Mirror Earth Read online

  A few days before his guests arrived, Drake realized he needed some sort of structure for the meeting, so he thought for a bit, then wrote this down on the blackboard: N = R* fp ne f1 fi fc L. N—the number you are trying to figure out—represents the number of civilizations capable of communicating across interstellar space. The letters on the right side represent, in order: R*, the rate at which Sun-like stars form; fp, the fraction of stars that form planets; ne, the number of planets per solar system hospitable to life; f1 the number of planets where life emerges; fi, the fraction of life-bearing planets where intelligence evolves; fc, the fraction of these planets that have developed interstellar communication; and L, the average lifetime of such civilizations (if they arose and then died out quickly, there would be few of them around). If N is large, it makes sense to search for alien signals; if not, it does not.

  The equation is so well known by now that people accost Drake in restaurants to have him write it down for them, along with his autograph (he once said that Japanese tourists often want him to write it on their clothing, for some reason he hasn’t been able to figure out). But while it’s useful as an organizing principle, nobody has a clue what N actually is. In 1961, the only term on the righthand side that anyone could put a number to was the rate of star formation. Today, thanks to Kepler and the radial-velocity searches conducted by Mayor and Marcy, exoplaneteers are on the verge of nailing down eta-sub-Earth, in one form or another.

  But how often life might arise on exoplanets is still a complete mystery. At Frank Drake’s 1961 conference, Carl Sagan suggested it would happen 100 percent of the time: Life should arise on every Earth-like planet in the habitable zone of a Sun-like star. It’s not a crazy proposition: The basic components of life as we know it on Earth are water and complex, carbon-based molecules, both of which are plentiful in the Milky Way. Astronomers have even found such organic molecules as formaldehyde and alcohol floating in interstellar space (to a chemist, organic doesn’t mean living, or produced by living things; it simply means carbon-based).

  It seems plausible that water and organic chemicals must inevitably give rise to life, but that’s a long way from proof. The only reason to believe such a thing is that life seems to have arisen on Earth by around 3.5 billion years ago, just a few hundred million years after the surface had cooled to tolerable temperatures in the aftermath of a bombardment by asteroids. If life arose so quickly, goes the argument, its appearance must have been pretty much inevitable. And if that’s the case here, it should be the same everywhere.

  Another line of reasoning that supports the “life is everywhere” theory, unknown at the time of Drake’s conference, has emerged over the past few decades: Scientists have found living organisms—bacteria, mostly—thriving in an enormous range of harsh and improbable environments, including floating sea ice in the high Arctic; pools of water that are boiling hot, or harshly acidic, or salty, or even radioactive; and solid rock a mile or more underground. If life can survive in such awful places, it could easily exist on planets that are barely Earth-like at best. This could suggest Sagan’s optimism may have been even more fully justified than he knew at the time.

  But again, this is purely circumstantial evidence. The truth is that nobody has a clue about how life first arose on Earth, or even where. Charles Darwin suggested in passing that it might have happened in a “warm little pond.” Since the 1950s, scientists have offered other ideas: It happened in the atmosphere, or in superheated water gushing from cracks in the ocean floor, or in beds of clay, or in lightning-charged clouds of gases spewing from ancient volcanoes. The best understanding of how it happened is similarly murky. The emergence of life must have involved a complex interplay between organic compounds that somehow organized themselves into self-replicating molecules. Here’s an excerpt from the Wikipedia article on “RNA world hypothesis” that nicely captures current thinking about just one of several theories of how it all happened:

  The RNA world hypothesis proposes that life based on ribonucleic acid (RNA) pre-dates the current world of life based on deoxyribonucleic acid (DNA), RNA and proteins. RNA is able both to store genetic information, like DNA, and to catalyze chemical reactions, like an enzyme … It may therefore have supported pre-cellular life and been a major step in the evolution of cellular life.

  In a 2011 review of the evidence, Thomas Čech suggests that multiple self-replicating molecular systems probably preceded RNA … The RNA world hypothesis suggests that RNA in modern cells is an evolutionary remnant of the RNA world that preceded ours.

  Note the words hypothesis, may, suggests, and probably. Also note that the RNA-world hypothesis isn’t the only one making the rounds. There’s also the “lipid world hypothesis” and the “iron-sulfur world hypothesis,” and a few more. Rather than go into the details of each, let’s just say that the question of how and where life arose on Earth is a massively complex puzzle. The puzzle pieces themselves—the physical evidence of what really happened—have long since vanished. The best biologists can do is to try reconstructing what the pieces might have looked like, and how they might have fitted together. Every breakthrough in origin-of-life studies to date has been an important but very small step toward a convincing explanation of how it really happened. It may be that life is inevitable, given the right conditions, as Sagan thought. It may equally be that life is terribly, terribly unlikely to happen, even under the best of circumstances. The fact that life on Earth survives in so many harsh environments, moreover, doesn’t prove that life arises easily. It proves only that that life can adapt like crazy after it arises.

  If you’re a pessimist, therefore, you might conclude that the search for extraterrestrial life might well prove to be fruitless. If you need further ammunition to bolster your pessimism, you might take a look at the book Rare Earth, published by paleontologist Peter Ward and astronomer Don Brownlee in 2000. The authors advance a series of arguments to suggest that while life might well be common in the Milky Way, the sort of advanced life we’d really love to find is very rare. Each argument by itself sounds pretty convincing; taken together, they appear at first to be devastating.

  Take Jupiter, for example. If our biggest planet had spiraled in toward the Sun to become a hot Jupiter, it would probably have disrupted Earth’s orbit. But if we had no Jupiter at all, that could be a problem as well. The reason, argue Ward and Brownlee, is that Jupiter shields the Earth from comet impacts. Comets originate from the outer solar system, and most of them stay there. When one does fall in toward the Sun, however, it’s almost always flung away by Jupiter before it can get anywhere near Earth. The astronomer George Wetherill showed decades ago that if Jupiter didn’t exist, we would get about ten thousand times more comets smashing into Earth than we do—not a good thing for the emergence and evolution of anything more advanced than bacteria.

  Ward and Brownlee also point out that our Moon is much bigger in relation to Earth than any planet-moon pair in the solar system. It’s so massive that its gravity helps stabilize the tilt of the Earth. Mars, whose moons are tiny, wobbles something like a spinning top that’s close to falling over. Without the Moon, our planet would do the same, making the seasons highly unstable and making it hard for plants and animals to adapt.

  And then there’s plate tectonics, which recycles the Earth’s crust back into the interior over hundreds of millions of years. That process also recycles carbon dioxide after it binds chemically to surface rocks, ensuring that the atmosphere doesn’t undergo a runaway greenhouse effect, turning our planet into a hothouse like Venus. Of all the rocky bodies in the solar system, only Earth has plate tectonics, so it’s probably rare in the universe. And then there’s Earth’s magnetic field, which protects us against energetic particles streaming in from the Sun or from deep space. And then … well, suffice it to say that Rare Earth makes a sobering read.

  It does, that is, until you talk to Jim Kasting. “A lot of people read [Rare Earth] and believed it,” he told me during our conversation at
that Vietnamese restaurant in Seattle. “I think they sold a lot of copies because it was the anti–Carl Sagan. It appealed to people who didn’t want to believe this whole line of stuff that Carl had been selling.”

  One by one, Kasting addressed the arguments in Rare Earth and made it clear that he wasn’t impressed. For example, he said, it’s true that if you eliminated the Moon, Earth’s tilt would wobble chaotically. But if Earth were spinning faster—if the day were twelve hours long rather than twenty-four—the chaos would go away. “So you have to ask,” said Kasting, “How fast would the Earth be spinning if you didn’t have the Moon? And that’s complicated.” In short, Ward and Brownlee raise a plausible argument, but hardly a definitive one.

  It’s also true, continued Kasting, that Jupiter protects Earth from comet impacts. But it actually raises the odds we’ll be struck by asteroids. That’s because the asteroid belt is just Sunward of Jupiter, so it’s relatively easy for the giant planet to nudge a mountain-size chunk of rock into an Earth-crossing orbit. “It appears,” Kasting writes in his 2010 book How to Find a Habitable Planet, where he devotes a full chapter to presenting counterarguments to Rare Earth, “that having a Jupiter-sized planet … is a mixed blessing.”

  As for plate tectonics, he said, Venus is the only other planet in our solar system besides Earth big enough to have them in the first place (a planet smaller than Venus would have cooled off by now, so it wouldn’t have the semi-molten rock that allows continents to slide around). But Venus lacks the water it would need to lubricate the motion of crustal plates, which could be why, despite its adequate size, it doesn’t have plate tectonics. Out of two planets that might have plate tectonics, one of them does, and Kasting sees no reason at all to assume that Venus is somehow typical of exoplanets while Earth isn’t. The bottom line, he said, is that “there are a lot of things that we don’t know, so we make conjectures. Ultimately, if we can do TPF and follow up with post-TPF missions, we’ll figure out what happens, and where.” “I’m an optimist,” he admitted. “I agree with Carl Sagan. I think there’s probably life all over the place, and there are probably other intelligent beings. I’m just not as good at speculating as he was.”

  There’s another reason you might lean in the direction of optimism. The concept of the habitable zone applies if you’re assuming life is confined to the surface of a planet. If you discard that assumption and consider places where conditions are favorable beneath the surface, you’ve suddenly got a lot more places to look. In our own solar system, Earth has the only habitable surface, but planetary scientists think the Martian subsurface might be habitable as well. In November 2011, NASA launched its biggest, most capable rover toward Mars, where the six-wheeled, SUV-size Curiosity will, among other things, drill into the Martian soil to look for organic chemicals (but not, on this mission, for life itself).

  The right conditions for life could also exist on even more exotic worlds. Astronomers have known for years that Jupiter’s moon Europa and Saturn’s moon Enceladus both have subsurface water. The energy to keep the former from freezing solid right down to the core comes from tidal squeezing, as it orbits through the powerful gravitational field of Jupiter; Enceladus’s heat source is a mystery. More recently, theorists have suggested that even Pluto might harbor liquid water, one hundred miles or so beneath its icy surface—the heat in this case coming from the decay of radioactive potassium. As for complex carbon molecules, they’re abundant in the bodies of both comets and asteroids, which have been crashing into the moons and the outer planets for billions of years.

  Yet another plausible reason for optimism arises from the fact that the universe is under no obligation to follow the “life as we know it” rule. Carbon is abundant in the Milky Way and combines easily with other atoms to form the elaborate organic molecules that underlie all of terrestrial biology. Water is abundant as well, and acts as a versatile solvent. So it’s not absurd to think that carbon-based life might be universal, and is exactly what astrobiologists should be looking for. “It may turn out to be universal,” Dimitar Sasselov, Sara Seager’s grad school thesis adviser, said on a visit to his Harvard office. “There may be some basic law of chemistry, which always leads you to the use of amino acids and nucleic acids for coding and the use of particular metabolic cycles for energy.” “But,” he added, “it also may be environmentally dependent.”

  Sasselov now directs Harvard’s Origins of Life Initiative, an interdisciplinary effort to understand where life comes from, and under what conditions, in order to guide future observations. This sort of astrobiology collaboration, in which biologists and geologists and astronomers and planetary scientists try to work together, is very popular nowadays; Debra Fischer has a similar collaboration at Yale, for example.

  At Harvard, Sasselov’s group is thinking about alternate biologies—particularly, he said, on planets where the global geochemical cycle is based not on carbon but on sulfur. “We showed in a paper last year,” he said, “that a sulfur cycle on a nearby Earth or super-Earth would be easily detectable with the James Webb Space Telescope. Not only is it detectable, but you’ll be able to measure the relative concentrations of sulfur dioxide and carbon dioxide and water, which is what the chemists in our group need in order to set up the experiments.” In the meantime, the biochemists in the group are doing a sort of practice run, trying to create an alternate biology in the lab that’s still based on carbon, but whose DNA and amino acids twist in the opposite direction from those in all Earthly organisms. As far as anyone knows, this mirror life would violate no rules of biochemistry, and while Earth biology is based on “left-handed” amino acids, their mirror-image right-handed counterparts also exist in nature.

  The goal, explained Sasselov, is to create a primitive living cell. “That’s why we’re doing the mirror project. It’s trivial from a planetary point of view,” he said, since it’s still a form of carbon-based life, “but it’s the easiest way to develop the basic methodology, which we’ll then use for a more weird biochemistry—weird in the sense of an alternative system.” The project, he said, is “pretty well along. George [Church, a geneticist and biochemist at Harvard Medical School] thinks that we’re within months of finishing it, and Jack [Szostak, ditto] thinks maybe a year and a half.” “What we expect from the experiment is a chemically functioning system,” he added, “not something that’s going to walk on this table. But that’s all we need.”

  It hasn’t escaped Sasselov that making a functioning cell with a biochemistry found nowhere on Earth sounds like science fiction. “That’s why I am attracted to this field,” he said. “I see a direct connection between what I do and the big questions. It would be exciting to say, ‘Well, I managed to detect water and sulfur dioxide on that exoplanet,’ but it’s not exactly one of the big questions of science. ‘What is the nature of life?’ is a big question.”

  The other big question that Sasselov is trying to answer, in his role as a Kepler co-investigator, is how big a planet can be and still be habitable. If a world has to be a true Mirror Earth in size, there will obviously be a relatively small number of planets to choose from: You’re going back down the road of pessimism. Sasselov isn’t going down that road. “If you’re asking what the optimal size is for life, I don’t see a dividing line,” he said, “between one Earth mass and five Earth masses. In fact, if you ask me, bigger is better. Smaller is not. Mars is definitely too small. If you go much below an Earth mass, you can’t have plate tectonics, and you don’t have a stable atmosphere because it evaporates too easily [because there’s less gravity to hold onto it].” Earth is not a Goldilocks planet from this point of view, he said. It’s not “just right” for life: It’s at the small end of the habitable range.

  All these factors—how likely life is to arise in the first place, what environments allow it to arise most easily, how many planets there are, of what size, in what orbits, with what geology and geochemistry, with what other sorts of planets in the same system—feed into the question o
f whether life exists anywhere but Earth, and if so, whether that life is more advanced than a bacterium. Nobody really knows the answer to any of these questions. Nobody can give even a ballpark solution to the Drake Equation, a half century after it was first written down. Nobody knows what habitable really means.

  When the question comes up at conferences, as it frequently does, this lack of knowledge doesn’t keep exoplaneteers from weighing in with their own best guesses, of course. At one meeting, however, I heard a different response from Dave Charbonneau. How do you define habitable? someone asked. “I don’t much care,” he said. “I want to build an experiment that can find things as small as the Earth and that are roughly at the same irradiance taking into account the luminosity of the stars, and then you can ask me in ten years what makes a planet habitable. We probably won’t know but I think that’s the way that we’ll make the greatest progress.”

  It’s also possible that the small handful of astronomers who are still doing SETI searches will detect an alien signal long before Dave Charbonneau’s ten years are up. This could, in principle, happen tomorrow. It’s been a half century since Frank Drake wrote down the Drake Equation and in all that time not a single verified transmission has been picked up. That hardly means the search has been a failure, however, argues Jill Tarter, director of the Center for SETI Research at the SETI Institute. The search has suffered from limited resources from the start, she says. “If you dipped a drinking glass into the ocean once,” she likes to ask, “and came up without a fish, would you conclude that there are no fish in the sea?”