on its way to meet dwarf planet Ceres in 2015.

Thousands of objects, including comets and asteroids, are zooming around our solar system; some could be on a collision course with Earth. A meteor is a streak of light across the sky. People call them shooting stars, but they are small pieces of matter burning up as they enter Earth’s atmosphere from space.

Meteors are called meteoroids before they reach Earth’s atmosphere. Meteoroids are smaller than asteroids and range from the size of boulders down to the size of tiny sand grains. Still smaller objects are called interplanetary dust. When Earth passes through a cluster of meteoroids, there is a meteor shower. These clusters are often remnants left behind by comet tails.


Although most meteors burn up in the atmosphere, larger meteoroids may strike the Earth’s surface to create a meteorite. Meteorites are valuable to scientists because they provide clues about our solar system. Many meteorites are from asteroids that formed when the solar system formed. A few meteorites are made of rocky material that is thought to have come from Mars when an asteroid impact shot material off the Martian surface and into space.


Comets are small, icy objects that have very elliptical orbits around the Sun. Their orbits carry them from the outer solar system to the inner solar system, close to the Sun. Early in Earth’s history, comets may have brought water and other substances to Earth during collisions.

Comet tails form the outer layers of ice melt and evaporate as the comet flies close to the Sun. The ice from the comet vaporizes and forms a glowing coma, which reflects light from the Sun. Radiation and particles streaming from the Sun push this gas and dust into a long tail that always points away from the Sun. Comets appear for only a short time when they are near the Sun; they seem to disappear again as they move back to the outer solar system.

The time between one appearance of a comet and the next is called the comet’s period. Halley’s comet, with a period of 75 years, will next be seen in 2061. The first mention of the comet in historical records may go back as much as two millennia. Short-period comets, with periods of about 200 years or less, come from a region beyond the orbit of Neptune. The Kuiper belt (pronounced “KI-per”) contains not only comets, but asteroids, and at least two dwarf planets.

Comets, with periods as long as thousands or even millions of years, come from a very distant region of the solar system called the Oort cloud, about 50,000–100,000 AU from the Sun (50,000–100,000 times the distance from the Sun to Earth).

2.15 Dwarf Planets

The dwarf planets of our solar system are exciting proof of how much we are learning about our solar system. With the discovery of many new objects in our solar system, in 2006, astronomers refined the definition of a planet. Their subsequent reclassification of Pluto to the new category dwarf planet stirred up a great deal of controversy. How the classification of Pluto has evolved is an interesting story in science. The question is: What is and is not a planet?


From the time it was discovered in 1930 until the early 2000s, Pluto was considered the ninth planet. When astronomers first located Pluto, the telescopes were not as good as they are today. Back then, astronomers believe Pluto and its moon, Charon, were one large object. With better telescopes, astronomers realized that Pluto was much smaller than they had initially been thought. Better technology also allowed astronomers to discover many smaller objects like Pluto that orbit the Sun. One of them, Eris, discovered in 2005, is even larger than Pluto.

Even when it was considered a planet, Pluto was an oddball. Unlike the other outer planets in the solar system, which are all gas giants, it is small, icy, and rocky. With a diameter of about 2,400 km, it is only about one-fifth the mass of Earth’s Moon. Pluto’s orbit is tilted relative to the other planets and is shaped like a long, narrow ellipse. Pluto’s orbit sometimes even passes inside Neptune’s orbit.

In 1992 Pluto’s orbit was recognized to be part of the Kuiper belt. With more than 200 million Kuiper belt objects, Pluto has failed the test of clearing other bodies out its orbit. In 2006, the International Astronomical Union decided that there be too many questions surrounding what could be called a planet and so refined the definition of a planet.

According to the new definition, a planet must:

  • Orbit a star.
  • Be big enough that its gravity causes it to be shaped as a sphere.
  • Be small enough that it is not a star itself.
  • Have cleared the area of its orbit of smaller objects.

A dwarf planet is an object that meets items the first three items in the list above, but not but not the fourth. Pluto is now called a dwarf planet, along with the objects Ceres, Makemake, and Eris.

According to the IAU, a dwarf planet must:

  • Orbit a star.
  • Have enough mass to be nearly spherical.
  • Not have cleared the area around its orbit of smaller objects.
  • Not be a moon.

Pluto has three moons of its own. The largest, Charon, is big enough that the Pluto-Charon system is sometimes considered to be a double dwarf planet. Two smaller moons, Nix and Hydra, were discovered in 2005. However, having moons is not enough to make an object a planet.


Ceres is the largest object in the asteroid belt. Before 2006, Ceres was considered the largest of the asteroids, with only about 1.3% of the mass of the Earth’s Moon. However, unlike the asteroids, Ceres has enough mass that its gravity causes it to be shaped like a sphere. Like Pluto, Ceres is rocky. Ceres orbits the Sun, is round, and is not a moon. As part of the asteroid belt, its orbit is full of other smaller bodies, so Ceres fails the fourth criterion for being a planet.


Makemake is the third largest and second brightest dwarf planet we have discovered so far. With a diameter estimated to be between 1,300 and 1,900 km, it is about three-quarters the size of Pluto. Makemake orbits the Sun in 310 years at a distance between 38.5 to 53 AU. It is thought to be made of methane, ethane, and nitrogen ices.

Eris is the most massive known dwarf planet in the solar system — about 27% more massive than Pluto. The object was not discovered until 2003 because it is about three times farther from the Sun than Pluto, and almost 100 times farther from the Sun than Earth is. For a short time, Eris was considered the “tenth planet” in the solar system, but its discovery helped to prompt astronomers to better define planets and dwarf planets in 2006. Eris also has a small moon, Dysnomia that orbits it once about every 16 days.

Astronomers know there may be other dwarf planets in the outer reaches of the solar system. Haumea was made a dwarf planet in 2008, and so now the total is five. Quaoar, Varuna, and Orcus may be added to the list of dwarf planets in the future. We still have a lot to discover and explore.


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Chapter 2 - The Universe and the Solar System by R. Adam Dastrup, MA, and GISP is licensed under a Creative Commons Attribution 4.0 International License, except where otherwise noted.

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