Mirror Earth (27 page)

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 of 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?”

Our failure to hear transmissions, in other words, doesn't mean they're not out there. The men and women who have continued to listen for alien radio signals for the past fifty years and, more recently, for possible flashes of light from alien signaling lasers, have always been mindful of Philip Morrison and Giuseppe Cocconi's observation that the probability
of success is difficult to estimate. Even if millions of habitable planets exist, which is looking more likely all the time, and even if some of them have given rise to technologically advanced life, nobody really knows whether radio waves or laser beams are the standard way of talking across interstellar space. Maybe what we think of as advanced communications is their equivalent of smoke signals—a stepping stone on the way to technologies we can't yet imagine.

By the time the American Astronomical Society's winter meeting rolled around in January 2011, the astronomers who had gathered in Seattle were itching to get their hands on the four hundred planet candidates the Kepler team had dangled, then snatched away, six months earlier. It wasn't quite time, though. Bill Borucki had promised the four hundred would be set free on February 1, and he wasn't going to move the date forward.

The conference-goers wouldn't have to leave empty-handed, however. In what was turning out to be a pattern, the Kepler team wouldn't release candidates, but they would announce actual, confirmed planets, just as they had at the Washington meeting a year earlier. In Washington, the big news was simply that Kepler could find planets, and that radial-velocity measurements could figure out the masses and densities for some of them. The only announcement in the year since then had been Kepler-9, the first planetary system where the mass of the planets had been measured, not with radial
velocities, but with transit timing, the powerful new technique nobody had even thought about when the Kepler Mission had first been approved.

This time, Natalie Batalha would be making the big reveal, at a press conference on the first day of the meeting. She agreed to brief me the day before, however, as long as I promised, on pain of her extreme disapproval, that I wouldn't say anything about it before the official presentation. We met at a hotel a few blocks from the main conference hotel; the Kepler team was having a preconference workshop to talk about the upcoming data release the next month and to work through various routine issues that inevitably came up in analyzing and following up what the satellite was telling them. When I showed up, an Ames press officer went into the workshop to get Batalha. I could see a couple dozen astronomers, including Dave Latham, Dave Charbonneau, and Bill Borucki, listening to someone making a presentation. I asked if I could just sit in on the meeting. The press officer and Batalha both looked startled for a moment, then realized I couldn't be serious. We all knew there was no way I was getting inside that room.

“So what we're announcing tomorrow,” she said, when we sat down, “is the discovery of our first rocky planet, Kepler-10b. It's in a very short period of less than a day, which makes it very similar to CoRoT-7b, if you're familiar with that result.” What made this discovery special, though, she said, “is that our error bars on all of our measurements are very tight. We know unquestionably that this is a rocky world.” An error bar is another term for uncertainty, the plus-or-minus that goes with astronomical measurements of pretty much anything—the
age of the universe, the distance between the Earth and the Sun, whatever. “And the reason that we know,” she said, “is because we've got all of our best capabilities coming together for this one discovery. We've got this exquisite photometry [the change in brightness as the planet moves in front of its star]. We've got a very high-precision radial velocity. And this star is bright enough that we can use astroseismology, which allows us to derive the fundamental stellar properties to within 2 to 6 percent accuracy.”

The team had known about this planet, or, at least, had known it was a good candidate, almost as soon as Kepler was launched. “We actually saw it during the commissioning phase,” she said, “before science operations had formally started.” Geoff Marcy's group did its first radial-velocity measurements from the Keck in August 2009, about the time Marcy was willing to admit publicly only that the satellite was working properly. “So the mass of this planet is 4.65 Earth masses,” Batalha continued, “and the radius is 1.4 times that of the Earth.” The density, she said, works out to 8.8 grams per cubic centimeter, which makes Kepler-10b half again as dense as Earth. “This seems kind of high,” she said. “If you google it, you'll find it's about the density of an iron dumbbell.”

But that doesn't mean it's made of iron. Kepler-10b is so massive that it crushes itself down under its own weight. If you take the material the Earth is made out of—the same proportions of silicate rocks and iron and nickel—and just add more of them, the planet grows heavier at a faster rate than it grows larger, thanks to the crushing; and at 4.5 times the mass of the real Earth it would in fact be about as dense as a dumbbell.
A dumbbell that heavy would be even denser. If you graph the size versus the mass of an Earth-type planet, and locate Kepler-10b on the graph, she said, “the error bar almost kisses that line of Earth composition. It's just a little higher in density, so it does seem to be a little bit iron-rich. Kind of like Mercury.” CoRoT-7b kisses the Earth-composition line too, she said, echoing what Marcy had told me earlier. The difference is that, unlike Kepler-10b, it has huge error bars. “We've got three different papers on CoRoT-7b with mass estimates that range from, like, zero to ten. Zero is a funny way of saying it, but it's basically a nondetection all the way up to ten Earth masses.”

For that reason, she was arguing, Kepler-10b, not CoRoT-7b, should be counted as the first unambiguously rocky world ever found beyond our solar system. It was too big and far too hot to be a true Mirror Earth, but it was another step closer. The star itself, Kepler-10, is about 560 light-years from Earth, which got Batalha thinking. “I told this story to the science team this morning,” she said. “Just out of curiosity, I subtracted 560 from the year 2010 and got 1450. The year 1450 is when the light left the star. And I googled the year 1450 to see what was happening—in Wikipedia it's listed as being the beginning of the age of discovery. Isn't that amazing? I love that. So Europeans were crossing the Atlantic for the first time when light left the star. I thought that was kind of a nice tie-in.” (It was now 2011, but since the distance to Kepler-10 isn't known down to the light-year, it's a legitimate bit of scientific poetry.)

A videographer named Dana Berry, who has done work for
National Geographic, had been commissioned to create a video animation of what Kepler-10b might look like—a rocky planet, hugging its star with one face always turned to the searing light glowing red with heat. “When I saw it,” Batalha told me, “I thought about our small telescope up at the Lick Observatory.” This was the Vulcan telescope, which Bill Borucki used to prove to NASA that he could detect multiple transits, named for the planet astronomers once thought orbited Sunward of Mercury. “So when I saw the animation,” said Batalha, “the first thought that came to my mind was, ‘Wow, this is our planet Vulcan.' ”