The Life of Super-Earths

by Dimitar Sasselov

  These are not anomalous creatures, either. The “bottom” line is that life on Earth appears indestructible today because this subsurface environment hosts a large fraction of the planet’s total life. Some scientists, such as the late Thomas Gold at Cornell University, have argued that indeed most of the biomass on planet Earth is below the surface. Recent estimates are up to 300 billion tons of carbon biomass, which is comparable to the entire continental surface biomass, which is mostly plant life.8 Most of this deep biosphere consists of microbial communities living in rocks and sediments roughly 500 to 1,000 meters below the ocean floor; a case at 1,600 meters below the Atlantic seabed is the current record holder.9 With the ocean floor covering more than 70 percent of our planet, and a measured million cells in every cubic centimeter of subfloor sediment, this would make for more than half of all microbial cells on Earth.10

  Most recently, deep biosphere hunters discovered the first nonmicrobial life-form from a 1- to 3.5-kilometer depth in South Africa—a tiny worm that feeds on subsurface bacteria. 11 This emphasizes the richness of life in the deep crust of the Earth.

  How can microbes survive in miles of rock without sunlight or oxygen, and having scarce nutrients and water? The drill samples show a predominance of microbes that are resilient to stress and especially skillful in conserving energy by growing (doubling) extremely slowly—on timescales of centuries! If they had any cares, they would not be about us, the surface dwellers, and yet it is clear that they descended, albeit hundreds of millions of years ago, from ocean floor and surface dwellers. This is revealed in their genome maps. They are not that extraordinary, after all.

  The history of life on Earth shows rapid adaptation and colonization of any place where there is water, regardless of extremes in temperature, pressure, and acidity. The deep water cycle—the water from the surface that reaches deep into the crust and below the oceans—has brought life along with it, probably as soon as life existed on this planet.

  What dangers there are to life mostly come from outside Earth. The most dramatic threats are cosmic: asteroid and comet collisions, as well as major atmospheric change, including the loss of the entire atmosphere. Dramatic, yes, but not necessarily a death blow to life on the planet.

  Asteroid impacts are a part of life in any planetary system. Asteroids, the mile-size (sometimes many miles) fragments left over from the accumulation of the rocky planets, have orbits that are prone to be influenced by the big planets. Many of them, as a result, have been “swept up” by larger planets in the Solar System, including Jupiter, but many remain, especially in a large belt that exists between Mars and Jupiter. Over long periods of time, the gravitational influence of the planets is enough to put the asteroids on a collision course with a planet or another asteroid.

  An impact by a two kilometer asteroid would be a catastrophe for humankind, but most of the microbes in the deep biosphere would not even notice the event. It would take an impacting body almost the size of Mercury to destroy Earth’s crust and oceans and perhaps sterilize all colonies of microorganisms that are miles below the surface. However, in our Solar System, at the start of the twenty-first century, astronomers have a pretty complete census of asteroids crossing Earth’s orbit. We know all bodies larger than two kilometers that could hit us. Astronomers know of no such planet threatening to impact the Earth.

  Collisions between asteroids and planets would have been very common during the period of planet formation and about 500 million years after. We know this from observing other solar systems. Were large collisions more common, the amount of small particles lingering among the planetary orbits would be more than enough to notice in our remote census of known nearby planetary systems. Such “debris disks,” as they are called, are well-known and easy to detect with modern infrared technology. The Spitzer space telescope, an infrared cousin to the Hubble space telescope, has provided evidence that our Solar System is quite typical in that respect.

  What about catastrophic climate change—the total loss of the Earth’s atmosphere and the loss or freezing over of any remaining oceans. This could occur due to a massive impact by an asteroid comparable to the Moon or a nearby stellar explosion: a supernova or a gamma ray burst.

  Gamma rays are the most energetic electromagnetic waves. Gamma ray bursts are among the most violent events we know and they occur infrequently in any given galaxy. Nevertheless, astronomers detect them often—once a day. This is because of their sheer brightness and the penetration of gamma rays. We are able to see a burst across the entire visible Universe. They emanate from the final explosions of very weighty stars (a special case of a supernova explosion) and the spiraling-in of two neutron stars. Nothing rivals the power of the explosion that brings about a gamma ray burst. However, such explosions are both rare and far apart. At the typical rate and average distance, the only effect we should worry about on Earth is ozone layer loss. In the unlikely event that one occurred within fifty light-years of the Solar System, however, we would be in trouble. The Earth’s atmosphere would be completely lost and all life on the surface would become extinct.12 But not life inside the crust.

  A sudden loss of the entire atmosphere would likely deprive the Earth of an atmosphere just temporarily, on geological timescales. Because the internal structure of the Earth is not going to change much at all, the basic plate tectonics and the release of gas from the planet’s interior through volcanic activity would continue. Carbon dioxide from volcanoes would replenish the atmosphere, which, because carbon dioxide is a greenhouse gas, would melt the frozen oceans (or whatever was left of them). Even if they melted partially, the evaporation of water into the atmosphere, followed by rain and erosion, would restart the carbonate silicate cycle.

  The carbonate silicate cycle is almost identical to what is commonly called the inorganic carbon cycle, or the carbon dioxide cycle. It is a fundamental planetary cycle of the abundant gas carbon dioxide as it rises from the Earth’s interior, undergoes transformations in the atmosphere, on land, and in the oceans, and returns back inside at the end of it. The carbonate silicate cycle has a typical timescale—a typical time for changes to take hold. For planet Earth this timescale is about 400,000 years. So, should a gamma ray burst destroy Earth’s atmosphere, it might take “just” a few million years for it to return and stabilize, perhaps less. Any microbes that survived deep in the crust—and there should be many—would have ample opportunity to recolonize the Earth’s surface. For example, microbial communities discovered in deep drilling in Texas appear to have been cut off from the surface 80 million years ago, much longer than the million years needed to recolonize.

  Of course, if such a calamity were to happen, the Earth’s atmosphere would be changed dramatically: its two main constituent gases today, nitrogen and oxygen, would be gone and could not be replenished by volcanoes and evaporation from the oceans. Of course, this wouldn’t be a big problem for any subterranean microorganisms remigrating to the surface; they have no need for oxygen gas in their present location, and would do just fine on the “new old Earth,” as long as some access to sources of nitrogen remain in the crust. They might even put those gases back, as they are byproducts of microbial life, if given another billion years—as they already did on Earth about 2 billion years ago. 13

  It is humans and complex life forms that live a precarious existence subject to the vagaries of cosmic change. Earth life, as represented by its most numerous and ancient forms—the microbes—is permanently entrenched on our little planet, at least until the Sun retires and engulfs it. Think about what would happen if the Earth’s orbit became unstable and a near collision with Jupiter were to fling Earth out of the Solar System. 14 Sounds like the end of days, literally, as darkness and deep freeze would cover the surface. However, hydrothermal activity—those same black smokers in the middle of the Atlantic Ocean—would continue without interruption. Much of life that calls black smokers home would survive, and for quite some time—the crust makes an excellent blanket, trapping the
remnant heat from Earth’s formation, as well as the heat emitted by radioactive decay of elements like uranium, potassium 40, and thorium. In fact, the rate of loss of internal heat on Earth today is measured to be 87 milliwatts per square meter. 15 This is nearly a thousand times weaker than the rate at which a household lightbulb uses energy, and you would have to collect all the internal heat from an area larger than a college classroom to light up just a feeble 25-watt lightbulb. This seems like a drop in the ocean for our energy-hungry twenty-first-century human society, but is entirely sufficient for microbes that live deep in the crust and near hydrothermal vents at the bottom of the ocean. At its present rate of cooling, Earth, even lost in space, could keep its hydrothermal habitats alive for at least 5 billion years.16

  Earth life is extremely resilient, and has been for most of its history. One reason is its inherent ability to adapt to changing conditions and take advantage of varied, even extreme environments. In doing so, it also modifies and creates new environments, eventually transforming the surface of the whole planet. Planet Earth today is very different from the planet on which life emerged—precisely because life emerged on it. Moreover, Earth has been harshly inhospitable to the emergence of new forms of life for billions of years, courtesy of current Earth life and the highly oxidizing atmosphere it has created. Life is truly a planetary phenomenon, as my colleague Andrew Knoll showed convincingly: life and planet evolved together, dynamic and inexorably linked.17 Planets may be good places to sustain life, but we have not yet answered the question, Is Earth the ideal place for life to emerge?

  CHAPTER TEN

  PLACES WE COULD CALL HOME

  A long time ago, in a galaxy a lot like ours, a star formed. Soon several planets formed too. Three of the planets were close to the star. They weren’t very big, and they were almost entirely rocky.

  The smallest and the coldest of the three had an atmosphere of carbon dioxide and sulfur dioxide produced by volcanoes. They acted like a greenhouse that kept its surface warm, and so some of the surface water was able to liquefy. Together, the atmosphere and the oceans made the chemistry on the planet’s surface varied and fun—and gave life a good chance to emerge. The party ended after a few hundred million years, in geological terms nothing more than a summer vacation in Alaska. The planet, being small, had a hard time holding on to its atmosphere and supporting enough geological activity to replenish it. It just could not keep warm, inside or out.

  The other two planets, however, were larger, so they had more water and a more substantial atmosphere, though they too were all rock at heart. One of them, the planet closer to the star of the pair, was—like its smaller cousin—quick to change, although in an opposite sense. Its ocean began to evaporate under the star’s glare, and the addition of the water to the atmosphere—where it served as an excellent greenhouse gas—prompted a runaway loss of the rest of the water, leaving nothing but a rocky desert behind.1

  On both planets, however varied the chemistry might have been initially, the geochemistry became increasingly limited, and gradually settled into an inactive equilibrium. The future development of these planets soon became predictable because it was very simple—described by the basic laws of physics and chemistry. They joined most of the rest of the galaxy—from gas giants to stellar remnants—in a slow, dull process of cooling with no further chemical changes, simply whirling around under the influence of gravity.

  The third planet was lucky—large enough and warm enough to replenish and keep an atmosphere and to recycle its surface material, yet not too hot, so that it could keep most of its surface water liquid. The chemistry on this planet was fun and varied too, but even more so than on its little cousin. An existing geochemical cycle of carbon dioxide soon took the direction of planetary development farther and farther away from its geochemical equilibrium. Something fundamentally different was happening. About 2 billion years into this, the atmosphere and the oceans became increasingly richer in the very reactive free oxygen. Real fun for a chemist!

  While dull equilibrium ever more firmly gripped the first two planets, the third was becoming even wilder, reacting in complex ways to outside disturbances, such as the asteroid impacts that for a time incessantly bombarded the planet’s surface. This went on for about 4 billion years, although the asteroid impacts eventually petered out—until finally this complex chemistry harnessed enough energy locally to eject small pieces of the surface (and itself!) into orbit, eventually to its moon and back, eventually away from the star and into the galaxy.

  From the perspective just described, the fate of three planets as viewed from above, from the vantage point of the distant stars, might appear very natural. Just as stars are different, yet any star is much simpler than a planet, planets are different too and some develop a complex chemistry. Life as viewed from the stars would be indistinguishable from any other kind of chemical process, except for the complexity of its outputs. Much as we might speak of a water planet, because its chemistry outputs great oceans, we might, in a galactic sense, speak of a life planet because that’s what, given its constituents and its place in a solar system, its chemistry leads to. And just like liquid oceans might be only a part of the history of a type of planet, so it could be with life.

  You probably won’t be surprised to learn that the three planets I’ve been describing are planets we know well—Mars, Venus, and Earth. In this context, Earth seems like the place to be. But is Earth the ideal planet for life? I come back to the question I posed in the beginning of the book. Now I’ll try to answer it. And the answer is no. Super-Earths are better.

  For any planet we find, super-Earth or not, we try to assess habitability. It is a tricky concept. For example, if we define as habitable any place we humans can inhabit without special protection, there will be many places on Earth that fail to qualify. If we allow for areas where we could survive given our best technology, the equatorial regions of Mars would qualify but not a fiery furnace like Venus. Some Earth microbes are so hardy they could survive in the Martian soil. Clearly the habitability of a planet depends on its location: far from the Sun it is too cold, much closer to the Sun is too hot. This range of comfortable distance is called a habitable zone, and is indicated by the presence of surface water.2 Every star has one.

  I am looking for environments that make complex molecular chemistry viable. I am interested in pathways to (origins of) life in the Universe, so I am looking for planets that maintain surface temperatures in which large molecules can survive and can attain chemical concentrations and can be sufficiently stable over time. Although the habitable zone concept is helpful in terms of temperature, it is not sufficient. There are many other factors that contribute to the habitability of a planet. Given that our knowledge of the conditions on super-Earths will be very limited until we actually visit them one day in the future, I prefer to talk about their habitable potential.

  What is the habitable potential of the first known super-Earths? Let’s visit some of them. Gliese 876d orbits a small cool M dwarf star once every two days! That means that its orbit is only 2 percent of the Earth-Sun distance (or 0.02 AU). Its close proximity means that, although its star is only a third of the weight and size of the Sun and only half as hot, the surface temperature on Gliese 876d must exceed that of sweltering Venus (about 730 K). Super-Earth Gliese 876d is not in the habitable zone and has no habitable potential (Figure 10.1).

  There are two other, Jupiter-like, planets in this planetary system.3 They were discovered in 2000 by Geoff Marcy and the California-Carnegie team and have the designations Gliese 876b and 876c. They have masses equal to about 2 and 0.6 Jupiters, respectively. Despite being inside the habitable zone, these planets are gas giants. Lacking any solid surface, they have no habitable potential. However, either of them might have large moons that could have excellent habitable potential. Unfortunately, these two Jupiter-like planets have close orbits and interact gravitationally with each other, so it is likely that they could not have and retain large moon
s.

  Another M dwarf star, Gliese 581, is known to have three super-Earths (Figure 10.1). A hot Neptune of 25 ME (called Gliese 581b) has been known since 2005; it orbits the star with a period of 5.4 days. Later, Michel Mayor’s Geneva team found two smaller planets that could be the closest resemblance to Earth yet, as they seem to be barely inside the habitable zone. Because Gliese 581 and 876 are very similar stars, it is easy to compare their planetary systems and habitable zones. The simple assumption that super-Earths ought to have atmospheres leads to the habitable zone shown in the figure, derived from the work of Franck Selsis and colleagues.4 The super-Earth with the best habitable potential is the planet Gliese 581d, which has a mass of eight Earths and is farther from the star; its atmosphere helps keep the planet warm. Gliese 581c, which has a mass of five Earths, is likely too hot, and there is an even hotter super-Earth E inside the orbit of B.

  FIGURE 10.1. Two planetary systems and their habitable zones. The super-Earth in Gliese 876 is the inner D planet. In Gliese 581 , the inner B planet is actually a hot Neptune. Planet Gliese 581d is the only super-Earth that happens to orbit within a habitable zone, barely; it has a slightly eccentric orbit. Gliese 581 e, a fourth planet not shown here, is barely two Earths but orbits the star inside the orbit of planet B and is too hot.

  The planetary system Gliese 581 is interesting in another way: most, if not all, of the planets must have formed farther from the star than they currently are. First, there is not enough material in a proto-planetary disk for such a bulky planet as the hot Neptune, Gliese 581b, to have formed just a dozen stellar sizes away from the star. Second, the temperature in the disk is too high for the large planet to have accumulated. It is much more likely that the hot Neptune, just like the hot Jupiters described earlier, formed farther out and was pushed close to the star by the disk. If that’s true, then the two super-Earths must have formed even farther out in the disk. They could be ocean planets, as they must have formed outside the disk’s snow line, where water freezes and accumulates easily. Some of this surface water would have liquefied when the planets were pushed closer in. However, they could be gas-rich planets, similar to the recently discovered planets GJ 1214b by David Charbonneau’s MEarth Project, and the planets in the Kepler-11 system. These are all of similar masses—a few Earths, and they are all of very low density that requires them to be mini versions of Uranus and Neptune.