Mirror Earth (26 page)

Such a world would be pretty much impossible to detect, however. That's unfortunate, since if Stevenson is right about how common and how balmy they're likely to be, they could be ideal places for life to have taken hold. As long as we're dreaming, it's even conceivable that these lonely worlds, trapped in perpetual night, might be the most common places in the Milky Way where life has managed to thrive. We can't help thinking of Earth as the model for an inhabited planet. If this theory is correct, however, our world could be just a quirky oddball, far outside the biological mainstream.

When Dave Charbonneau originally coined the term
exoplaneteer
, it referred to someone who searches for planets around stars beyond the Sun. But Charbonneau, and Geoff Marcy, and Bill Borucki, and virtually every other planet hunter I ever spoke with was ultimately looking not simply for planets, but for life beyond Earth. That search covers a broader range of scientific disciplines than just pure astronomy, so
exoplaneteer
can just as accurately describe not just astronomers, but also planetary scientists, climate modelers, chemists, biologists, geologists, and more. Back in the 1960s, scientists who were interested in the question of extraterrestrial life (the scientific question, that is, not the question of whether UFOs were crashing in Arizona) began using a variety of names for this meta-discipline, including bioastronomy, exobiology, and astrobiology. By the late 1990s, everyone had pretty much settled on astrobiology, including NASA, which created the NASA Astrobiology Institute in 1998. It's a good, descriptive catchall term. But
exoplaneteer
is a lot more fun.

Whatever you call it, the best way to think about exoplanetology comes from an equation scribbled on a blackboard in rural West Virginia in 1961. That's when a young radio astronomer named Frank Drake first wrote down the Drake Equation, a series of symbols you can use to calculate the number of technologically advanced civilizations in the Milky Way. Drake was then a scientist at the National Radio Astronomy Observatory in Green Bank, West Virginia (the rural setting was chosen because it was far from artificial radio sources that could contaminate signals from space). He was intrigued with the idea of listening for alien radio broadcasts with the observatory's powerful radio dishes—Drake performed the very first search for extraterrestrial intelligence, or SETI, that same year—and he'd organized a small workshop to talk about the question. A young Carl Sagan was one of the participants.

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* f
_p
n
_e
f
₁
f
_i
f
_c
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; f
_p
, the fraction of stars that form planets; n
_e
, the number of planets per solar system hospitable to life; f
₁
the number of planets where life emerges; f
_i
, the fraction of life-bearing planets where intelligence evolves; f
_c
, 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).