Exoplanets—Planets Outside Our Solar System

Understanding the process by which the solar system was formed is being aided by observations of different stages of planet formation occurring around other stars. Although astronomers have been searching for planets outside the solar system for centuries, it was only in the 1980s that observations leading to their discovery were first made.

5-11 Planets and smaller debris that orbit other stars have been discovered

Stellar evolution and star formation are ongoing processes. You can see a cloud in the process of forming stars today with your naked eye, namely the middle “star” in Orion’s sword (see the figure opening this chapter), which looks like a fuzzy blob in the night sky. Called the Orion Nebula or Great Nebula of Orion, this “star” is actually part of a gigantic interstellar cloud region called the Orion Molecular Cloud in which new stars and planets are being created. Recall from Chapter 3 that visible light provides limited information; you can see in Figure 3-32c and Figure 5-2b that infrared images reveal even more of this region. Likewise, ultraviolet images provide more insight into star and planet formation than do visible ones.

As indicated earlier in this chapter, astronomers have discovered stars in early stages of formation that are surrounded by protoplanetary disks—disks of gas and dust. These disks are similar to what the solar nebula was believed to have been like more than 4.5 billion years ago. Indirect observational evidence for the existence of planets outside the solar system, called exoplanets or extrasolar planets, first came from distortions of protoplanetary disks.

If one or more planets orbit in a protoplanetary disk around a young star, their gravitational pull will affect the disk of gas and dust around the star, causing the disk to warp or become off-center from its star. An example of serious warping can be seen in the edge-on disk surrounding the star Beta Pictoris. This star and the material that orbits it formed only 20 million years ago. In 2006, astronomers discovered that Beta Pictoris actually has two disks (Figure 5-13), tilted slightly with respect to each other. Computer models show that a Jupiterlike planet orbiting out of the plane of the big disk would attract debris from that disk, thereby forming the smaller disk in the plane of its orbit. Beta Pictoris also has millions of comet nuclei, which were discovered in 2001. By studying such systems as Beta Pictoris, we can also begin addressing such questions as whether Earth acquired its water from comets in the young solar system.

This interaction between planets and disks is seen in other systems. The nearby star Fomalhaut has an off-centered disk of gas and dust. This star and its entourage are only some 200 million years old. Early observations suggested that this distortion is caused by one or more planets tugging on the gas as they orbit. One planet, Fomalhaut b, was first observed in 2004 (Figure 5-14). This exoplanet has an extremely eccentric orbit (much more so than any planet in our solar system). Its orbit ranges from 5 to 29 times further from its star than Saturn is from the Sun. The pull of the planet (and possibly others) is so great that the center of Fomalhaut’s disk is displaced 15 AU from the star. Furthermore, it appears Fomalhaut b will enter the disk in 2032, causing great havoc as it pulls some of the orbiting debris onto itself and flinging other debris out of orbit.

A slowly growing number of exoplanets are being observed directly as they orbit other stars. This is extremely challenging to do because even high-albedo planets like Jupiter are typically very dim compared to the stars that they orbit. Therefore it is perhaps not surprising that the first planetlike body orbiting another object outside the solar system, discovered in 2004, was one in which the second object is not quite a star, and is much dimmer than a star so that it does not overwhelm the light from the planetlike body (Figure 5-15). In order to be classified as a star, a young object has to have sufficient mass to compress its insides enough to force fusion of hydrogen into helium to occur in its core. The lowest mass for which this can happen is about 0.08 M_⊙ (solar masses), which is equal to about 75 M_Jupiter. Denoted 2M1207, the larger body in this first system with an orbiting planetlike body has only 0.025 M_⊙, so hydrogen to helium fusion does not occur in it. The smaller companion, denoted 2M1207b, has a mass between 5 and 8 times the mass of Jupiter. It was because the larger body does not shine brightly that the dimmer, planetlike body is easily visible. However, because the larger body is not a star, the smaller one is not considered a planet.

Driven by a desire to directly image planets orbiting actual stars, both observing techniques and analysis of the observations have developed profoundly in the past decade. Today at least a dozen exoplanets have been directly observed (Figure 5-16).

Most exoplanets are being discovered by their effects on the motions of their stars (Figure 5-17a). Some planets are discovered by measuring changes in the radial velocities of their stars as seen from Earth (defined in Section 4-7), which are calculated from the Doppler shifts of the stars’ spectra (Figure 5-17b). The Doppler shift changes cyclically as each star and its planet(s) orbit around their common center of mass (like two dancers waltzing around a point between them). The length of time of one cycle of Doppler shifts is the period of the planet’s orbit. Modern spectroscopic techniques are so good that we can detect stars approaching or receding at speeds as low as 4 km/h (2.5 mi/h).

Other planets are discovered from variations in a star’s proper motion, or motion among the background stars (also defined in Section 4-7). A planet’s gravitational attraction causes its star to deviate from motion in a straight line. This astrometric method of discovery looks for just such a wobble (Figure 5-17c).

Yet other planets are located by observing changes in the brightnesses of their stars. As a planet passes or transits between us and its star, it partially eclipses the star (Figure 5-17d). In 2003, astronomers used this transit method to detect a planet only one-sixteenth as far from its star as Mercury is from the Sun. That planet orbits once every 28 hours and 33 minutes. By observing transits, astronomers can also study the spectrum of a planet’s atmosphere. When a gas giant planet labeled HD 209458b passed in front of the star HD 209458 (Henry Draper catalog star number 209458) some 150 ly from Earth, hydrogen, sodium, oxygen, carbon, and carbon monoxide in the planet’s atmosphere absorbed certain wavelengths of starlight. Therefore, the star’s spectrum has extra absorption lines (introduced in Section 4-6). The Hubble Space Telescope further revealed that the outer layers of this planet are being heated so much that at least 10,000 tons of hydrogen are evaporating into space from it every second. In 2010, a tail created by this departing gas was observed. Based on this rate of mass loss, the planet has lost at least 0.1% of its mass over its lifetime of 5 billion years. Furthermore, observations in 2010 revealed winds in the outer cloud layers of this planet reaching 7000 km/h (4300 mi/h), more than 3 times faster than any winds known to occur in our solar system. The atmospheres of other exoplanets also include carbon dioxide and water vapor.

The transit method has also revealed that some exoplanets have much more high-density material in their cores than do our giant planets. The changing intensity of light from the star HD 149026 showed that the planet passing in front of it, which has a mass 115 times that of Earth, is much smaller than other planets of that mass. Calculations reveal that about 70 Earth masses (roughly two-thirds) of that planet is rocky material in its core. For comparison, Jupiter’s terrestrial core is about 4% of its total mass.

As you can see in Figure 5-18, many giant planets orbit surprisingly close to their stars, much closer than Earth is to the Sun. If the Nice model of how giant planets form is correct, then these planets are spiraling inward after having formed farther away from their stars than they are now.

A dramatic consequence of the inward spiraling of planets was discovered in 2001, when the remnants of at least one planet were discovered in the atmosphere of star HD 82943 that still has at least two other planets in orbit around it. This star’s atmosphere contained a rare form of lithium that is found in planets, but that is destroyed in stars within 30 million years after they form. The presence of this isotope, ⁶Li, means that at least one planet spiraled so close to the star that it was vaporized. In 2008, a roughly Jupiter-mass planet, WASP-12b, was discovered so close to its star, WASP-12, that tides caused by the star have made the planet egg-shaped. WASP-12b’s atmosphere is now being pulled onto its star at a rate of about 2 × 10¹⁷ tons per year. The planet will be devoured within 10 million years.

In 2010, observations of exoplanets transiting their stars revealed that some of the planets are orbiting in the opposite direction to which their stars rotate (or spin). This motion is intriguing because it seems to run counter to the predictions of the Nice model, in which planets should orbit in the same direction as the stellar rotation, as occurs here in our solar system.

The power of the transit method is demonstrated even further by its ability to reveal when a star has several planets orbiting it. To do this, astronomers watch the largest planet transit a star several consecutive times. The interval between transits is the length of the year on that planet. When there are other planets in a star system, including ones that don’t transit their star as seen from Earth, their gravitational tugs on the observed planet change the length of time it takes to orbit the star. So, if the length of a year for a transiting planet changes, then astronomers can deduce that there must be other planets in that system. In 2010, star Kepler 9 was observed to have two transiting planets and at least one nontransiting planet.

In 2004, astronomers began finding exoplanets by using a property of space that Albert Einstein discovered. His theory of general relativity, which we explore in detail in Chapter 14, shows that matter warps its surrounding space, causing, among other effects, passing light to change direction. This phenomenon is quite analogous to how light passing through a lens changes direction and is focused—light can also be focused by gravity, an effect called microlensing (or gravitational microlensing). Figure 5-19 shows how a star with a planet, passing between Earth and a distant star, can focus the light from the distant star, causing it to appear to change brightness. This change occurs twice, once as the distant star’s light is focused toward us by the closer star and again when the light is focused toward us by the planet and the star together.

The vast majority of known exoplanets have masses ranging between a half and a few times the mass of Jupiter (see Figure 5-18). However, the smallest-mass exoplanet discovered so far, Kepler 37b, has a mass only slightly greater than that of our Moon. It is one of at least three planets orbiting the star, Kepler 37, which has a mass of .8 M_⊙.

In 1999, astronomers began observing some stars wobbling in ways too complicated to be caused by a single planet, implying the presence of several planets. In a multiple-planet system, each planet contributes a tug on the star. By combining the effects of two or more planets, the observed pattern of the star’s motion (Doppler or astrometric) can be reproduced. The first star discovered with multiple planets was Upsilon Andromedae (Figure 5-20). One of its planets, labeled Upsilon Andromedae B, is 10 times closer to its star than Mercury is to the Sun. Planets that close to their stars are believed to be in synchronous rotation. A planet in synchronous orbit rotates at the same rate that it orbits its star, thus keeping the same side facing its star all the time. This side is swelteringly hot, while the other side is ice cold. Using the infrared-sensitive Spitzer Space Telescope, astronomers have determined that the difference in temperature between the daytime and nighttime sides of Upsilon Andromedae B is 1400 K, whereas the average difference on Earth is 11 K. A direct image of three planets orbiting HR 8799 is shown in Figure 5-20c. The star 55 Cancri A, in the constellation Cancer, has at least five planets and several stars with six planets each have been observed.

In 2005, microlensing was used to detect a terrestrial (rocky) planet labeled Gliese 876d orbiting star Gliese 876. (The Gliese catalogs list nearby stars and their known planets.) This planet has 6.8 times Earth’s mass. At least four (and possibly six) similar-mass planets, Gliese 581b, c, d, e, and possibly f and g, have been discovered orbiting another Gliese catalog star, Gliese 581, since 2007. All of these planets orbit their stars closer than Mercury orbits the Sun, and, hence, they are likely to be in synchronous orbits. Most, if not all, of these planets are believed to be composed primarily of rock and metal, with water on or near their surfaces. The stars they orbit are much cooler than our Sun. As of 2013, 146 stars have been observed with two or more planets that orbit them; several are shown in Figure 5-18.

The number of exoplanets that are being discovered has exploded with the activation of NASA’s Kepler orbiting observatory, which was designed specifically to search for Earthlike planets. It began discovering them shortly after its launch in 2009. In the first six weeks of operation, Kepler had identified five previously unknown exoplanets. In its four years of observations it found more than 2700 other candidate exoplanets, of which over 150 have been confirmed. Compare this rate to the discovery by other technologies of just 506 exoplanets between 1992 and 2010.

Along with disks of gas and dust, large numbers of comets, and the exoplanets already mentioned, astronomers have discovered asteroids orbiting other stars. The first system discovered to have asteroids is Zeta Leporis, a young star just 70 ly from Earth. Its asteroid belt, located about the same distance from that star as our asteroid belt is from the Sun, contains an estimated 200 times as much mass as ours. The pieces of asteroid debris have not been directly observed, but were inferred from the existence of hot (room-temperature) dust surrounding Zeta Leporis. The most likely explanation for this high-temperature dust is that it is being generated by the collisions of asteroids. As noted in Section 5-6, our solar system has one asteroid belt, located between Mars and Jupiter. Observations reveal that the star Epsilon Eridani has two asteroid belts, along with planets and a ring of icy debris. In 2013, astronomers discovered an asteroid belt around the bright star Vega, in the constellation Lyra.

5-12 Exoplanets orbit a breathtaking variety of stars

While some planets are orbiting stars similar to the Sun, many planets orbit burned-out remnants of stars and even orbit pairs or trios of stars. Most of the planets we have found are around stars in the disk of the Milky Way Galaxy (which is where our solar system resides), but a few have been found in large clumps of stars outside our Galaxy’s disk. (The word galaxy when used alone is capitalized only when referring to our Milky Way.) These clumps, called globular clusters, contain very old stars that formed shortly after the universe began. Because it is in a globular cluster, the oldest known planet dates back to the cluster’s formation, 13 billion years ago.

The vast majority of the more than 1000 exoplanets discovered so far have highly elliptical orbits, with eccentricities up to at least e = 0.71 (see Section 2-5). Fomalhaut b, discussed in the preceding section, is such a planet. Star systems with massive planets in highly eccentric orbits are unlikely to have life-sustaining Earthlike planets, because the changing location of the massive planet is likely to prevent smaller planets from staying in stable orbits.

Given the growing number of exoplanets that are being discovered orbiting a wide range of stars, astronomers are beginning to make estimates for the total number of planets in our Milky Way Galaxy and the number of those planets orbiting Sunlike stars. The results are impressive: On average, every five stars have, among them, eight planets. At least one in every six stars has an Earthlike planet orbiting it closer than Mercury is orbiting the Sun. Our Galaxy is estimated to have at least 100 billion planets. All of these numbers will be refined as better estimates are determined for the percentage of star systems with planets.

Although astronomers had expected to find disks of gas and dust orbiting young stars, consistent with the theory explaining the formation of the solar system, they have also found disks of dusty debris orbiting stars as old as the Sun (so they are over 4 billion years old). These stars are all known to have planets, and the rubble surrounding them was most likely formed by the collisions of asteroids and comets with each other and perhaps with the planets in these systems.

We have started finding Earthlike planets orbiting Sunlike stars. Which of these planets will be the best candidates for supporting life? We are most likely to discover life (of the sort we are familiar with on Earth) on exoplanets orbiting at about 1 AU from stars similar in mass (within about a factor of 2) to the Sun. At this distance, such stars provide enough heat to keep surface water liquid, and these stars shine long enough for life to evolve. The region around each star where water can be liquid on terrestrial planets in those locations is called the habitable zone or goldilocks zone. The latter name comes about because such regions are not too hot (for water to be liquid) and not too cold. Such planets are beginning to be discovered! We discuss the issues related to other life in the universe in Chapter 19.

5-13 Planets that are not orbiting stars have also been observed

While our discussion so far centers on planets orbiting stars, astronomers have begun observing Jupiter-sized planets that are unconnected to stars. Named, for now, free-floating planets, interstellar planets, or nomad planets, they are believed to have formed around stars and then were ejected by gravitational effects of other planets or nearby stars.

While only a handful of free-floating planets have been observed, their very existence allows astrophysicists to estimate how frequently planets get expelled from orbit around stars. Combined with the result that Earth-mass planets should be ejected even more commonly than higher mass planets, calculations suggest that there may be more free-floating planets in the Milky Way than there are stars.

5-14 Frontiers yet to be discovered

The formations of the solar system and other planetary systems are so complex that we are continually making refinements to our models of how these processes started and evolved. Our understanding of the development of the solar system is likely to be greatly advanced by further study of protoplanetary disks and planets that orbit other stars. For example, observations of these systems should help us pin down the timescales involved in forming planetary systems. Is the Nice model of planet formation correct? Did the collision of Moon-sized bodies create the terrestrial planets in the solar system?

Another major challenge in solar system astronomy is identifying and determining the properties of the myriad Kuiper belt and Oort cloud objects. What are their compositions? Are there many Kuiper belt objects like Pluto? Although the most poorly understood bodies in the solar system to date are the TNOs, every other object in the solar system also has secrets to be uncovered.

The exoplanets and their environs present many mysteries, such as how Jupiter-mass planets came to be closer to their stars than any such bodies in our solar system, how some planets ended up orbiting in the opposite direction to their stars’ rotation, how planets get ejected from orbit to become free-floating planets, and how to detect life on terrestrial planets orbiting in their stars’ habitable zones.

We begin a detailed exploration of the solar system in Chapter 6 by examining the two bodies we know best—Earth and its Moon. By understanding them, we will be better able to make some sense of the remarkably alien neighboring worlds we encounter in the following three chapters.