The Life of Super-Earths

by Dimitar Sasselov

  FIGURE1.2. The orbit of the first “hot Jupiter” planet, 51 Peg b. The two orbits are shown at the same scale. The distance from the Earth to the Sun is 93 million miles, from 51 Peg b to 51 Peg, 5 million miles.

  So while my experience in the beautiful old city of the Medicis took some time to sink in, when it did, I was deeply inspired to find answers to the questions that just days before I had taken for granted.

  Thirteen years later Michel and I met again at the same conference. This time Michel described a bounty of small planets, perhaps like Earth, that he had discovered. I reported, based on computer calculations, on the strange worlds some of them might be. These smaller planets are more numerous and diverse than anyone had expected—searing hot planets with iron rain, atmospheres with 1,000 mile an hour winds, planetary systems with two suns, a planet that literally skims the surface of its star once every three months, and more.

  Today we stand on the threshold to new worlds—planets that we could call home, planets that someone else might call home already. The search for them has spawned a new space race: the race to discover an Earth twin planet. The zeal and effort going into this race may seem odd and unjustified. Even for scientists there is no overwhelming benefit in discovering an Earth twin, because to study the properties of Earth-like planets they could rely on bigger ones, which are much easier to find. Yet everyone agrees that this is a historic moment. What gives rise to the extraordinary excitement of this race is the human yearning for meaning and belonging. It is the twenty-first-century version of the age-old question of “the Other,” but on a grand scale.

  The question of the Other is about how a conscious human being perceives his own identity: Who am I and how do I relate to others? It arises front and center during first encounters. Human history is full of first encounters: Homo sapiens encounters Homo neanderthalensis somewhere in today’s Europe, Mayans encounter Spanish conquistadors in Central America, and so on.7 The time of first encounters on our planet is now over. For better or worse, we humans—all of us—know about each other. The present generation of Homo sapiens has a global awareness, a sense of social connectedness, and an understanding of a common genetic makeup. The end of the twentieth century marked a real watershed in this sense.

  The discovery of new worlds orbiting distant stars offers a fresh opportunity to contemplate a first encounter. As in the past, humans approach it with both insatiable curiosity and fear, with mixed, very strong emotions. As in the past—amazingly, despite all our modern technology and the visions of Star Trek—the new worlds we have just begun to uncover are enshrouded in mystery and full of surprises. And we will never stop exploring, as T. S. Eliot famously wrote: “We must never cease from exploration. And the end of all our exploring will be to arrive where we began and to know the place for the first time.”

  CHAPTER TWO

  THE WORLD OF PLANETS

  In the mid-1990s the world of planets was a small one that comprised the nine planets of the Solar System; Pluto’s “planethood” had not yet been challenged. Still, those planets represented a diversity of environments not imagined. The cameras aboard the flotilla of spaceships exploring our Solar System had shown us those exotic places and taught us the basics of comparative planetology. We couldn’t be sure just how important planets, let alone life, were to the Universe at large. Today, for the first time, scientists can look at both planets and life as integral parts of the Universe and its history. In what follows we’ll do just that.

  The planets in our Solar System form two groups—the gas giants and the terrestrial planets. Jupiter, by name and by physical stature, is the ruler of them all. How the ancients could have sensed that is a mystery, since it took scientists more than two and a half millennia to measure Jupiter’s mass and size and confirm its enormity for a fact.1 Four hundred years ago in Padua, Galileo Galilei first used his unusual optical device, the telescope, to look at Jupiter. Galileo saw a planet, not a point-like star, orbited by four moons. He called them stars—Medicean stars, naming them for his Florentine patrons, the Medicis—the distinction between stars and planets not having been clarified yet. Now we call them the Galilean moons of Jupiter; their orbits help us discern the planet’s gravitational pull and measure its mass.b That measurement was one of the triumphs of Isaac Newton’s law of gravity in the generation of scientists that followed Galileo, and it inspired young thinkers like Immanuel Kant to figure out how the planets formed. It showed that Jupiter had the mass of more than 300 Earths and more than two times the mass of all the other planets taken together. Jupiter is a giant indeed, rivaled only by its distant second—the ringed planet Saturn.

  Jupiter is a gas giant planet—we know that from its average, or mean, density. Its radius is ten times larger than Earth’s, which makes Jupiter’s volume a thousandfold larger. Given that Jupiter has only 300 times more mass, it must be made of stuff that is less dense than our rocky Earth. Indeed, Jupiter and Saturn are composed mostly of hydrogen and helium, the two most common and lightest elements in the Universe, very similar to the makeup of the Sun and the stars (Figure 2.1).

  The largest planets in our planetary system resemble the Sun in another important way—they have no solid surface or geography. From the top of the atmosphere that we see going down, it is all clouds and more clouds, getting denser and hotter as we sink deeper. Most of Jupiter’s interior is hydrogen and helium under pressures a million times higher than we are used to on Earth. One reason why the pressure inside is so much higher is that the larger the planet the stronger it pulls itself together by its own gravity—you and I would weigh 2.4 times more on Jupiter. If we were to venture deeper inside the planet, like diving in the ocean, the pressure would become higher as well. No wonder, then, that things can get a bit out of hand inside Jupiter. The hydrogen gas turns into a liquid known as metallic hydrogen. It conducts electricity, which is why we call it that; otherwise the substance has the least bit of resemblance to the copper wire in your bedside lamp. Studying the properties of this exotic material is a challenge in a lab—it was produced on Earth about ten years ago. Today we know it sufficiently well to describe—in computer calculations—more or less confidently the interiors of Jupiter and Saturn, and consequently hot Jupiters like 51 Peg b as well.

  Both Jupiter and Saturn have a small core (small for them, but enormous by Earth standards) made of elements heavier than hydrogen, helium, and neon. A core is typical of a planet, left over from its birth and formative years. For comparison, stars are born without cores and live long without them. As they age, stars grow a core, as lighter elements are fused into heavier ones, which simply pile up inside the star. Surprisingly, Saturn’s core, with a mass of about fifteen planet Earths, is bigger than Jupiter’s, at three to ten Earth masses. Or at least we think so. Jupiter is so much bigger, with so much metallic hydrogen, that the content of its core is difficult to determine. If its core indeed turns out to be smaller than Saturn’s, that could have happened by birth, or it could have eroded slowly and gotten mixed into the upper layers. More importantly, both Jupiter and Saturn have a similar fivefold excess of heavy elements compared to the Sun in their core and mixed in throughout, revealing in no uncertain terms their planetary ancestry.

  Uranus and Neptune are a different story. While Jupiter and Saturn, like the Sun, are mostly hydrogen and helium, Uranus and Neptune have only 10 percent of their mass in hydrogen and helium. The rest contains lots more oxygen, carbon, and nitrogen, in the form of frozen water, ammonia, and carbon dioxide. Although ten to twenty times less massive than Jupiter and Saturn, they are giants compared to Earth, and so they are known as the ice giants. Pluto is compositionally quite similar to Uranus and Neptune, but much smaller.

  FIGURE 2.1. Proportions of the most abundant elements in the Universe today. This is the makeup of our Sun and most of our Milky Way Galaxy.

  Much closer to the Sun is the province of the terrestrial planets, where Earth (a.k.a. Terra) rules in size and mass over Mercury, Venus, a
nd Mars. Here hydrogen is almost gone—less than 0.1 percent by mass—and helium is virtually nonexistent. 2 The terrestrial planets are mostly oxygen, iron, and silicon, although iron predominates on Mercury. Most of the iron in these planets resides in central cores. During the planets’ formative periods, iron (and a few other metals, such as nickel, that could not be part of the rocks) precipitated in large droplets in the center of the planets. The opposite is true of water: some is bound in rocks, but the rest, rather than sinking, stays at the surface. If the temperature and atmospheric pressure are right, a terrestrial planet will have liquid oceans.

  An obvious question, given how different these planetary groups seem, is whether they could have come from the same “stock.” The modern Kant-Laplace model teaches that planets form from material left over from the making of the star, which consequently ought to have the same proportions of heavy and light elements. Images taken with the Hubble space telescope and the infrared Spitzer space telescope show that planet-forming disks are just 1 percent as massive as their stars, and that less than 2 percent of that mass is in all the elements heavier than hydrogen and helium. So why don’t the small planets have any hydrogen or helium? Because of their mass. It takes the gravitational pull of a very large planetary seed to catch and keep those light gases. Smaller planets, such as Earth or Pluto, just can’t hold them, and the intermediate-mass ice giants formed so far out in the disk that they could only grow slowly. By the time they were ready to catch the hydrogen and helium, it had all but dissipated.

  Now we can project the knowledge we have gained about these planets onto the planets discovered around other stars. We have seen among them Jupiters and Saturns, with small or large cores, and we have seen Neptunes as well. And we have seen more diversity than we ever imagined. As we hone our techniques to discover and study smaller planets, we are in for more surprises.

  In the hierarchy of structures and objects in the Universe, planets occupy a place at the bottom of a sequence that starts with clusters of galaxies and continues through galaxies and stars. All of these structures assemble and develop under the pull of gravity—their own weight keeps them together. All except the planets have similar compositions that are dominated by hydrogen and helium. Thus planets, in breaking with this uniformity, are more than just the products of gravity: they present the richness of form that the full table of elements—chemistry—can afford.

  Imagine a planet that is larger and more massive than Earth but smaller than Uranus. Would a planet like this have deep water oceans—being a true water world—or would it be a dry planet with huge volcanoes billowing smoke high into a thin atmosphere? This is what we are about to explore. First, however, we have to find them.

  CHAPTER THREE

  COMPLETING THE COPERNICAN REVOLUTION

  In 1543 Nicolaus Copernicus set in motion events that transformed science and, through technology, human society. His insight—simplifying the architecture of the cosmos and placing the Sun, not Earth, in the center of the planetary system—was essential to the scientists of the next two generations (particularly Galileo and Newton) and the creation of modern physics. The Copernican revolution went directly to the heart of the question about humankind’s place in the world. Many thinkers, most famously Dutch physicist Christiaan Huygens (1629–1695), jumped from the Copernican view of Earth as just another planet to the possibility of life on other planets.1 In 1686 Bernard de Fontenelle popularized the possibility of extraterrestrial life in Conversations on the Plurality of Worlds, and it reached a culmination 300 years later in books and movies like War of the Worlds and Star Trek.

  Ironically, these conjectured other planets did not materialize for 450 years. Even the nearest stars turned out to be very, very far away; discerning the tiny planets that may orbit them required four centuries of technological development. Now we are finally within reach of completing the Copernican revolution by discovering analogs of the Earth and the Solar System.

  What makes extrasolar planets difficult to find is their distance and the fact that they are orbiting stars that are far bigger and brighter than they are. Typically a star is 1 billion to 10 billion times brighter than any orbiting planet, at least in visible light. This is a huge contrast ratio. To make things worse, since the observer is far away, star and planet appear very close to each other in the telescope. Taken separately, the high contrast ratio and the apparent closeness of star and planet are solvable. Together, they have been nearly intractable.

  The telescopes that have been in operation during the past twenty years, including the Hubble space telescope, are capable of collecting light from objects fainter than 10 billion times the brightness of the nearby stars. This is done in the same way that a photographer takes a picture at dusk—by keeping the camera’s shutter open longer. Taking a longer exposure allows more light to accumulate on the detector inside the camera, revealing very faint objects. The famous image of the Hubble Deep Field was obtained by taking a thirty-three-hour exposure in visible light, revealing thousands of distant galaxies.2

  Many of the extrasolar planets known today could be detected by a very long exposure like that, except that the star makes a huge bright smudge in the middle of the image. The star will be “overexposed,” as a photographer would say, and its light would be scattered all over the image. In fact, it could even damage the detector. Somewhere, lost in this scattered stellar light, is the faint light speck of the planet. That is why discovering a planet orbiting a normal star is such a big challenge.

  Solutions have been proposed.3 One method is to try observing the star and planet in other types of light. The star-to-planet ratio might be a billion to 1 or 10 billion to 1 in visible light. But as we know, light is a mixture of colors—waves of different length (or wavelength). These waves, when spread out according to wavelength (as done by a prism, for example) comprise a spectrum, as when water droplets turn sunlight into a rainbow. Consequently, applying a prism and looking for wavelengths in which the ratio between star and planet isn’t so great might help.

  This does work in some cases. For very hot planets, such as 51 Peg b, the star-to-planet contrast ratio improves a thousandfold (down to 107) when observed in infrared light. Infrared is light of longer and longer wavelength, beyond what our eyes see as red light; our skin detects it as heat. A hot planet stands out better in infrared light next to its star because it “shines” with its own heat. A hot Jupiter can have a temperature of 1,500 to 2,000 K, which is much hotter than Earth (at 287 K) but is comparable to the Sun (at 5,800 K). Nevertheless, the 107 contrast ratio is still daunting. Recently, infrared observations of known extrasolar planets have succeeded in special cases, but they still don’t yield images, and the method is still not used for discovery.4

  What other star-to-planet comparisons could we exploit? First, there is mass and then size; for both of these the star-to-planet ratios are much more favorable. For example, the Sun is 1,050 times more massive than Jupiter—so their star-to-planet mass ratio is 103. That is much more manageable than 107. With sizes, things get even better—the Sun is “just” ten times the size of Jupiter (and just 109 times the size of Earth)! This sounds good in theory, but how can we use it in practice?

  Let’s look at the star-to-planet mass ratio, since methods that exploit it have been the most successful and popular so far. The mass of an object determines its gravity—a more massive body exerts a stronger force (or pull). Thus the Sun makes Jupiter revolve around it in an eternal bind. But wait! Is Jupiter orbiting around the Sun like an anonymous slave, or are Sun and Jupiter waltzing their way through the Galaxy?

  A waltz it is! To every force there is an equal and opposite reaction force, so the Sun and Jupiter balance each other around their “center of mass,” which is a virtual point that is always on the line that joins them. They both orbit around the center of mass, just like a dancing couple. The Sun, being a thousandfold more massive, keeps their center of mass very close to itself, yet that virtual center is not insid
e the Sun. The Sun-Jupiter center of mass is about 7 percent of the solar radius above the surface of the Sun. To a careless observer from a distant star this might seem indistinguishable from Jupiter just revolving around the center of the Sun. But an astute and observant astronomer would see the waltz (or wobble, if you will) of the Sun as it orbits around the center of mass with Jupiter. The beauty of this trick is that the astronomer could observe the wobble of the Sun even if unable to see Jupiter in any other way! This is an indirect method of discovering a planet.

  There are several practical ways to exploit the star-to-planet mass ratio in order to discover extrasolar planets. Three of them make use of detecting the wobble of the parent star and one exploits the mass ratio in a snapshot of sorts. A star’s wobble can be detected directly by carefully observing the position of the star with respect to other stars over a period of time longer than the orbital period of the putative planet. This method—astrometry—allows us to isolate influences on a star’s behavior caused by orbiting planets, and not by the Universe at larger scales. It has been tried for many years, at least since the early twentieth century, but turned out to be very demanding. Recently, the Jet Propulsion Laboratory at NASA developed the technology needed to achieve astrometry at the required precision from space, so the method might still deliver in the future.5

  Another wobble method delivered the first new extrasolar planets. It relies on the influence of the Doppler effect on the spectrum of light coming from a star. The Doppler effect, as described in 1842 by Austrian physicist Christian Doppler, is something you are sure to have experienced. It happens anytime an object is moving and emitting waves at the same time. Sounds are waves in the air that we hear, so the sound of a passing car (or an ambulance siren) changes its pitch because of the Doppler effect: you hear a slightly higher sound as the ambulance approaches, and a slightly lower one after it passes by. Moreover, it is the relative motion that matters. So if the ambulance is stopped with its siren on and you drive by it, you’ll experience exactly the same Doppler effect. The Doppler effect can be used in a practical way to measure the speed of objects, as, for example, when police radar catches you speeding on the highway, or when meteorologists measure shifting wind speeds in an attempt to detect tornadoes.