{"id":640,"date":"2017-08-08T13:13:21","date_gmt":"2017-08-08T17:13:21","guid":{"rendered":"https:\/\/pressbooks.bccampus.ca\/astronomy1105\/chapter\/21-4-planets-beyond-the-solar-system-search-and-discovery\/"},"modified":"2021-05-01T08:32:30","modified_gmt":"2021-05-01T12:32:30","slug":"21-4-planets-beyond-the-solar-system-search-and-discovery","status":"publish","type":"chapter","link":"https:\/\/pressbooks.bccampus.ca\/astronomy1105\/chapter\/21-4-planets-beyond-the-solar-system-search-and-discovery\/","title":{"raw":"21.4 Planets beyond the Solar System: Search and Discovery","rendered":"21.4 Planets beyond the Solar System: Search and Discovery"},"content":{"raw":"<div class=\"bcc-box bcc-highlight\">\r\n<h3>Learning Objectives<\/h3>\r\n<p id=\"fs-id1168583453740\">By the end of this section, you will be able to:<\/p>\r\n\r\n<ul id=\"fs-id1163975768573\">\r\n \t<li>Describe the orbital motion of planets in our solar system using <span class=\"no-emphasis\">Kepler<\/span>\u2019s laws<\/li>\r\n \t<li>Compare the indirect and direct observational techniques for exoplanet detection<\/li>\r\n<\/ul>\r\n<\/div>\r\nFor centuries, astronomers have dreamed of finding planets around other stars, including other planets like Earth. Direct observations of such distant planets are very difficult, however. You might compare a planet orbiting a star to a mosquito flying around one of those giant spotlights at a shopping centre opening. From close up, you might spot the mosquito. But imagine viewing the scene from some distance away\u2014say, from an airplane. You could see the spotlight just fine, but what are your chances of catching the mosquito in that light? Instead of making direct images, astronomers have relied on indirect observations and have now succeeded in detecting a multitude of planets around other stars.\r\n<p id=\"fs-id1163975634752\">In 1995, after decades of effort, we found the first such exoplanet (a planet outside our solar system) orbiting a main-sequence star, and today we know that most stars form with planets. This is an example of how persistence and new methods of observation advance the knowledge of humanity. By studying exoplanets, astronomers hope to better understand our solar system in context of the rest of the universe. For instance, how does the arrangement of our solar system compare to planetary systems in the rest of the universe? What do exoplanets tell us about the process of planet formation? And how does knowing the frequency of exoplanets influence our estimates of whether there is life elsewhere?<\/p>\r\n\r\n<div class=\"textbox shaded\">\r\n<p style=\"text-align: LEFT\"><b>MOST: Microvariability and Oscillations of STars. <\/b><\/p>\r\n<p style=\"text-align: LEFT\">The MOST Space Telescope was launched in 2003 and was the first space telescope to be entirely designed and built in <a href=\"http:\/\/www.asc-csa.gc.ca\/eng\/satellites\/most\/Default.asp\">Canada<\/a>. Roughly the size of a suitcase and weighing 54 kilograms, this satellite is entirely dedicated to the study of stellar vibrations. This study is called asteroseismology and not only reveals details about stars, but also the planets around them.<\/p>\r\n\r\n\r\n[caption id=\"attachment_2819\" align=\"aligncenter\" width=\"245\"]<img class=\"wp-image-2819 size-medium\" src=\"https:\/\/pressbooks.bccampus.ca\/astronomy1105\/wp-content\/uploads\/sites\/235\/2017\/08\/most_beauty-245x300.jpg\" alt=\"\" width=\"245\" height=\"300\" \/> <strong>Figure 1.<\/strong> The MOST Space Telescope (credit: C.S.A.)[\/caption]\r\n<p style=\"text-align: LEFT\">MOST orbits the earth once every 101 minutes and spends 60 days looking at each star. Forecast to last one year and study 10 stars, MOST exceeded expectations as it has now been in operation for over 10 years and been involved in the study of more than 5000 stars!<\/p>\r\n<p style=\"text-align: LEFT\">The <a href=\"http:\/\/astro-canada.ca\/le_telescope_spatial_most-the_most_space_telescope-eng\">Virtual Museum of Canada<\/a> lists some of MOST's most important discoveries, including a new variable star class (slowly pulsating B supergiants) and the confirmation of the 55 Cancri e exoplanet.<\/p>\r\n\r\n<\/div>\r\n<section>\r\n<h1>Searching for Orbital Motion<\/h1>\r\n<p id=\"fs-id1163975726855\">Most exoplanet detections are made using techniques where we observe the <em>effect<\/em> that the planet exerts on the host star. For example, the gravitational tug of an unseen planet will cause a small wobble in the host star. Or, if its orbit is properly aligned, a planet will periodically cross in front of the star, causing the brightness of the star to dim.<\/p>\r\n<p id=\"fs-id1163975837596\">To understand how a planet can move its host star, consider a single Jupiter-like planet. Both the planet and the star actually revolve about their <em>common centre of mass<\/em>. Remember that gravity is a mutual attraction. The star and the planet each exert a force on the other, and we can find a stable point, the centre of mass, between them about which both objects move. The smaller the mass of a body in such a system, the larger its orbit. A massive star barely swings around the centre of mass, while a low-mass planet makes a much larger \u201ctour.\u201d<\/p>\r\n<p id=\"fs-id1163975429234\">Suppose the planet is like Jupiter and has a mass about one-thousandth that of its star; in this case, the size of the star\u2019s orbit is one-thousandth the size of the planet\u2019s. To get a sense of how difficult observing such motion might be, let\u2019s see how hard Jupiter would be to detect in this way from the distance of a nearby star. Consider an alien astronomer trying to observe our own system from Alpha Centauri, the closest star system to our own (about 4.3 light-years away). There are two ways this astronomer could try to detect the orbital motion of the Sun. One way would be to look for changes in the Sun\u2019s position on the sky. The second would be to use the <span class=\"no-emphasis\">Doppler effect<\/span> to look for changes in its velocity. Let\u2019s discuss each of these in turn.<\/p>\r\nThe diameter of Jupiter\u2019s apparent orbit viewed from Alpha Centauri is 10 seconds of arc, and that of the Sun\u2019s orbit is 0.010 seconds of arc. (Remember, 1 second of arc is 1\/3600 degree.) If they could measure the apparent position of the Sun (which is bright and easy to detect) to sufficient precision, they would describe an orbit of diameter 0.010 seconds of arc with a period equal to that of Jupiter, which is 12 years.\r\n<p id=\"fs-id1163975551084\">In other words, if they watched the Sun for 12 years, they would see it wiggle back and forth in the sky by this minuscule fraction of a degree. From the observed motion and the period of the \u201cwiggle,\u201d they could deduce the mass of Jupiter and its distance using Kepler\u2019s laws. (To refresh your memory about these laws, see the chapter on <a class=\"target-chapter\" href=\"https:\/\/pressbooks.bccampus.ca\/astronomy1105\/chapter\/3-4-orbits-in-the-solar-system\/\">Orbits and Gravity<\/a>.)<\/p>\r\n<p id=\"fs-id1163975371532\">Measuring positions in the sky this accurately is extremely difficult, and so far, astronomers have not made any confirmed detections of planets using this technique. However, we have been successful in using spectrometers to measure the changing velocity of stars with planets around them.<\/p>\r\nAs the star and planet orbit each other, part of their motion will be in our line of sight (toward us or away from us). Such motion can be measured using the <em>Doppler effect<\/em> and the star\u2019s spectrum. As the star moves back and forth in orbit around the system\u2019s centre of mass in response to the gravitational tug of an orbiting planet, the lines in its spectrum will shift back and forth.\r\n<p id=\"fs-id1163975729482\">Let\u2019s again consider the example of the Sun. Its <em>radial velocity<\/em> (motion toward or away from us) changes by about 13 meters per second with a period of 12 years because of the gravitational pull of Jupiter. This corresponds to about 30 miles per hour, roughly the speed at which many of us drive around town. Detecting motion at this level in a star\u2019s spectrum presents an enormous technical challenge, but several groups of astronomers around the world, using specialized spectrographs designed for this purpose, have succeeded. Note that the change in speed does not depend on the distance of the star from the observer. Using the Doppler effect to detect planets will work at any distance, as long as the star is bright enough to provide a good spectrum and a large telescope is available to make the observations as illustrated in <a class=\"autogenerated-content\" href=\"#OSC_Astro_21_04_Doppler\">Figure 2<\/a>.<\/p>\r\n\r\n<figure id=\"OSC_Astro_21_04_Doppler\">\r\n<div class=\"title\" style=\"text-align: center\"><strong>Doppler Method of Detecting Planets.<\/strong><\/div>\r\n\r\n[caption id=\"\" align=\"aligncenter\" width=\"975\"]<img src=\"https:\/\/pressbooks.bccampus.ca\/astronomy1105\/wp-content\/uploads\/sites\/235\/2017\/08\/OSC_Astro_21_04_Doppler-1.jpg\" alt=\"Diagram of the Doppler Method of Detecting Planets. On the lower left of this illustration is a telescope pointing toward the upper right where there are two stars. Both stars lie on a circle with an x in the middle of the circle marking the center of mass of the system. This circle with two stars represents a single star seen at two different points on its orbit around the center of mass. A wavy blue line (for blueshifted light) connects the telescope and the star on the left hand side of the circle. This depicts the star when it is moving toward the observer on that part of its orbit. The star on the right hand side of the circle is connected to the telescope by a wavy red line (for redshifted light). At that part of the orbit the star is moving away from the observer. Below the stars is drawn a planet, with a small arrow indicating its motion around the star.\" width=\"975\" height=\"593\" \/> <strong>Figure 2.<\/strong> The motion of a star around a common centre of mass with an orbiting planet can be detected by measuring the changing speed of the star. When the star is moving away from us, the lines in its spectrum show a tiny redshift; when it is moving toward us, they show a tiny blueshift. The change in color (wavelength) has been exaggerated here for illustrative purposes. In reality, the Doppler shifts we measure are extremely small and require sophisticated equipment to be detected.[\/caption]<\/figure>\r\nThe first successful use of the Doppler effect to find a planet around another star was in 1995. Michel <span class=\"no-emphasis\">Mayor<\/span> and Didier <span class=\"no-emphasis\">Queloz<\/span> of the Geneva Observatory, pictured in <a class=\"autogenerated-content\" href=\"#OSC_Astro_21_04_Major\">Figure 3<\/a>, used this technique to find a planet orbiting a star resembling our Sun called 51 Pegasi, about 40 light-years away. (The star can be found in the sky near the great square of Pegasus, the flying horse of Greek mythology, one of the easiest-to-find star patterns.) To everyone\u2019s surprise, the planet takes a mere 4.2 days to orbit around the star. (Remember that Mercury, the innermost planet in our solar system, takes 88 days to go once around the Sun, so 4.2 days seems fantastically short.)\r\n<figure id=\"OSC_Astro_21_04_Major\">\r\n<div class=\"title\" style=\"text-align: center\"><strong>Planet Discoverers.<\/strong><\/div>\r\n\r\n[caption id=\"\" align=\"aligncenter\" width=\"487\"]<img src=\"https:\/\/pressbooks.bccampus.ca\/astronomy1105\/wp-content\/uploads\/sites\/235\/2017\/08\/OSC_Astro_21_04_Major-1.jpg\" alt=\"Planet Discoverers. An image of Didier Queloz and Michel Mayor, with an observatory in the background.\" width=\"487\" height=\"355\" \/> <strong>Figure 3.<\/strong> In 1995, Didier Queloz and Michel Mayor of the Geneva Observatory were the first to discover a planet around a regular star (51 Pegasi). They are seen here at an observatory in Chile where they are continuing their planet hunting. (credit: Weinstein\/Ciel et Espace Photos)[\/caption]<\/figure>\r\n<p id=\"fs-id1163975531437\">Mayor and Queloz\u2019s findings mean the planet must be very close to 51 Pegasi, circling it about 7 million kilometers away as shown in <a class=\"autogenerated-content\" href=\"#OSC_Astro_21_04_HotJup\">Figure 4<\/a>. At that distance, the energy of the star should heat the planet\u2019s surface to a temperature of a few thousand degrees Celsius (a bit hot for future tourism). From its motion, astronomers calculate that it has at least half the mass of Jupiter<a href=\"#footnote1\" name=\"footnote-ref1\"><sup>1<\/sup><\/a>, making it clearly a jovian and not a terrestrial-type planet.<\/p>\r\n\r\n<figure id=\"OSC_Astro_21_04_HotJup\">\r\n<div class=\"title\" style=\"text-align: center\"><strong>Hot Jupiter.<\/strong><\/div>\r\n\r\n[caption id=\"\" align=\"aligncenter\" width=\"731\"]<img src=\"https:\/\/pressbooks.bccampus.ca\/astronomy1105\/wp-content\/uploads\/sites\/235\/2017\/08\/OSC_Astro_21_04_HotJup-1.jpg\" alt=\"Hot Jupiter. An artist\u2019s impression of a hot Jupiter-type planet on the right and in the foreground, and a large bright sun to the left and in the background.\" width=\"731\" height=\"487\" \/> <strong>Figure 4.<\/strong> Artist Greg Bacon painted this impression of a hot, Jupiter-type planet orbiting close to a sunlike star. The artist shows bands on the planet like Jupiter, but we only estimate the mass of most hot, Jupiter-type planets from the Doppler method and don\u2019t know what conditions on the planet are like. (credit: ESO)[\/caption]<\/figure>\r\n<p id=\"fs-id1163975686891\">Since that initial planet discovery, the rate of progress has been breathtaking. Hundreds of giant planets have been discovered using the Doppler technique. Many of these giant planets are orbiting close to their stars\u2014astronomers have called these <em>hot Jupiters<\/em>.<\/p>\r\n<p id=\"fs-id1163975565972\">The existence of giant planets so close to their stars was a surprise, and these discoveries have forced us to rethink our ideas about how planetary systems form. But for now, bear in mind that the Doppler-shift method\u2014which relies on the pull of a planet making its star \u201cwiggle\u201d back and forth around the centre of mass\u2014is most effective at finding planets that are both close to their stars and massive. These planets cause the biggest \u201cwiggles\u201d in the motion of their stars and the biggest Doppler shifts in the spectrum. Plus, they will be found sooner, since astronomers like to monitor the star for at least one full orbit (and perhaps more) and <span class=\"no-emphasis\">hot Jupiters<\/span> take the shortest time to complete their orbit.<\/p>\r\n<p id=\"fs-id1163975430936\">So if such planets exist, we would expect to be finding this type first. Scientists call this a <em>selection effect<\/em>\u2014where our technique of discovery selects certain kinds of objects as \u201ceasy finds.\u201d As an example of a selection effect in everyday life, imagine you decide you are ready for a new romantic relationship in your life. To begin with, you only attend social events on campus, all of which require a student ID to get in. Your selection of possible partners will then be limited to students at your college. That may not give you as diverse a group to choose from as you want. In the same way, when we first used the Doppler technique, it selected massive planets close to their stars as the most likely discoveries. As we spend longer times watching target stars and as our ability to measure smaller Doppler shifts improves, this technique can reveal more distant and less massive planets too.<\/p>\r\n\r\n<div id=\"fs-id1163975345218\" class=\"note astronomy link-to-learning\">\r\n<div class=\"textbox shaded\">View a <a href=\"https:\/\/openstaxcollege.org\/l\/30keplawsolarani\">series of animations<\/a>\u00a0(direct link <a href=\"http:\/\/zingale.github.io\/astro_animations\/\">http:\/\/zingale.github.io\/astro_animations\/<\/a>) demonstrating solar system motion and Kepler\u2019s laws, and select animation 1 (Kepler\u2019s laws) from the dropdown playlist. To view an animation demonstrating the radial velocity curve for an exoplanet, select animation 29 (radial velocity curve for an exoplanet) and animation 30 (radial velocity curve for an exoplanet\u2014elliptical orbit) from the dropdown playlist.<\/div>\r\n<\/div>\r\n<\/section><section id=\"fs-id1163975709820\">\r\n<h1>Transiting Planets<\/h1>\r\n<p id=\"fs-id1163974241735\">The second method for indirect detection of exoplanets is based not on the motion of the star but on its brightness. When the orbital plane of the planet is tilted or inclined so that it is viewed edge-on, we will see the planet cross in front of the star once per orbit, causing the star to dim slightly; this event is known as transit. <a class=\"autogenerated-content\" href=\"#OSC_Astro_21_04_Transit\">Figure 5<\/a> shows a sketch of the transit at three time steps: (1) out of transit, (2) the start of transit, and (3) full transit, along with a sketch of the light curve, which shows the drop in the brightness of the host star. The amount of light blocked\u2014the depth of the transit\u2014depends on the area of the planet (its size) compared to the star. If we can determine the size of the star, the transit method tells us the size of the planet.<\/p>\r\n\r\n<figure id=\"OSC_Astro_21_04_Transit\">\r\n<div class=\"title\" style=\"text-align: center\"><strong>Planet Transits.<\/strong><\/div>\r\n\r\n[caption id=\"\" align=\"aligncenter\" width=\"601\"]<img src=\"https:\/\/pressbooks.bccampus.ca\/astronomy1105\/wp-content\/uploads\/sites\/235\/2017\/08\/OSC_Astro_21_04_Transit-1.jpg\" alt=\"Illustration of a Planet Transits. At the bottom of the figure is a graph. The vertical axis is labeled \u201cBrightness\u201d, in arbitrary units increasing upward, and the horizontal axis is labeled \u201cTime\u201d, in arbitrary units increasing to the right. A curve is plotted showing the brightness of the star as constant. After a time the brightness suddenly drops for a short duration before returning to its original value. At the top of the figure the disk of a star surrounded by an ellipse representing the orbit of a planet is shown. On the ellipse are drawn three dots representing the position of a planet at three different times in its orbit around the star. At position 1 the planet is to the left of the star. A dashed line connects the planet to the plotted curve. At this position the dashed line intersects the curve at a point of constant brightness. At position 2 the planet is just beginning to cross the face of the star. A dashed line connects the planet at position 2 to the curve where the brightness begins to drop. Finally, at position 3, the planet is fully in front of the star and the dashed line from the planet intersects the curve where the brightness is at minimum.\" width=\"601\" height=\"354\" \/> <strong>Figure 5.<\/strong> As the planet transits, it blocks out some of the light from the star, causing a temporary dimming in the brightness of the star. The top figure shows three moments during the transit event and the bottom panel shows the corresponding light curve: (1) out of transit, (2) transit ingress, and (3) the full drop in brightness.[\/caption]<\/figure>\r\n<p id=\"fs-id1163974216387\">The interval between successive transits is the length of the year for that planet, which can be used (again using Kepler\u2019s laws) to find its distance from the star. Larger planets like Jupiter block out more starlight than small earthlike planets, making transits by giant planets easier to detect, even from ground-based observatories. But by going into space, above the distorting effects of Earth\u2019s atmosphere, the transit technique has been extended to exoplanets as small as Mars.\u00a0 [latexpage]<\/p>\r\n\r\n<div id=\"fs-id1163975535457\" class=\"example\">\r\n<div class=\"textbox shaded\">\r\n<p id=\"fs-id1163975338920\"><strong>Transit Depth<\/strong>\r\nIn a transit, the planet\u2019s circular disk blocks the light of the star\u2019s circular disk. The area of a circle is \u03c0<em>R<\/em><sup>2<\/sup>. The amount of light the planet blocks, called the <span class=\"no-emphasis\">transit depth<\/span>, is then given by<\/p>\r\n\r\n<div id=\"fs-id1163976486577\" class=\"equation unnumbered\" style=\"text-align: center\">$$\\frac{\\pi{{R}^{2}}_{\\text{planet}}}{\\pi{{R}^{2}}_{\\text{star}}}=\\frac{{{R}^{2}}_{\\text{planet}}}{{{R}^{2}}_{\\text{star}}}={\\left(\\frac{{R}_{\\text{planet}}}{{R}_{\\text{star}}}\\right)}^{2}$$<\/div>\r\n<p id=\"fs-id1163974244471\">Now calculate the transit depth for a star the size of the Sun with a gas giant planet the size of Jupiter.<\/p>\r\n<p id=\"fs-id1163975329098\"><strong>Solution<\/strong>\r\nThe radius of Jupiter is 71,400 km, while the radius of the Sun is 695,700 km. Substituting into the equation, we get $${\\left(\\frac{{R}_{\\text{planet}}}{{R}_{\\text{star}}}\\right)}^{2}={\\left(\\frac{\\text{71,400 km}}{\\text{695,700 km}}\\right)}^{2}=0.01$$ or 1%, which can easily be detected with the instruments on board the Kepler spacecraft.<\/p>\r\n<p id=\"fs-id1163974360754\"><strong>Check Your Learning<\/strong>\r\nWhat is the transit depth for a star half the size of the Sun with a much smaller planet, like the size of Earth?<\/p>\r\n\r\n<div id=\"fs-id1163976942978\" class=\"note\">\r\n<div class=\"title\"><strong>Answer:<\/strong><\/div>\r\n<p id=\"fs-id1163976542930\">The radius of Earth is 6371 km. Therefore,<\/p>\r\n\r\n<div><\/div>\r\n$${\\left(\\frac{{R}_{\\text{planet}}}{{R}_{\\text{star}}}\\right)}^{2}={\\left(\\frac{\\text{6371 km}}{695,700\\text{\/}2\\phantom{\\rule{0.2em}{0ex}}\\text{km}}\\right)}^{2}={\\left(\\frac{\\text{6371 km}}{347,850\\phantom{\\rule{0.2em}{0ex}}\\text{km}}\\right)}^{2}=0.0003$$, or significantly less than 1%.\r\n\r\n<\/div>\r\n<\/div>\r\n<\/div>\r\n<p id=\"fs-id1163975351235\">The Doppler method allows us to estimate the mass of a planet. If the same object can be studied by both the Doppler and transit techniques, we can measure both the mass and the size of the exoplanet. This is a powerful combination that can be used to derive the average density (mass\/volume) of the planet. In 1999, using measurements from ground-based telescopes, the first transiting planet was detected orbiting the star HD 209458. The planet transits its parent star for about 3 hours every 3.5 days as we view it from Earth. Doppler measurements showed that the planet around HD 209458 has about 70% the mass of Jupiter, but its radius is about 35% larger than Jupiter\u2019s. This was the first case where we could determine what an exoplanet was made of\u2014with that mass and radius, HD 209458 must be a gas and liquid world like Jupiter or Saturn.<\/p>\r\n<p id=\"fs-id1163975562747\">It is even possible to learn something about the planet\u2019s atmosphere. When the planet passes in front of HD 209458, the atoms in the planet\u2019s atmosphere absorb starlight. Observations of this absorption were first made at the wavelengths of yellow sodium lines and showed that the atmosphere of the planet contains sodium; now, other elements can be measured as well.<\/p>\r\n\r\n<div id=\"fs-id1163974265664\" class=\"note astronomy link-to-learning\">\r\n<div class=\"textbox shaded\">Try a <a href=\"https:\/\/openstaxcollege.org\/l\/30transimul\">transit simulator<\/a> that demonstrates how a planet passing in front of its parent star can lead to the planet\u2019s detection. Follow the instructions to run the animation on your computer.<\/div>\r\n<\/div>\r\n<p id=\"fs-id1163975462014\">Transiting planets reveal such a wealth of information that the French Space Agency (CNES) and the European Space Agency (ESA) launched the CoRoT space telescope in 2007 to detect transiting exoplanets. CoRoT discovered 32 transiting exoplanets, including the first transiting planet with a size and density similar to Earth. In 2012, the spacecraft suffered an onboard computer failure, ending the mission. Meanwhile, NASA built a much more powerful transit observatory called Kepler.<\/p>\r\n<p id=\"fs-id1163975465813\">In 2009, NASA launched the Kepler space telescope, dedicated to the discovery of transiting exoplanets. This spacecraft stared continuously at more than 150,000 stars in a small patch of sky near the constellation of Cygnus\u2014just above the plane of our Milky Way Galaxy as pictured in <a class=\"autogenerated-content\" href=\"#OSC_Astro_21_04_KepField\">Figure 6<\/a>. Kepler\u2019s cameras and ability to measure small changes in brightness very precisely enabled the discovery of thousands of exoplanets, including many multi-planet systems. The spacecraft required three reaction wheels\u2014a type of wheel used to help control slight rotation of the spacecraft\u2014to stabilize the pointing of the telescope and monitor the brightness of the same group of stars over and over again. Kepler was launched with four reaction wheels (one a spare), but by May 2013, two wheels had failed and the telescope could no longer be accurately pointed toward the target area. Kepler had been designed to operate for 4 years, and ironically, the pointing failure occurred exactly 4 years and 1 day after it began observing.<\/p>\r\n\r\n<figure id=\"OSC_Astro_21_04_KepField\">\r\n<div class=\"title\" style=\"text-align: center\"><strong>Kepler\u2019s Field of View.<\/strong><\/div>\r\n\r\n[caption id=\"\" align=\"aligncenter\" width=\"975\"]<img src=\"https:\/\/pressbooks.bccampus.ca\/astronomy1105\/wp-content\/uploads\/sites\/235\/2017\/08\/OSC_Astro_21_04_KepField-1.jpg\" alt=\"Image of Kepler\u2019s Field of View. A wide view of the area imaged by the Kepler spacecraft, with boxes outlining the regions where stars were imaged regularly. An artist\u2019s illustration of the Kepler spacecraft is in the lower right hand corner.\" width=\"975\" height=\"619\" \/> <strong>Figure 6.<\/strong> The boxes show the region where the Kepler spacecraft cameras took images of over 150,000 stars regularly, to find transiting planets. (credit \u201cfield of view\u201d: modification of work by NASA\/Kepler mission; credit \u201cspacecraft\u201d: modification of work by NASA\/Kepler mission\/Wendy Stenzel)[\/caption]<\/figure>\r\n<p id=\"fs-id1163974213661\">What do we mean, exactly, by \u201cdiscovery\u201d of transiting exoplanets? A single transit shows up as a very slight drop in the brightness of the star, lasting several hours. However, astronomers must be on guard against other factors that might produce a false transit, especially when working at the limit of precision of the telescope. We must wait for a second transit of similar depth. But when another transit is observed, we don\u2019t initially know whether it might be due to another planet in a different orbit. The \u201cdiscovery\u201d occurs only when a third transit is found with similar depth and the same spacing in time as the first pair.<\/p>\r\n<p id=\"fs-id1163974220846\">Computers normally conduct the analysis, which involves searching for tiny, periodic dips in the light from each star, extending over 4 years of observation. But the Kepler mission also has a program in which non-astronomers\u2014citizen scientists\u2014can examine the data. These dedicated volunteers have found several transits that were missed by the computer analyses, showing that the human eye and brain sometimes recognize unusual events that a computer was not programmed to look for.<\/p>\r\n<p id=\"fs-id1163975409318\">Measuring three or four evenly spaced transits is normally enough to \u201cdiscover\u201d an exoplanet. But in a new field like exoplanet research, we would like to find further independent verification. The strongest confirmation happens when ground-based telescopes are also able to detect a Doppler shift with the same period as the transits. However, this is generally not possible for Earth-size planets. One of the most convincing ways to verify that a dip in brightness is due to a planet is to find more planets orbiting the same star\u2014a <em>planetary system<\/em>. Multi-planet systems also provide alternative ways to estimate the masses of the planets, as we will discuss in the next section.<\/p>\r\n<p id=\"fs-id1163975454229\">The selection effects (or biases) in the Kepler data are similar to those in Doppler observations. Large planets are easier to find than small ones, and short-period planets are easier than long-period planets. If we require three transits to establish the presence of a planet, we are of course limited to discovering planets with orbital periods less than one-third of the observing interval. Thus, it was only in its fourth and final year of operation that Kepler was able to find planets with orbits like Earth\u2019s that require 1 year to go around their star.<\/p>\r\n\r\n<\/section><section id=\"fs-id1163975571648\">\r\n<h1>Direct Detection<\/h1>\r\n<p id=\"fs-id1163975635316\">The best possible evidence for an earthlike planet elsewhere would be an image. After all, \u201cseeing is believing\u201d is a very human prejudice. But imaging a distant planet is a formidable challenge indeed. Suppose, for example, you were a great distance away and wished to detect reflected light from Earth. Earth intercepts and reflects less than one billionth of the Sun\u2019s radiation, so its apparent brightness in visible light is less than one billionth that of the Sun. Compounding the challenge of detecting such a faint speck of light, the planet is swamped by the blaze of radiation from its parent star.<\/p>\r\n<p id=\"fs-id1163975841224\">Even today, the best telescope mirrors\u2019 optics have slight imperfections that prevent the star\u2019s light from coming into focus in a completely sharp point.<\/p>\r\n<p id=\"fs-id1163975330034\">Direct imaging works best for young gas giant planets that emit infrared light and reside at large separations from their host stars. Young giant planets emit more infrared light because they have more internal energy, stored from the process of planet formation. Even then, clever techniques must be employed to subtract out the light from the host star. In 2008, three such young planets were discovered orbiting HR 8799, a star in the constellation of Pegasus, shown in <a class=\"autogenerated-content\" href=\"#OSC_Astro_21_04_HR8799\">Figure 7<\/a>. Two years later, a fourth planet was detected closer to the star. Additional planets may reside even closer to HR 8799, but if they exist, they are currently lost in the glare of the star.<\/p>\r\n<p id=\"fs-id1163974216506\">Since then, a number of planets around other stars have been found using direct imaging. However, one challenge is to tell whether the objects we are seeing are indeed planets or if they are brown dwarfs (failed stars) in orbit around a star.<\/p>\r\n\r\n<figure id=\"OSC_Astro_21_04_HR8799\">\r\n<div class=\"title\" style=\"text-align: center\"><strong>Exoplanets around HR 8799.<\/strong><\/div>\r\n\r\n[caption id=\"\" align=\"aligncenter\" width=\"975\"]<img src=\"https:\/\/pressbooks.bccampus.ca\/astronomy1105\/wp-content\/uploads\/sites\/235\/2017\/08\/OSC_Astro_21_04_HR8799-1.jpg\" alt=\"Image of Exoplanets Around HR 8799. In this image North is up and East is to the left. At center is the position of the star, which has been removed from the image to reveal the planets. Scattered around the center are the 4 directly imaged planets, with 3 on the right and one on the left. Each has a semi-circular arrow attached indicating its direction of motion around the star. At lower right a scale of 20 AU \/ 0.5\u201d is shown.\" width=\"975\" height=\"495\" \/> <strong>Figure 7.<\/strong> This image shows Keck telescope observations of four directly imaged planets orbiting HR 8799. A size scale for the system gives the distance in AU (remember that one astronomical unit is the distance between Earth and the Sun.) (credit: modification of work by Ben Zuckerman)[\/caption]<\/figure>\r\n<p id=\"fs-id1163974269426\">Direct imaging is an important technique for characterizing an exoplanet. The brightness of the planet can be measured at different wavelengths. These observations provide an estimate for the temperature of the planet\u2019s atmosphere; in the case of HR 8799 planet 1, the color suggests the presence of thick clouds. Spectra can also be obtained from the faint light to analyze the atmospheric constituents. A spectrum of HR 8799 planet 1 indicates a hydrogen-rich atmosphere, while the closer planet 4 shows evidence for methane in the atmosphere.<\/p>\r\n<p id=\"fs-id1163975450522\">Another way to overcome the blurring effect of Earth\u2019s atmosphere is to observe from space. Infrared may be the optimal wavelength range in which to observe because planets get brighter in the infrared while stars like our Sun get fainter, thereby making it easier to detect a planet against the glare of its star. Special optical techniques can be used to suppress the light from the central star and make it easier to see the planet itself. However, even if we go into space, it will be difficult to obtain images of Earth-size planets.<\/p>\r\n\r\n<\/section>\r\n<div class=\"textbox shaded\"><strong>Canadians in Astronomy <\/strong>\r\nProfessor Jaymie M. Matthews oversees research into stellar astrophysics, asteroseismology, and exoplanetary science at the University of British Columbia. His published papers on space science have earned him recognition worldwide in the academic community and the brave reader can find one of his published papers <a href=\"https:\/\/www.researchgate.net\/publication\/237279640_SPOT_MODULATION_OF_HD_189733_MOST1DETECTS_MODERATE_SPIN-ORBIT_MISALIGNMENT_OF_THE_TRANSITING_EXOPLANETARY_SYSTEM_HD_189733\">here<\/a>. A fascinatingly unique lecturer, Jaymie has earned the UBC Killam Teaching Prize and the Canadian Association of Physicists Medal for Excellence in Teaching. He is a recipient of the Order of Canada and was lead researcher of the <a href=\"http:\/\/astro-canada.ca\/le_telescope_spatial_most-the_most_space_telescope-eng\">MOST Space Telescope Project<\/a> previously mentioned.\r\n\r\n[caption id=\"attachment_2818\" align=\"aligncenter\" width=\"300\"]<img class=\"wp-image-2818 size-medium\" src=\"https:\/\/pressbooks.bccampus.ca\/astronomy1105\/wp-content\/uploads\/sites\/235\/2017\/08\/jaymiemattyhews-300x300.jpg\" alt=\"\" width=\"300\" height=\"300\" \/> <strong>Figure 8.<\/strong> Professor Jaymie Mark Matthews. (credit: University of British Columbia)[\/caption]\r\n<p style=\"text-align: LEFT\">Professor Matthews is an enthusiastic advocate of astronomy education and public outreach. His numerous media appearances have included some out-of-the-world <a href=\"https:\/\/www.youtube.com\/watch?v=-2yz72UU3mE\">outfits<\/a> along with the science to accompany them. Other times, his meaning has been humorously muddled:<\/p>\r\n<p style=\"text-align: LEFT\">\u201cI have yet to live down being quoted in Discover Magazine as saying 'Exploding Star Contains Atoms From Elvis Presley's Brain - Scientists Confirm The King of Rock &amp; Roll Lived In Another Galaxy 160,000 Years Ago!'\u201d<\/p>\r\n<p style=\"text-align: LEFT\">- Professor Jaymie Matthews<\/p>\r\n\r\n<\/div>\r\n<section id=\"fs-id1163975454016\" class=\"summary\">\r\n<h1>Key Concepts and Summary<\/h1>\r\n<p id=\"fs-id1163974189553\">Several observational techniques have successfully detected planets orbiting other stars. These techniques fall into two general categories\u2014direct and indirect detection. The Doppler and transit techniques are our most powerful indirect tools for finding exoplanets. Some planets are also being found by direct imaging.<\/p>\r\n\r\n<\/section>\r\n<div>\r\n<h2>Footnotes<\/h2>\r\n<ol>\r\n \t<li><a href=\"#footnote-ref1\" name=\"footnote1\">1<\/a> The Doppler method only allows us to find the minimum mass of a planet. To determine the exact mass using the Doppler shift and Kepler\u2019s laws, we must also have the angle at which the planet\u2019s orbit is oriented to our view\u2014something we don\u2019t have any independent way of knowing in most cases. Still, if the minimum mass is half of Jupiter\u2019s, the actual mass can only be larger than that, and we are sure that we are dealing with a jovian planet.<\/li>\r\n<\/ol>\r\n<\/div>\r\n<div>\r\n<h2>Glossary<\/h2>\r\n<dl id=\"fs-id1163976927423\" class=\"definition\">\r\n \t<dt>exoplanet<\/dt>\r\n \t<dd id=\"fs-id1163976557007\">a planet orbiting a star other than our Sun<\/dd>\r\n<\/dl>\r\n<dl id=\"fs-id1163976647066\" class=\"definition\">\r\n \t<dt>transit<\/dt>\r\n \t<dd id=\"fs-id1163974270301\">when one astronomical object moves in front of another<\/dd>\r\n<\/dl>\r\n<\/div>","rendered":"<div class=\"bcc-box bcc-highlight\">\n<h3>Learning Objectives<\/h3>\n<p id=\"fs-id1168583453740\">By the end of this section, you will be able to:<\/p>\n<ul id=\"fs-id1163975768573\">\n<li>Describe the orbital motion of planets in our solar system using <span class=\"no-emphasis\">Kepler<\/span>\u2019s laws<\/li>\n<li>Compare the indirect and direct observational techniques for exoplanet detection<\/li>\n<\/ul>\n<\/div>\n<p>For centuries, astronomers have dreamed of finding planets around other stars, including other planets like Earth. Direct observations of such distant planets are very difficult, however. You might compare a planet orbiting a star to a mosquito flying around one of those giant spotlights at a shopping centre opening. From close up, you might spot the mosquito. But imagine viewing the scene from some distance away\u2014say, from an airplane. You could see the spotlight just fine, but what are your chances of catching the mosquito in that light? Instead of making direct images, astronomers have relied on indirect observations and have now succeeded in detecting a multitude of planets around other stars.<\/p>\n<p id=\"fs-id1163975634752\">In 1995, after decades of effort, we found the first such exoplanet (a planet outside our solar system) orbiting a main-sequence star, and today we know that most stars form with planets. This is an example of how persistence and new methods of observation advance the knowledge of humanity. By studying exoplanets, astronomers hope to better understand our solar system in context of the rest of the universe. For instance, how does the arrangement of our solar system compare to planetary systems in the rest of the universe? What do exoplanets tell us about the process of planet formation? And how does knowing the frequency of exoplanets influence our estimates of whether there is life elsewhere?<\/p>\n<div class=\"textbox shaded\">\n<p style=\"text-align: LEFT\"><b>MOST: Microvariability and Oscillations of STars. <\/b><\/p>\n<p style=\"text-align: LEFT\">The MOST Space Telescope was launched in 2003 and was the first space telescope to be entirely designed and built in <a href=\"http:\/\/www.asc-csa.gc.ca\/eng\/satellites\/most\/Default.asp\">Canada<\/a>. Roughly the size of a suitcase and weighing 54 kilograms, this satellite is entirely dedicated to the study of stellar vibrations. This study is called asteroseismology and not only reveals details about stars, but also the planets around them.<\/p>\n<figure id=\"attachment_2819\" aria-describedby=\"caption-attachment-2819\" style=\"width: 245px\" class=\"wp-caption aligncenter\"><img loading=\"lazy\" decoding=\"async\" class=\"wp-image-2819 size-medium\" src=\"https:\/\/pressbooks.bccampus.ca\/astronomy1105\/wp-content\/uploads\/sites\/235\/2017\/08\/most_beauty-245x300.jpg\" alt=\"\" width=\"245\" height=\"300\" \/><figcaption id=\"caption-attachment-2819\" class=\"wp-caption-text\"><strong>Figure 1.<\/strong> The MOST Space Telescope (credit: C.S.A.)<\/figcaption><\/figure>\n<p style=\"text-align: LEFT\">MOST orbits the earth once every 101 minutes and spends 60 days looking at each star. Forecast to last one year and study 10 stars, MOST exceeded expectations as it has now been in operation for over 10 years and been involved in the study of more than 5000 stars!<\/p>\n<p style=\"text-align: LEFT\">The <a href=\"http:\/\/astro-canada.ca\/le_telescope_spatial_most-the_most_space_telescope-eng\">Virtual Museum of Canada<\/a> lists some of MOST&#8217;s most important discoveries, including a new variable star class (slowly pulsating B supergiants) and the confirmation of the 55 Cancri e exoplanet.<\/p>\n<\/div>\n<section>\n<h1>Searching for Orbital Motion<\/h1>\n<p id=\"fs-id1163975726855\">Most exoplanet detections are made using techniques where we observe the <em>effect<\/em> that the planet exerts on the host star. For example, the gravitational tug of an unseen planet will cause a small wobble in the host star. Or, if its orbit is properly aligned, a planet will periodically cross in front of the star, causing the brightness of the star to dim.<\/p>\n<p id=\"fs-id1163975837596\">To understand how a planet can move its host star, consider a single Jupiter-like planet. Both the planet and the star actually revolve about their <em>common centre of mass<\/em>. Remember that gravity is a mutual attraction. The star and the planet each exert a force on the other, and we can find a stable point, the centre of mass, between them about which both objects move. The smaller the mass of a body in such a system, the larger its orbit. A massive star barely swings around the centre of mass, while a low-mass planet makes a much larger \u201ctour.\u201d<\/p>\n<p id=\"fs-id1163975429234\">Suppose the planet is like Jupiter and has a mass about one-thousandth that of its star; in this case, the size of the star\u2019s orbit is one-thousandth the size of the planet\u2019s. To get a sense of how difficult observing such motion might be, let\u2019s see how hard Jupiter would be to detect in this way from the distance of a nearby star. Consider an alien astronomer trying to observe our own system from Alpha Centauri, the closest star system to our own (about 4.3 light-years away). There are two ways this astronomer could try to detect the orbital motion of the Sun. One way would be to look for changes in the Sun\u2019s position on the sky. The second would be to use the <span class=\"no-emphasis\">Doppler effect<\/span> to look for changes in its velocity. Let\u2019s discuss each of these in turn.<\/p>\n<p>The diameter of Jupiter\u2019s apparent orbit viewed from Alpha Centauri is 10 seconds of arc, and that of the Sun\u2019s orbit is 0.010 seconds of arc. (Remember, 1 second of arc is 1\/3600 degree.) If they could measure the apparent position of the Sun (which is bright and easy to detect) to sufficient precision, they would describe an orbit of diameter 0.010 seconds of arc with a period equal to that of Jupiter, which is 12 years.<\/p>\n<p id=\"fs-id1163975551084\">In other words, if they watched the Sun for 12 years, they would see it wiggle back and forth in the sky by this minuscule fraction of a degree. From the observed motion and the period of the \u201cwiggle,\u201d they could deduce the mass of Jupiter and its distance using Kepler\u2019s laws. (To refresh your memory about these laws, see the chapter on <a class=\"target-chapter\" href=\"https:\/\/pressbooks.bccampus.ca\/astronomy1105\/chapter\/3-4-orbits-in-the-solar-system\/\">Orbits and Gravity<\/a>.)<\/p>\n<p id=\"fs-id1163975371532\">Measuring positions in the sky this accurately is extremely difficult, and so far, astronomers have not made any confirmed detections of planets using this technique. However, we have been successful in using spectrometers to measure the changing velocity of stars with planets around them.<\/p>\n<p>As the star and planet orbit each other, part of their motion will be in our line of sight (toward us or away from us). Such motion can be measured using the <em>Doppler effect<\/em> and the star\u2019s spectrum. As the star moves back and forth in orbit around the system\u2019s centre of mass in response to the gravitational tug of an orbiting planet, the lines in its spectrum will shift back and forth.<\/p>\n<p id=\"fs-id1163975729482\">Let\u2019s again consider the example of the Sun. Its <em>radial velocity<\/em> (motion toward or away from us) changes by about 13 meters per second with a period of 12 years because of the gravitational pull of Jupiter. This corresponds to about 30 miles per hour, roughly the speed at which many of us drive around town. Detecting motion at this level in a star\u2019s spectrum presents an enormous technical challenge, but several groups of astronomers around the world, using specialized spectrographs designed for this purpose, have succeeded. Note that the change in speed does not depend on the distance of the star from the observer. Using the Doppler effect to detect planets will work at any distance, as long as the star is bright enough to provide a good spectrum and a large telescope is available to make the observations as illustrated in <a class=\"autogenerated-content\" href=\"#OSC_Astro_21_04_Doppler\">Figure 2<\/a>.<\/p>\n<figure id=\"OSC_Astro_21_04_Doppler\">\n<div class=\"title\" style=\"text-align: center\"><strong>Doppler Method of Detecting Planets.<\/strong><\/div>\n<figure style=\"width: 975px\" class=\"wp-caption aligncenter\"><img loading=\"lazy\" decoding=\"async\" src=\"https:\/\/pressbooks.bccampus.ca\/astronomy1105\/wp-content\/uploads\/sites\/235\/2017\/08\/OSC_Astro_21_04_Doppler-1.jpg\" alt=\"Diagram of the Doppler Method of Detecting Planets. On the lower left of this illustration is a telescope pointing toward the upper right where there are two stars. Both stars lie on a circle with an x in the middle of the circle marking the center of mass of the system. This circle with two stars represents a single star seen at two different points on its orbit around the center of mass. A wavy blue line (for blueshifted light) connects the telescope and the star on the left hand side of the circle. This depicts the star when it is moving toward the observer on that part of its orbit. The star on the right hand side of the circle is connected to the telescope by a wavy red line (for redshifted light). At that part of the orbit the star is moving away from the observer. Below the stars is drawn a planet, with a small arrow indicating its motion around the star.\" width=\"975\" height=\"593\" \/><figcaption class=\"wp-caption-text\"><strong>Figure 2.<\/strong> The motion of a star around a common centre of mass with an orbiting planet can be detected by measuring the changing speed of the star. When the star is moving away from us, the lines in its spectrum show a tiny redshift; when it is moving toward us, they show a tiny blueshift. The change in color (wavelength) has been exaggerated here for illustrative purposes. In reality, the Doppler shifts we measure are extremely small and require sophisticated equipment to be detected.<\/figcaption><\/figure>\n<\/figure>\n<p>The first successful use of the Doppler effect to find a planet around another star was in 1995. Michel <span class=\"no-emphasis\">Mayor<\/span> and Didier <span class=\"no-emphasis\">Queloz<\/span> of the Geneva Observatory, pictured in <a class=\"autogenerated-content\" href=\"#OSC_Astro_21_04_Major\">Figure 3<\/a>, used this technique to find a planet orbiting a star resembling our Sun called 51 Pegasi, about 40 light-years away. (The star can be found in the sky near the great square of Pegasus, the flying horse of Greek mythology, one of the easiest-to-find star patterns.) To everyone\u2019s surprise, the planet takes a mere 4.2 days to orbit around the star. (Remember that Mercury, the innermost planet in our solar system, takes 88 days to go once around the Sun, so 4.2 days seems fantastically short.)<\/p>\n<figure id=\"OSC_Astro_21_04_Major\">\n<div class=\"title\" style=\"text-align: center\"><strong>Planet Discoverers.<\/strong><\/div>\n<figure style=\"width: 487px\" class=\"wp-caption aligncenter\"><img loading=\"lazy\" decoding=\"async\" src=\"https:\/\/pressbooks.bccampus.ca\/astronomy1105\/wp-content\/uploads\/sites\/235\/2017\/08\/OSC_Astro_21_04_Major-1.jpg\" alt=\"Planet Discoverers. An image of Didier Queloz and Michel Mayor, with an observatory in the background.\" width=\"487\" height=\"355\" \/><figcaption class=\"wp-caption-text\"><strong>Figure 3.<\/strong> In 1995, Didier Queloz and Michel Mayor of the Geneva Observatory were the first to discover a planet around a regular star (51 Pegasi). They are seen here at an observatory in Chile where they are continuing their planet hunting. (credit: Weinstein\/Ciel et Espace Photos)<\/figcaption><\/figure>\n<\/figure>\n<p id=\"fs-id1163975531437\">Mayor and Queloz\u2019s findings mean the planet must be very close to 51 Pegasi, circling it about 7 million kilometers away as shown in <a class=\"autogenerated-content\" href=\"#OSC_Astro_21_04_HotJup\">Figure 4<\/a>. At that distance, the energy of the star should heat the planet\u2019s surface to a temperature of a few thousand degrees Celsius (a bit hot for future tourism). From its motion, astronomers calculate that it has at least half the mass of Jupiter<a href=\"#footnote1\" name=\"footnote-ref1\"><sup>1<\/sup><\/a>, making it clearly a jovian and not a terrestrial-type planet.<\/p>\n<figure id=\"OSC_Astro_21_04_HotJup\">\n<div class=\"title\" style=\"text-align: center\"><strong>Hot Jupiter.<\/strong><\/div>\n<figure style=\"width: 731px\" class=\"wp-caption aligncenter\"><img loading=\"lazy\" decoding=\"async\" src=\"https:\/\/pressbooks.bccampus.ca\/astronomy1105\/wp-content\/uploads\/sites\/235\/2017\/08\/OSC_Astro_21_04_HotJup-1.jpg\" alt=\"Hot Jupiter. An artist\u2019s impression of a hot Jupiter-type planet on the right and in the foreground, and a large bright sun to the left and in the background.\" width=\"731\" height=\"487\" \/><figcaption class=\"wp-caption-text\"><strong>Figure 4.<\/strong> Artist Greg Bacon painted this impression of a hot, Jupiter-type planet orbiting close to a sunlike star. The artist shows bands on the planet like Jupiter, but we only estimate the mass of most hot, Jupiter-type planets from the Doppler method and don\u2019t know what conditions on the planet are like. (credit: ESO)<\/figcaption><\/figure>\n<\/figure>\n<p id=\"fs-id1163975686891\">Since that initial planet discovery, the rate of progress has been breathtaking. Hundreds of giant planets have been discovered using the Doppler technique. Many of these giant planets are orbiting close to their stars\u2014astronomers have called these <em>hot Jupiters<\/em>.<\/p>\n<p id=\"fs-id1163975565972\">The existence of giant planets so close to their stars was a surprise, and these discoveries have forced us to rethink our ideas about how planetary systems form. But for now, bear in mind that the Doppler-shift method\u2014which relies on the pull of a planet making its star \u201cwiggle\u201d back and forth around the centre of mass\u2014is most effective at finding planets that are both close to their stars and massive. These planets cause the biggest \u201cwiggles\u201d in the motion of their stars and the biggest Doppler shifts in the spectrum. Plus, they will be found sooner, since astronomers like to monitor the star for at least one full orbit (and perhaps more) and <span class=\"no-emphasis\">hot Jupiters<\/span> take the shortest time to complete their orbit.<\/p>\n<p id=\"fs-id1163975430936\">So if such planets exist, we would expect to be finding this type first. Scientists call this a <em>selection effect<\/em>\u2014where our technique of discovery selects certain kinds of objects as \u201ceasy finds.\u201d As an example of a selection effect in everyday life, imagine you decide you are ready for a new romantic relationship in your life. To begin with, you only attend social events on campus, all of which require a student ID to get in. Your selection of possible partners will then be limited to students at your college. That may not give you as diverse a group to choose from as you want. In the same way, when we first used the Doppler technique, it selected massive planets close to their stars as the most likely discoveries. As we spend longer times watching target stars and as our ability to measure smaller Doppler shifts improves, this technique can reveal more distant and less massive planets too.<\/p>\n<div id=\"fs-id1163975345218\" class=\"note astronomy link-to-learning\">\n<div class=\"textbox shaded\">View a <a href=\"https:\/\/openstaxcollege.org\/l\/30keplawsolarani\">series of animations<\/a>\u00a0(direct link <a href=\"http:\/\/zingale.github.io\/astro_animations\/\">http:\/\/zingale.github.io\/astro_animations\/<\/a>) demonstrating solar system motion and Kepler\u2019s laws, and select animation 1 (Kepler\u2019s laws) from the dropdown playlist. To view an animation demonstrating the radial velocity curve for an exoplanet, select animation 29 (radial velocity curve for an exoplanet) and animation 30 (radial velocity curve for an exoplanet\u2014elliptical orbit) from the dropdown playlist.<\/div>\n<\/div>\n<\/section>\n<section id=\"fs-id1163975709820\">\n<h1>Transiting Planets<\/h1>\n<p id=\"fs-id1163974241735\">The second method for indirect detection of exoplanets is based not on the motion of the star but on its brightness. When the orbital plane of the planet is tilted or inclined so that it is viewed edge-on, we will see the planet cross in front of the star once per orbit, causing the star to dim slightly; this event is known as transit. <a class=\"autogenerated-content\" href=\"#OSC_Astro_21_04_Transit\">Figure 5<\/a> shows a sketch of the transit at three time steps: (1) out of transit, (2) the start of transit, and (3) full transit, along with a sketch of the light curve, which shows the drop in the brightness of the host star. The amount of light blocked\u2014the depth of the transit\u2014depends on the area of the planet (its size) compared to the star. If we can determine the size of the star, the transit method tells us the size of the planet.<\/p>\n<figure id=\"OSC_Astro_21_04_Transit\">\n<div class=\"title\" style=\"text-align: center\"><strong>Planet Transits.<\/strong><\/div>\n<figure style=\"width: 601px\" class=\"wp-caption aligncenter\"><img loading=\"lazy\" decoding=\"async\" src=\"https:\/\/pressbooks.bccampus.ca\/astronomy1105\/wp-content\/uploads\/sites\/235\/2017\/08\/OSC_Astro_21_04_Transit-1.jpg\" alt=\"Illustration of a Planet Transits. At the bottom of the figure is a graph. The vertical axis is labeled \u201cBrightness\u201d, in arbitrary units increasing upward, and the horizontal axis is labeled \u201cTime\u201d, in arbitrary units increasing to the right. A curve is plotted showing the brightness of the star as constant. After a time the brightness suddenly drops for a short duration before returning to its original value. At the top of the figure the disk of a star surrounded by an ellipse representing the orbit of a planet is shown. On the ellipse are drawn three dots representing the position of a planet at three different times in its orbit around the star. At position 1 the planet is to the left of the star. A dashed line connects the planet to the plotted curve. At this position the dashed line intersects the curve at a point of constant brightness. At position 2 the planet is just beginning to cross the face of the star. A dashed line connects the planet at position 2 to the curve where the brightness begins to drop. Finally, at position 3, the planet is fully in front of the star and the dashed line from the planet intersects the curve where the brightness is at minimum.\" width=\"601\" height=\"354\" \/><figcaption class=\"wp-caption-text\"><strong>Figure 5.<\/strong> As the planet transits, it blocks out some of the light from the star, causing a temporary dimming in the brightness of the star. The top figure shows three moments during the transit event and the bottom panel shows the corresponding light curve: (1) out of transit, (2) transit ingress, and (3) the full drop in brightness.<\/figcaption><\/figure>\n<\/figure>\n<p id=\"fs-id1163974216387\">The interval between successive transits is the length of the year for that planet, which can be used (again using Kepler\u2019s laws) to find its distance from the star. Larger planets like Jupiter block out more starlight than small earthlike planets, making transits by giant planets easier to detect, even from ground-based observatories. But by going into space, above the distorting effects of Earth\u2019s atmosphere, the transit technique has been extended to exoplanets as small as Mars.\u00a0 <\/p>\n<div id=\"fs-id1163975535457\" class=\"example\">\n<div class=\"textbox shaded\">\n<p id=\"fs-id1163975338920\"><strong>Transit Depth<\/strong><br \/>\nIn a transit, the planet\u2019s circular disk blocks the light of the star\u2019s circular disk. The area of a circle is \u03c0<em>R<\/em><sup>2<\/sup>. The amount of light the planet blocks, called the <span class=\"no-emphasis\">transit depth<\/span>, is then given by<\/p>\n<div id=\"fs-id1163976486577\" class=\"equation unnumbered\" style=\"text-align: center\">\n<p class=\"ql-center-displayed-equation\" style=\"line-height: 46px;\"><span class=\"ql-right-eqno\"> &nbsp; <\/span><span class=\"ql-left-eqno\"> &nbsp; <\/span><img loading=\"lazy\" decoding=\"async\" src=\"https:\/\/pressbooks.bccampus.ca\/astronomy1105\/wp-content\/ql-cache\/quicklatex.com-c8e667db4c358ce533fc25fee6f7ef35_l3.png\" height=\"46\" width=\"271\" class=\"ql-img-displayed-equation quicklatex-auto-format\" alt=\"&#92;&#091;&#92;&#102;&#114;&#97;&#99;&#123;&#92;&#112;&#105;&#123;&#123;&#82;&#125;&#94;&#123;&#50;&#125;&#125;&#95;&#123;&#92;&#116;&#101;&#120;&#116;&#123;&#112;&#108;&#97;&#110;&#101;&#116;&#125;&#125;&#125;&#123;&#92;&#112;&#105;&#123;&#123;&#82;&#125;&#94;&#123;&#50;&#125;&#125;&#95;&#123;&#92;&#116;&#101;&#120;&#116;&#123;&#115;&#116;&#97;&#114;&#125;&#125;&#125;&#61;&#92;&#102;&#114;&#97;&#99;&#123;&#123;&#123;&#82;&#125;&#94;&#123;&#50;&#125;&#125;&#95;&#123;&#92;&#116;&#101;&#120;&#116;&#123;&#112;&#108;&#97;&#110;&#101;&#116;&#125;&#125;&#125;&#123;&#123;&#123;&#82;&#125;&#94;&#123;&#50;&#125;&#125;&#95;&#123;&#92;&#116;&#101;&#120;&#116;&#123;&#115;&#116;&#97;&#114;&#125;&#125;&#125;&#61;&#123;&#92;&#108;&#101;&#102;&#116;&#40;&#92;&#102;&#114;&#97;&#99;&#123;&#123;&#82;&#125;&#95;&#123;&#92;&#116;&#101;&#120;&#116;&#123;&#112;&#108;&#97;&#110;&#101;&#116;&#125;&#125;&#125;&#123;&#123;&#82;&#125;&#95;&#123;&#92;&#116;&#101;&#120;&#116;&#123;&#115;&#116;&#97;&#114;&#125;&#125;&#125;&#92;&#114;&#105;&#103;&#104;&#116;&#41;&#125;&#94;&#123;&#50;&#125;&#92;&#093;\" title=\"Rendered by QuickLaTeX.com\" \/><\/p>\n<\/div>\n<p id=\"fs-id1163974244471\">Now calculate the transit depth for a star the size of the Sun with a gas giant planet the size of Jupiter.<\/p>\n<p id=\"fs-id1163975329098\"><strong>Solution<\/strong><br \/>\nThe radius of Jupiter is 71,400 km, while the radius of the Sun is 695,700 km. Substituting into the equation, we get <\/p>\n<p class=\"ql-center-displayed-equation\" style=\"line-height: 46px;\"><span class=\"ql-right-eqno\"> &nbsp; <\/span><span class=\"ql-left-eqno\"> &nbsp; <\/span><img loading=\"lazy\" decoding=\"async\" src=\"https:\/\/pressbooks.bccampus.ca\/astronomy1105\/wp-content\/ql-cache\/quicklatex.com-06342ecd8162a95650831d6d5f6c23a4_l3.png\" height=\"46\" width=\"290\" class=\"ql-img-displayed-equation quicklatex-auto-format\" alt=\"&#92;&#091;&#123;&#92;&#108;&#101;&#102;&#116;&#40;&#92;&#102;&#114;&#97;&#99;&#123;&#123;&#82;&#125;&#95;&#123;&#92;&#116;&#101;&#120;&#116;&#123;&#112;&#108;&#97;&#110;&#101;&#116;&#125;&#125;&#125;&#123;&#123;&#82;&#125;&#95;&#123;&#92;&#116;&#101;&#120;&#116;&#123;&#115;&#116;&#97;&#114;&#125;&#125;&#125;&#92;&#114;&#105;&#103;&#104;&#116;&#41;&#125;&#94;&#123;&#50;&#125;&#61;&#123;&#92;&#108;&#101;&#102;&#116;&#40;&#92;&#102;&#114;&#97;&#99;&#123;&#92;&#116;&#101;&#120;&#116;&#123;&#55;&#49;&#44;&#52;&#48;&#48;&#32;&#107;&#109;&#125;&#125;&#123;&#92;&#116;&#101;&#120;&#116;&#123;&#54;&#57;&#53;&#44;&#55;&#48;&#48;&#32;&#107;&#109;&#125;&#125;&#92;&#114;&#105;&#103;&#104;&#116;&#41;&#125;&#94;&#123;&#50;&#125;&#61;&#48;&#46;&#48;&#49;&#92;&#093;\" title=\"Rendered by QuickLaTeX.com\" \/><\/p>\n<p> or 1%, which can easily be detected with the instruments on board the Kepler spacecraft.<\/p>\n<p id=\"fs-id1163974360754\"><strong>Check Your Learning<\/strong><br \/>\nWhat is the transit depth for a star half the size of the Sun with a much smaller planet, like the size of Earth?<\/p>\n<div id=\"fs-id1163976942978\" class=\"note\">\n<div class=\"title\"><strong>Answer:<\/strong><\/div>\n<p id=\"fs-id1163976542930\">The radius of Earth is 6371 km. Therefore,<\/p>\n<div><\/div>\n<p class=\"ql-center-displayed-equation\" style=\"line-height: 47px;\"><span class=\"ql-right-eqno\"> &nbsp; <\/span><span class=\"ql-left-eqno\"> &nbsp; <\/span><img loading=\"lazy\" decoding=\"async\" src=\"https:\/\/pressbooks.bccampus.ca\/astronomy1105\/wp-content\/ql-cache\/quicklatex.com-06a1e7fbbe330caa586056332625ef2c_l3.png\" height=\"47\" width=\"478\" class=\"ql-img-displayed-equation quicklatex-auto-format\" alt=\"&#92;&#091;&#123;&#92;&#108;&#101;&#102;&#116;&#40;&#92;&#102;&#114;&#97;&#99;&#123;&#123;&#82;&#125;&#95;&#123;&#92;&#116;&#101;&#120;&#116;&#123;&#112;&#108;&#97;&#110;&#101;&#116;&#125;&#125;&#125;&#123;&#123;&#82;&#125;&#95;&#123;&#92;&#116;&#101;&#120;&#116;&#123;&#115;&#116;&#97;&#114;&#125;&#125;&#125;&#92;&#114;&#105;&#103;&#104;&#116;&#41;&#125;&#94;&#123;&#50;&#125;&#61;&#123;&#92;&#108;&#101;&#102;&#116;&#40;&#92;&#102;&#114;&#97;&#99;&#123;&#92;&#116;&#101;&#120;&#116;&#123;&#54;&#51;&#55;&#49;&#32;&#107;&#109;&#125;&#125;&#123;&#54;&#57;&#53;&#44;&#55;&#48;&#48;&#92;&#116;&#101;&#120;&#116;&#123;&#47;&#125;&#50;&#92;&#112;&#104;&#97;&#110;&#116;&#111;&#109;&#123;&#92;&#114;&#117;&#108;&#101;&#123;&#48;&#46;&#50;&#101;&#109;&#125;&#123;&#48;&#101;&#120;&#125;&#125;&#92;&#116;&#101;&#120;&#116;&#123;&#107;&#109;&#125;&#125;&#92;&#114;&#105;&#103;&#104;&#116;&#41;&#125;&#94;&#123;&#50;&#125;&#61;&#123;&#92;&#108;&#101;&#102;&#116;&#40;&#92;&#102;&#114;&#97;&#99;&#123;&#92;&#116;&#101;&#120;&#116;&#123;&#54;&#51;&#55;&#49;&#32;&#107;&#109;&#125;&#125;&#123;&#51;&#52;&#55;&#44;&#56;&#53;&#48;&#92;&#112;&#104;&#97;&#110;&#116;&#111;&#109;&#123;&#92;&#114;&#117;&#108;&#101;&#123;&#48;&#46;&#50;&#101;&#109;&#125;&#123;&#48;&#101;&#120;&#125;&#125;&#92;&#116;&#101;&#120;&#116;&#123;&#107;&#109;&#125;&#125;&#92;&#114;&#105;&#103;&#104;&#116;&#41;&#125;&#94;&#123;&#50;&#125;&#61;&#48;&#46;&#48;&#48;&#48;&#51;&#92;&#093;\" title=\"Rendered by QuickLaTeX.com\" \/><\/p>\n<p>, or significantly less than 1%.<\/p>\n<\/div>\n<\/div>\n<\/div>\n<p id=\"fs-id1163975351235\">The Doppler method allows us to estimate the mass of a planet. If the same object can be studied by both the Doppler and transit techniques, we can measure both the mass and the size of the exoplanet. This is a powerful combination that can be used to derive the average density (mass\/volume) of the planet. In 1999, using measurements from ground-based telescopes, the first transiting planet was detected orbiting the star HD 209458. The planet transits its parent star for about 3 hours every 3.5 days as we view it from Earth. Doppler measurements showed that the planet around HD 209458 has about 70% the mass of Jupiter, but its radius is about 35% larger than Jupiter\u2019s. This was the first case where we could determine what an exoplanet was made of\u2014with that mass and radius, HD 209458 must be a gas and liquid world like Jupiter or Saturn.<\/p>\n<p id=\"fs-id1163975562747\">It is even possible to learn something about the planet\u2019s atmosphere. When the planet passes in front of HD 209458, the atoms in the planet\u2019s atmosphere absorb starlight. Observations of this absorption were first made at the wavelengths of yellow sodium lines and showed that the atmosphere of the planet contains sodium; now, other elements can be measured as well.<\/p>\n<div id=\"fs-id1163974265664\" class=\"note astronomy link-to-learning\">\n<div class=\"textbox shaded\">Try a <a href=\"https:\/\/openstaxcollege.org\/l\/30transimul\">transit simulator<\/a> that demonstrates how a planet passing in front of its parent star can lead to the planet\u2019s detection. Follow the instructions to run the animation on your computer.<\/div>\n<\/div>\n<p id=\"fs-id1163975462014\">Transiting planets reveal such a wealth of information that the French Space Agency (CNES) and the European Space Agency (ESA) launched the CoRoT space telescope in 2007 to detect transiting exoplanets. CoRoT discovered 32 transiting exoplanets, including the first transiting planet with a size and density similar to Earth. In 2012, the spacecraft suffered an onboard computer failure, ending the mission. Meanwhile, NASA built a much more powerful transit observatory called Kepler.<\/p>\n<p id=\"fs-id1163975465813\">In 2009, NASA launched the Kepler space telescope, dedicated to the discovery of transiting exoplanets. This spacecraft stared continuously at more than 150,000 stars in a small patch of sky near the constellation of Cygnus\u2014just above the plane of our Milky Way Galaxy as pictured in <a class=\"autogenerated-content\" href=\"#OSC_Astro_21_04_KepField\">Figure 6<\/a>. Kepler\u2019s cameras and ability to measure small changes in brightness very precisely enabled the discovery of thousands of exoplanets, including many multi-planet systems. The spacecraft required three reaction wheels\u2014a type of wheel used to help control slight rotation of the spacecraft\u2014to stabilize the pointing of the telescope and monitor the brightness of the same group of stars over and over again. Kepler was launched with four reaction wheels (one a spare), but by May 2013, two wheels had failed and the telescope could no longer be accurately pointed toward the target area. Kepler had been designed to operate for 4 years, and ironically, the pointing failure occurred exactly 4 years and 1 day after it began observing.<\/p>\n<figure id=\"OSC_Astro_21_04_KepField\">\n<div class=\"title\" style=\"text-align: center\"><strong>Kepler\u2019s Field of View.<\/strong><\/div>\n<figure style=\"width: 975px\" class=\"wp-caption aligncenter\"><img loading=\"lazy\" decoding=\"async\" src=\"https:\/\/pressbooks.bccampus.ca\/astronomy1105\/wp-content\/uploads\/sites\/235\/2017\/08\/OSC_Astro_21_04_KepField-1.jpg\" alt=\"Image of Kepler\u2019s Field of View. A wide view of the area imaged by the Kepler spacecraft, with boxes outlining the regions where stars were imaged regularly. An artist\u2019s illustration of the Kepler spacecraft is in the lower right hand corner.\" width=\"975\" height=\"619\" \/><figcaption class=\"wp-caption-text\"><strong>Figure 6.<\/strong> The boxes show the region where the Kepler spacecraft cameras took images of over 150,000 stars regularly, to find transiting planets. (credit \u201cfield of view\u201d: modification of work by NASA\/Kepler mission; credit \u201cspacecraft\u201d: modification of work by NASA\/Kepler mission\/Wendy Stenzel)<\/figcaption><\/figure>\n<\/figure>\n<p id=\"fs-id1163974213661\">What do we mean, exactly, by \u201cdiscovery\u201d of transiting exoplanets? A single transit shows up as a very slight drop in the brightness of the star, lasting several hours. However, astronomers must be on guard against other factors that might produce a false transit, especially when working at the limit of precision of the telescope. We must wait for a second transit of similar depth. But when another transit is observed, we don\u2019t initially know whether it might be due to another planet in a different orbit. The \u201cdiscovery\u201d occurs only when a third transit is found with similar depth and the same spacing in time as the first pair.<\/p>\n<p id=\"fs-id1163974220846\">Computers normally conduct the analysis, which involves searching for tiny, periodic dips in the light from each star, extending over 4 years of observation. But the Kepler mission also has a program in which non-astronomers\u2014citizen scientists\u2014can examine the data. These dedicated volunteers have found several transits that were missed by the computer analyses, showing that the human eye and brain sometimes recognize unusual events that a computer was not programmed to look for.<\/p>\n<p id=\"fs-id1163975409318\">Measuring three or four evenly spaced transits is normally enough to \u201cdiscover\u201d an exoplanet. But in a new field like exoplanet research, we would like to find further independent verification. The strongest confirmation happens when ground-based telescopes are also able to detect a Doppler shift with the same period as the transits. However, this is generally not possible for Earth-size planets. One of the most convincing ways to verify that a dip in brightness is due to a planet is to find more planets orbiting the same star\u2014a <em>planetary system<\/em>. Multi-planet systems also provide alternative ways to estimate the masses of the planets, as we will discuss in the next section.<\/p>\n<p id=\"fs-id1163975454229\">The selection effects (or biases) in the Kepler data are similar to those in Doppler observations. Large planets are easier to find than small ones, and short-period planets are easier than long-period planets. If we require three transits to establish the presence of a planet, we are of course limited to discovering planets with orbital periods less than one-third of the observing interval. Thus, it was only in its fourth and final year of operation that Kepler was able to find planets with orbits like Earth\u2019s that require 1 year to go around their star.<\/p>\n<\/section>\n<section id=\"fs-id1163975571648\">\n<h1>Direct Detection<\/h1>\n<p id=\"fs-id1163975635316\">The best possible evidence for an earthlike planet elsewhere would be an image. After all, \u201cseeing is believing\u201d is a very human prejudice. But imaging a distant planet is a formidable challenge indeed. Suppose, for example, you were a great distance away and wished to detect reflected light from Earth. Earth intercepts and reflects less than one billionth of the Sun\u2019s radiation, so its apparent brightness in visible light is less than one billionth that of the Sun. Compounding the challenge of detecting such a faint speck of light, the planet is swamped by the blaze of radiation from its parent star.<\/p>\n<p id=\"fs-id1163975841224\">Even today, the best telescope mirrors\u2019 optics have slight imperfections that prevent the star\u2019s light from coming into focus in a completely sharp point.<\/p>\n<p id=\"fs-id1163975330034\">Direct imaging works best for young gas giant planets that emit infrared light and reside at large separations from their host stars. Young giant planets emit more infrared light because they have more internal energy, stored from the process of planet formation. Even then, clever techniques must be employed to subtract out the light from the host star. In 2008, three such young planets were discovered orbiting HR 8799, a star in the constellation of Pegasus, shown in <a class=\"autogenerated-content\" href=\"#OSC_Astro_21_04_HR8799\">Figure 7<\/a>. Two years later, a fourth planet was detected closer to the star. Additional planets may reside even closer to HR 8799, but if they exist, they are currently lost in the glare of the star.<\/p>\n<p id=\"fs-id1163974216506\">Since then, a number of planets around other stars have been found using direct imaging. However, one challenge is to tell whether the objects we are seeing are indeed planets or if they are brown dwarfs (failed stars) in orbit around a star.<\/p>\n<figure id=\"OSC_Astro_21_04_HR8799\">\n<div class=\"title\" style=\"text-align: center\"><strong>Exoplanets around HR 8799.<\/strong><\/div>\n<figure style=\"width: 975px\" class=\"wp-caption aligncenter\"><img loading=\"lazy\" decoding=\"async\" src=\"https:\/\/pressbooks.bccampus.ca\/astronomy1105\/wp-content\/uploads\/sites\/235\/2017\/08\/OSC_Astro_21_04_HR8799-1.jpg\" alt=\"Image of Exoplanets Around HR 8799. In this image North is up and East is to the left. At center is the position of the star, which has been removed from the image to reveal the planets. Scattered around the center are the 4 directly imaged planets, with 3 on the right and one on the left. Each has a semi-circular arrow attached indicating its direction of motion around the star. At lower right a scale of 20 AU \/ 0.5\u201d is shown.\" width=\"975\" height=\"495\" \/><figcaption class=\"wp-caption-text\"><strong>Figure 7.<\/strong> This image shows Keck telescope observations of four directly imaged planets orbiting HR 8799. A size scale for the system gives the distance in AU (remember that one astronomical unit is the distance between Earth and the Sun.) (credit: modification of work by Ben Zuckerman)<\/figcaption><\/figure>\n<\/figure>\n<p id=\"fs-id1163974269426\">Direct imaging is an important technique for characterizing an exoplanet. The brightness of the planet can be measured at different wavelengths. These observations provide an estimate for the temperature of the planet\u2019s atmosphere; in the case of HR 8799 planet 1, the color suggests the presence of thick clouds. Spectra can also be obtained from the faint light to analyze the atmospheric constituents. A spectrum of HR 8799 planet 1 indicates a hydrogen-rich atmosphere, while the closer planet 4 shows evidence for methane in the atmosphere.<\/p>\n<p id=\"fs-id1163975450522\">Another way to overcome the blurring effect of Earth\u2019s atmosphere is to observe from space. Infrared may be the optimal wavelength range in which to observe because planets get brighter in the infrared while stars like our Sun get fainter, thereby making it easier to detect a planet against the glare of its star. Special optical techniques can be used to suppress the light from the central star and make it easier to see the planet itself. However, even if we go into space, it will be difficult to obtain images of Earth-size planets.<\/p>\n<\/section>\n<div class=\"textbox shaded\"><strong>Canadians in Astronomy <\/strong><br \/>\nProfessor Jaymie M. Matthews oversees research into stellar astrophysics, asteroseismology, and exoplanetary science at the University of British Columbia. His published papers on space science have earned him recognition worldwide in the academic community and the brave reader can find one of his published papers <a href=\"https:\/\/www.researchgate.net\/publication\/237279640_SPOT_MODULATION_OF_HD_189733_MOST1DETECTS_MODERATE_SPIN-ORBIT_MISALIGNMENT_OF_THE_TRANSITING_EXOPLANETARY_SYSTEM_HD_189733\">here<\/a>. A fascinatingly unique lecturer, Jaymie has earned the UBC Killam Teaching Prize and the Canadian Association of Physicists Medal for Excellence in Teaching. He is a recipient of the Order of Canada and was lead researcher of the <a href=\"http:\/\/astro-canada.ca\/le_telescope_spatial_most-the_most_space_telescope-eng\">MOST Space Telescope Project<\/a> previously mentioned.<\/p>\n<figure id=\"attachment_2818\" aria-describedby=\"caption-attachment-2818\" style=\"width: 300px\" class=\"wp-caption aligncenter\"><img loading=\"lazy\" decoding=\"async\" class=\"wp-image-2818 size-medium\" src=\"https:\/\/pressbooks.bccampus.ca\/astronomy1105\/wp-content\/uploads\/sites\/235\/2017\/08\/jaymiemattyhews-300x300.jpg\" alt=\"\" width=\"300\" height=\"300\" \/><figcaption id=\"caption-attachment-2818\" class=\"wp-caption-text\"><strong>Figure 8.<\/strong> Professor Jaymie Mark Matthews. (credit: University of British Columbia)<\/figcaption><\/figure>\n<p style=\"text-align: LEFT\">Professor Matthews is an enthusiastic advocate of astronomy education and public outreach. His numerous media appearances have included some out-of-the-world <a href=\"https:\/\/www.youtube.com\/watch?v=-2yz72UU3mE\">outfits<\/a> along with the science to accompany them. Other times, his meaning has been humorously muddled:<\/p>\n<p style=\"text-align: LEFT\">\u201cI have yet to live down being quoted in Discover Magazine as saying &#8216;Exploding Star Contains Atoms From Elvis Presley&#8217;s Brain &#8211; Scientists Confirm The King of Rock &amp; Roll Lived In Another Galaxy 160,000 Years Ago!&#8217;\u201d<\/p>\n<p style=\"text-align: LEFT\">&#8211; Professor Jaymie Matthews<\/p>\n<\/div>\n<section id=\"fs-id1163975454016\" class=\"summary\">\n<h1>Key Concepts and Summary<\/h1>\n<p id=\"fs-id1163974189553\">Several observational techniques have successfully detected planets orbiting other stars. These techniques fall into two general categories\u2014direct and indirect detection. The Doppler and transit techniques are our most powerful indirect tools for finding exoplanets. Some planets are also being found by direct imaging.<\/p>\n<\/section>\n<div>\n<h2>Footnotes<\/h2>\n<ol>\n<li><a href=\"#footnote-ref1\" name=\"footnote1\" id=\"footnote1\">1<\/a> The Doppler method only allows us to find the minimum mass of a planet. To determine the exact mass using the Doppler shift and Kepler\u2019s laws, we must also have the angle at which the planet\u2019s orbit is oriented to our view\u2014something we don\u2019t have any independent way of knowing in most cases. Still, if the minimum mass is half of Jupiter\u2019s, the actual mass can only be larger than that, and we are sure that we are dealing with a jovian planet.<\/li>\n<\/ol>\n<\/div>\n<div>\n<h2>Glossary<\/h2>\n<dl id=\"fs-id1163976927423\" class=\"definition\">\n<dt>exoplanet<\/dt>\n<dd id=\"fs-id1163976557007\">a planet orbiting a star other than our Sun<\/dd>\n<\/dl>\n<dl id=\"fs-id1163976647066\" class=\"definition\">\n<dt>transit<\/dt>\n<dd id=\"fs-id1163974270301\">when one astronomical object moves in front of another<\/dd>\n<\/dl>\n<\/div>\n","protected":false},"author":9,"menu_order":5,"template":"","meta":{"pb_show_title":"on","pb_short_title":"","pb_subtitle":"","pb_authors":[],"pb_section_license":""},"chapter-type":[],"contributor":[],"license":[],"class_list":["post-640","chapter","type-chapter","status-publish","hentry"],"part":612,"_links":{"self":[{"href":"https:\/\/pressbooks.bccampus.ca\/astronomy1105\/wp-json\/pressbooks\/v2\/chapters\/640","targetHints":{"allow":["GET"]}}],"collection":[{"href":"https:\/\/pressbooks.bccampus.ca\/astronomy1105\/wp-json\/pressbooks\/v2\/chapters"}],"about":[{"href":"https:\/\/pressbooks.bccampus.ca\/astronomy1105\/wp-json\/wp\/v2\/types\/chapter"}],"author":[{"embeddable":true,"href":"https:\/\/pressbooks.bccampus.ca\/astronomy1105\/wp-json\/wp\/v2\/users\/9"}],"version-history":[{"count":4,"href":"https:\/\/pressbooks.bccampus.ca\/astronomy1105\/wp-json\/pressbooks\/v2\/chapters\/640\/revisions"}],"predecessor-version":[{"id":2817,"href":"https:\/\/pressbooks.bccampus.ca\/astronomy1105\/wp-json\/pressbooks\/v2\/chapters\/640\/revisions\/2817"}],"part":[{"href":"https:\/\/pressbooks.bccampus.ca\/astronomy1105\/wp-json\/pressbooks\/v2\/parts\/612"}],"metadata":[{"href":"https:\/\/pressbooks.bccampus.ca\/astronomy1105\/wp-json\/pressbooks\/v2\/chapters\/640\/metadata\/"}],"wp:attachment":[{"href":"https:\/\/pressbooks.bccampus.ca\/astronomy1105\/wp-json\/wp\/v2\/media?parent=640"}],"wp:term":[{"taxonomy":"chapter-type","embeddable":true,"href":"https:\/\/pressbooks.bccampus.ca\/astronomy1105\/wp-json\/pressbooks\/v2\/chapter-type?post=640"},{"taxonomy":"contributor","embeddable":true,"href":"https:\/\/pressbooks.bccampus.ca\/astronomy1105\/wp-json\/wp\/v2\/contributor?post=640"},{"taxonomy":"license","embeddable":true,"href":"https:\/\/pressbooks.bccampus.ca\/astronomy1105\/wp-json\/wp\/v2\/license?post=640"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}