The Life of Super-Earths

by Dimitar Sasselov

  Finally, in December 2001, the Kepler mission, as it was now known, was approved. Seven years later, in March 2009, Kepler was launched from Cape Canaveral in Florida and two months later beamed down to Earth images and exquisite measurements from stars and planets hundreds of light-years away. Kepler is NASA’s first mission capable of finding Earth-size and smaller planets within the habitable zone of stars similar to the Sun (a topic we will return to later). It is a fairly modest telescope, about half the size of Hubble, but with a lens that allows a wide field of view, captured by the largest ever camera built for a NASA science mission—a 95 megapixel monster that can image millions of stars in a single shot.

  The NASA Kepler mission will use the transiting method to discover planets like Earth—of the same size and in similar orbits. The goal of the mission is to do so in a systematic and comprehensive manner, so we can find out what fraction of stars have Earth-like planets. The mission is designed to avoid most of the pitfalls of the standard transiting method by investing in several years of preparation—a comprehensive and carefully vetted input catalog of 200,000 stars that the Kepler telescope is going to search for planets. After checking most of these over the initial few months, Kepler is to settle on about 100,000 to 120,000 stars for the life of the mission, about three to four years. All the advance work should mean that we will get very few false positives among the planet candidates from the Kepler photometry; at the Kepler team we have a plan how to weed out the remaining false positives, using the same methods we applied to the OGLE catalog. As I write this the approach is clearly paying off; our preliminary evaluation is that the fraction of false positives is very small.

  Ultimately, the transiting method is more than just a successful technique to discover planets, including Earth-like ones, as valuable as this may be. Transiting planets are the best ones for us to study in the short term. Because we know their masses and radiuses precisely, we can infer their bulk composition from the mean density. In addition, observing transits enables us to remotely analyze a planet’s atmosphere. During a transit, the light of the star is colored by the presence of the planet’s atmosphere; we can compare the spectra then to the spectra observed when the planet is not transiting, and use spectrographic techniques to identify the chemicals—such as water, methane, or carbon dioxide—present in a planet’s atmosphere.

  History repeats itself. In terms of their significance, there are uncanny parallels between the first observed transits of Mercury and Venus in the seventeenth century and the first transits of extrasolar planets observed today. Today, as in the seventeenth century, the first transit observation convinced any remaining skeptics of the reality of a big new concept. Today, as in the seventeenth century, the first transit observation brought in an unexpected result as well. Today, as in the seventeenth century, the first transit observation opened the door for an important future use of transits. Perhaps the only difference is that back then it took a century to take advantage of that important utility of transits; for us it is already happening.

  It is no coincidence that Johannes Kepler, the astronomer, mathematician, astrologer, and mystic, should be the namesake of our mission to find new Earths. He discovered the laws that govern planetary motion—we still use the laws as he wrote them to calculate the orbits of the planets we discover with NASA’s Kepler telescope. But that is not all. A less known fact is that Johannes Kepler was the first to predict and calculate accurately the transits of Mercury and Venus.

  Johannes Kepler never saw a transit. He died exactly one year before the first transit he had predicted, that of Mercury in 1631. But at least one astronomer, Pierre Gassendi, heeded Kepler’s call and observed the transit from Paris on November 7, 1631.

  November weather is often cloudy in Paris; Gassendi was beset by rain and clouds the days before. He had planned to observe the sun before and after the predicted time of the transit, since Kepler had noted the large uncertainty in his calculations. On the morning of November 7 the clouds cleared briefly and Gassendi saw the tiny black dot of Mercury on the solar disk. He was projecting the sun on a white screen, as we would do nowadays at home or for public viewing. Gassendi knew how to distinguish Mercury’s dark image from sunspots.19 Just twenty years earlier Galileo Galilei had used his new telescope to discover sunspots and describe how they move and change; Gassendi was an avid follower of Galileo and his experimental methods, and knew it when he saw the tiny dot move with respect to the spots in the direction of Mercury’s orbit.20

  The observation of Mercury’s transit in 1631 was important for several reasons. Though most scholars had adopted the Copernican revolution, the rest of the world had not. After all, it was just thirty years since Giordano Bruno’s death at the stake in Rome, and much less since Galileo’s own troubles with the Inquisition. The transit was one more success for the heliocentric system, similar to Galileo’s observations of the phases of Venus in 1609. Kepler’s prediction of the time of transit was precise to within five hours, which was amazing for his time, and was a success for the Copernican system and its predictive power.

  However, what most excited both Kepler and Gassendi about the transits of Mercury and Venus was the opportunity to measure directly the sizes of these planets or of any planet, for that matter. Kepler thought he had “discovered” another law, namely, that the volumes of the planets are proportional to their distances from the Sun. Unlike his laws of planetary orbital motion, this one was based not on evidence but on harmonic proportion as part of Kepler’s worldview, his “harmony of the spheres.”21

  Gassendi was aware of this and was surprised to see a very tiny Mercury. He was well prepared however, and made precise measurements of its size, which he dutifully reported to a skeptical audience. Kepler’s harmonic law of planetary volumes made Mercury and Venus too big and Jupiter and Saturn too small, but the scholars of that time had a tough time accepting that Kepler was wrong. Eight years later, when Venus was first observed in transit, the same surprise was in order. It appeared smaller than expected, and scholars finally understood that planet size and orbit are not strictly related.22

  When the hot Jupiter planet HD 209458b was observed to transit its star in 1999, some skeptics argued that the new class of wobbling stars might be a phenomenon other than planets. The transit of HD 209458b removed once and for all any such lingering doubts. But the observation (as with Mercury in 1631) brought a surprise as well—the size of the planet was larger than expected, namely, Jupiter’s size.

  Ten years later the inflated radius of HD 209458b and a good dozen other such planets remains a mystery. The expectation, as we’ve seen, is that planets with the weight of Jupiter consist almost entirely of hydrogen and helium. With hydrogen and helium being the lightest gases in our Universe, there is a maximum size a planet could have after compressing the gases under its own weight. The presence of anything heavier, such as oxygen or metals, would cause the planet to shrink.

  Jupiter and Saturn obey that theory well, so it is very surprising to find several extrasolar planet analogs to Jupiter that are 30 to 50 percent larger in size. Even in the unlikely event that they are made of pure hydrogen, their size can only be explained by a strong, persistent source of internal heat. Hot Jupiters are all very hot, but so far scientists have failed to identify a way to account for such a heat source.23 Once we find transiting Jupiters with the Kepler telescope in a range of orbits away from their stars and measure their radiuses, we may glean a clue as to what makes puffed-up hot Jupiters so . . . puffed up.

  Johannes Kepler made one more mistake regarding the transits of Mercury and Venus: he predicted only one of the pair of Venus transits in the seventeenth century. Kepler predicted the transit of Venus on December 6, 1631, hot in the footsteps of the Mercury transit the month before. Our friend in Paris, Pierre Gassendi, observed the Sun for three days in a row, but in vain. It was not his fault: the transit was visible only in the Western Hemisphere, as Kepler had correctly suspected. What Kepler failed to predic
t, however, was that just eight years later—on December 4, 1639—Venus would transit again and this time would be visible from Europe. A young Englishman, Jeremiah Horrocks, uncovered Kepler’s mistake and went on to be the first to see Venus in transit.

  How could the sixteen-year-old Horrocks do a better planetary calculation than Kepler, the father of orbital laws? The answer lies in what was to become the overarching importance of Venus transits a century later—measuring the “astronomical unit” (the Sun-Earth distance). In Kepler’s time that distance was not known; there were estimates, and Kepler adopted a very short one. Thus for an observer on Earth, the angle subtended by Venus on December 4, 1639, was too large and Kepler predicted that Venus would skim the Sun, not cross in front of it. Horrocks redid the calculation with newly published tables that had adopted a longer Sun-Earth distance. The transit was a go!

  This little anecdote reveals an important point—Kepler’s laws are correct and precise, but they give us proportions of how one planet’s orbit measures with respect to another planet’s orbit. In order to get an actual (absolute) distance, say in miles, one needs to know at least one orbit in miles. Therefore, if we know the astronomical unit, we can measure the distances to all the planets in the Solar System. Today, knowing the astronomical unit precisely is still important because we need it to measure the distances to other stars and the planets around them.

  About twenty-five years after Horrocks observed the transit of Venus, James Gregory in Scotland proposed transits as a method to measure accurately the astronomical unit.24 In the meantime, precise navigation was becoming an increasingly important strategic issue for the imperial powers of the day. The astronomical tables, whose precision depended on a reliable astronomical unit, were central to good navigation. The next pair of Venus transits in 1761 and 1769 was no longer left in the hands of an occasional astronomer in a Paris apartment or a house in the English countryside. Chasing Venus’s shadow was now a matter of international competition.

  In order to appreciate the significance of the transits of Venus in the eighteenth century, consider some of the people who were involved and the global reach of their well funded expeditions. Charles Mason and Jeremiah Dixon, who later went on to draw the famous Mason-Dixon Line between Pennsylvania and Maryland, led one of the British expeditions to measure the first transit. Captain James Cook led the British expedition to measure the second transit and later went on to explore much of the Pacific.

  The transits of Mercury and Venus have all but lost their practical importance in the twenty-first century. However, our ships are still leaving port to chase transits—up in space and for alien planets. Johannes Kepler would be glad to see his name on one of them. Captain Cook would marvel at them and admonish us to explore new worlds.

  CHAPTER FIVE

  SUPER-EARTH

  A New Type of Planet

  We love our planet Earth. We should—it is our home, and there’s no place like home. There can’t ever be a better place than Earth. Plenty of serious science literature supports that view in an emotionally detached manner. It is often called the “Goldilocks hypothesis”: the Earth is just the right size (not too big, not too small) and just the right temperature (not too hot, not too cold) for life to emerge here. Life is a rare thing. Perched on our little planet, we can’t see any other out there, or at least not yet—so a certain dose of Earth-centrism seems justified.1 Or is it?

  Life is extremely resilient once it takes hold, but it requires rich chemistry, large energy sources, and stability, right from the beginning. The comparative planetology of our Solar System makes it seem like those initial conditions are hard to come by. Earth seems perfect, whereas the rest have obvious defects. Mars is on the smallish side, lacks a substantial atmosphere and water, and is very cold (although we still hope to find life there). Jupiter is too big; its crushing pressures and element-poor environment make interesting chemistry impossible. The trouble with such a comparative analysis, however, is that it leaves out a crucial class of planets that, purely by happenstance, doesn’t occur in our Solar System.

  These are the super-Earths, which we’ll examine in the next two chapters.2 A super-Earth is a planet that is more massive and larger than Earth, although still made of rocks—perhaps with continents and oceans—and an atmosphere. There is no such planet in our Solar System, but we know that they must be common in other planetary systems.3 Moreover, theory predicts that they might have all the nice attributes of Earth, and, in fact, provide a more stable environment on their surface. True super-Earths!

  A super-Earth is a planet defined by its mass—between 1 and 10 ME (where ME stands for Earth mass). The Earth’s mass is 6 × 1024 kg, or a 6 with 24 zeros. That’s pretty big, but the Sun’s mass is 2 × 1030 kg—a million times larger—lest we lose perspective.

  I limit super-Earths to about 10 ME for a couple of reasons. To start with, we have not seen any super-Earths above that limit yet. We know the mean densities of about a dozen small exoplanets above that limit, and they are all made of gas and ice mostly, like Neptune. Second, when planets form in a proto-planetary disk (as in Figure 1.1), 10 ME is roughly the critical mass at which hydrogen gas can be swept in and retained by the growing planet, turning it into a Neptune-like giant. Of course, right now this range is somewhat approximate, and the upper limit to the mass of super-Earths could be anywhere between about 10 and 20 ME.4

  The first super-Earth was found by the serendipitous work of Eugenio Rivera, Jack Lissauer, and the California-Carnegie team in 2005. It orbits the small star Gliese 876 and is about seven times more massive than Earth.5 It is also very hot because it orbits very close to its star—only seven stellar radii, or approximately 1.7 million kilometers, away! (For comparison, this is only 3 percent of the distance from our Sun to Mercury.) Another, much colder super-Earth that is about 5 ME was discovered by J. P. Beaulieu and his team at the Paris Institute of Astrophysics, unfortunately using the gravitational-lensing technique described in Chapter 3. In early 2007 Michel Mayor’s team in Geneva spotted at least three planets orbiting the star Gliese 581; two of them are super-Earths with minimum possible mass of 5 and 8 ME each, and orbital distances that are 7 percent and 25 percent of the distance from Earth to the Sun, respectively.6 A year later the same team reported several more super-Earths, some orbiting stars as big and hot as our Sun. The first transiting super-Earth was discovered by the CoRoT space mission in 2009: CoRoT-7b, a very hot small planet, probably similar to Gliese 876.

  With so little opportunity for direct observation of these planets, little besides a planet’s mass and orbit can be known empirically. So scientists turn to theory to answer their questions about what these planets are like. “Theory” is what practicing scientists in astrophysics and planetary science call the research done to explain results from experiments and observations, as well as research predicting phenomena and objects not yet seen. (It is shorthand, and it should not be confused with what is known as scientific theory, which includes things like Newton’s laws of mechanics, which are so well established that we treat them as natural laws.) Theorists are scientists who do not conduct experiments and do not make observations. In the past they worked with pencil and paper, but these days they stare at a computer screen. They model a process or calculate the properties of an object by solving mathematical equations that describe them. In the history of science, Albert Einstein and Stephen Hawking are theorists; Ernest Rutherford was an experimentalist; Edwin Hubble was an observer. Sometimes the distinctions are blurred.

  Although observations are limited, scientists can study super-Earths today by building theoretical models with a computer, the idea being that they will then be tested by observation. By comparing predictions and observations, we can refine our models even as we learn more about these unusual planets. This is no different from how science studied Earth’s interior and the other planets in the Solar System less than 100 years ago. In fact, what we know today about Jupiter-like exoplanets is about equiv
alent to our knowledge of Jupiter in the 1950s.

  Theorists have applied this sort of effort to super-Earths only since 2004. That they were not studied theoretically before 2004 is a matter of neglect. Theorists usually work on phenomena and objects that are known to exist. Why waste time on something that is not immediately available?

  I am a theorist, so I have to accept some of the responsibility. When Harvard planetary scientist Richard O’Connell and I first talked about computing a model for a planet like Earth but twice as massive, it did not seem to be a difficult project. We were just excited to see what would come out. It was no disappointment—the models of these big rocky planets were very interesting indeed—and by the end of the year Diana Valencia, the graduate student Rick had recruited to work on this project, was ready to give a short report at the American Geophysical Union meeting. I was still incredulous that no one had done such models before, so I asked Diana to find out as much as she could by talking to colleagues after her presentation. She came back very encouraged. People were very interested, and no one had computed such models.7 We were treading new territory, literally!

  My continuing motivation to find super-Earths had been boosted by experimental success with the method I had worked on for confirming the presence of planets as they transited their stars. The number of confirmed exoplanets continued to grow. And even more exciting was the advent of Kepler.

  As a member of the Kepler team and preparing for the mission, I was acutely aware that we were going to discover many—hundreds of—planets smaller than Neptune but bigger than Earth, yet we knew nothing about how their masses and radii should relate to each other. (This was the problem that had taken me to Rick O’Connell, a colleague but in the Harvard Earth and Planetary Sciences Department, in the first place.)

  There are two criteria for calling something a super-Earth, which Diana, Rick, and I established: (1) it is between 1 and 10 ME; (2) it is composed mostly of solids (such as rock and ice).8 This may not seem like a great recipe for variety, and it is true that some of the planets we modeled seemed quite familiar. Far more often, however, we found planets that were exotic and novel.