First Contact (19 page)

Many space scientists think Levin is something of a nuisance, someone who can't let go of a flawed experiment and result. Others see him as a pioneer and dedicated scientist who properly won't stand down. “Steelie,” for instance, who did so much to undercut the work of McKay, says that Levin is one of his heroes. “He got his findings, he trusted the instruments, and he held his ground,” he said. “I admire that.” Scientists will eventually come forward with evidence of what they are convinced is, or was, extraterrestrial life, probably of the microscopic variety. How will we know when it's the real thing?

“Extraordinary claims require extraordinary evidence.” Sagan's often-cited words have been used to bludgeon David McKay's Mars meteorite conclusions, Levin's Labeled Release results, Hoover's Murchison microfossils. The common refrain: How could they make such grand claims based on incomplete or controversial evidence? While Sagan's standard sounds right and may be entirely appropriate, the experience of at least one of these researchers shows just how difficult it will be to define and interpret. As McKay knows, but seldom says, one of the people given his initial Allan Hills 84001 paper to review for
Science
was none other than Carl
Sagan. He read it, no doubt had questions, criticisms, and suggestions, and ultimately recommended that it be published. He was terminally ill then, but still by all accounts alert and actively engaged in his work. Clearly, Sagan saw ALH 84001 as important—even “extraordinary”—science that was worthy of the attention it would soon receive. But fifteen years later, it remains just not extraordinary enough.

You would think that searching for exoplanets many light-years away would require the newest, most sophisticated telescopes. But Paul Butler, one of the world's great planet hunters, has done some of his best work at an observatory formally dedicated in 1974 by Prince Charles in the Australian countryside. The telescope's mirror is relatively small by today's standards and the observatory has none of the power and élan of the Paranal facility used by Mike Mumma. The skies are far more likely to be clouded over and the telescope unusable at night than at other new observatories. It's also in kangaroo country, which Butler and many Australians see as a less than pleasant feature since the animals frequent the nighttime pathways around the campus: A full-force kangaroo kick can kill a man.

But Butler loves the Anglo-Australian Observatory (AAO), twenty miles outside the two-street town of Coonabarabran in New South Wales and next door to the dramatic and ancient volcano remains of Warrumbungle National Park. Some of its perceived weaknesses, he says, are invaluable strengths. Yes, finding extrasolar planets is hard, but it's well within the capacity of the four-meter Anglo-Australian Telescope (AAT) glass. And because the observatory is no longer cutting-edge, and is in a distant location that American and European astronomers are not keen to put in the time to use, Butler has what he needs most—lots and lots of nights on the telescope. He spends three months a year at the AAO. That means he can study a star for hours, even days or weeks if he suspects there's a planet circling it. Since he began coming to Coonabarabran, the self-proclaimed
“Astronomy Capital of Australia,” Butler and the team he leads have identified and to some extent characterized 40 exoplanets. Worldwide, astronomers have found more than 500 exoplanets, and Butler has been in some way involved in about half those discoveries. He plans to planet-hunt for years to come, but now he has a new focus: the makeup or “architecture” of solar systems. That's because discovering where exoplanets are in relation to their suns and companion planets is essential to determining if they're habitable and could ever be home to extraterrestrial life.

The son of a Los Angeles policeman, Paul Butler is a tall, bearded man who loves jazz and wears long pants only when it's below freezing. He's been looking for exoplanets since the late 1980s, more than a half decade before a Swiss team announced the first detection, which soon after was confirmed by Butler and his colleagues. He's away from his wife, his home, and his office at the Carnegie Institution in Washington more than half the year at observatories in Chile, Hawaii, and in the land of kangaroos (or, as he terms them, giant-tailed rats). He calls the AAT control area his living room because he's been there so much and he feels that comfortable in it. It helps that the telescope is also just through a blackout door from the control room; at more sophisticated and more highly elevated observatories like the W. M. Keck in Hawaii, the control room is a two-hour ride down the mountain. The nighttime world of star- and planet-gazing exerts an almost gravitational pull on those captured by it, an endless desire to know more about the mysterious yet increasingly knowable vastness in which we live. “On clear nights, there's absolutely nowhere I'd rather be,” Butler says.

But our first night at the telescope was cloud covered—actually, was socked in and packing knock-you-down wind from a cyclone on the eastern coast—so instead of observing, we talked. Butler was eager to explain exactly why the AAT remains such a godsend even though significantly more sophisticated telescopes are available, and his discoveries about the planets orbiting the star called 61 Virginis were exhibit A. The research was published in 2009 and represents the teasing out of one of the first three-planet solar systems orbiting a sunlike star.

“Look at this curve,” he said, pointing at a computer screen full of initially indecipherable but nonetheless elegant graphs, the kind that allow him and other planet hunters to determine that a distant planet is present. Specifically, he motioned to the chart labeled “61 Vir,” which happens to be one of the closest bright stars to Earth and one that is visible to the naked eye. “We'd been observing that star for years, and now we were seeing something. But we had to pull out the signal, and it was very complicated.” Astronomers will one day be able to routinely see or “image” distant planets using considerably more sophisticated telescopes than those available today, but for now most planet hunting involves indirect measurements of the effects of extrasolar planets on their suns.

First Butler and his team found signs of a planet the size of five Earths orbiting 61 Vir in a breakneck four days. “But after a while the pattern changed—something that doesn't happen unless there's a good reason. We suspected there was another planet, and that one turned out to be Neptune-sized with a thirty-eight-day orbit.” But still something was off, and Butler wouldn't know what it was until he and his team observed for several more weeks. What they ultimately found was the signature of not one planet but of three: one orbiting closely, one at 38 days to circle its sun, and then another at 125 days. This only became clear because Butler had weeks of time on the telescope—forty-seven straight nights, a run that would be impossible at the bigger and more sophisticated observatories in Chile and Hawaii. And all those nights of observing allowed his team to put enough dots on its chart so it could read the complicated message being sent by the planets.

When astronomers began detecting exoplanets in the mid-1990s, it was very big news that landed a story about Butler and his colleague, Geoff Marcy, on the cover of
Time
magazine. The implications were as exciting as the discoveries themselves: If planets were found to orbit tens of billions of stars in the Milky Way alone, then it seemed entirely plausible that some contained liquid water, nutrients including carbon, and an atmosphere to keep out the most damaging cosmic and solar rays. In other words, the
basic conditions for life as we know it. And who knows, some of those planets could well be home to life as we don't know it, based on different chemicals and conditions. Suddenly the prospects for extraterrestrial existence increased dramatically. It was no coincidence that the emergence of astrobiology as a respected and, soon after, a hot field of research occurred in the mid- and late 1990s—right as it became clear that the discovery of the first extrasolar planet, 51 Pegasi, would be followed by many more. NASA started its formal Astrobiology Program in 1998 with these exoplanet discoveries, as well as that controversial announcement of signatures of life in a Martian meteorite, very much in mind.

But as more planets were found, it became clear that many, and probably most, were strikingly different than what almost all astronomers and planetary scientists expected, what Butler calls the “Everything You Know Is Wrong” phase of extrasolar planet research, borrowing from the Firesign Theatre comedy team of the 1970s and one of its iconic acts. The consensus of the astronomy community had been that distant solar systems would be similar to ours, that the known physics and dynamics of star and solar system creation required a certain kind of arrangement. Yet huge Jupiter-like planets were discovered revolving extraordinarily close to their suns. In fact, planets with highly eccentric orbits were found to be far more common than the near-circular orbits of planets in our solar system. Even more unexpectedly, solar systems were found that were somewhat like ours but with seemingly impossible variations—for instance, with a circular-orbiting Jupiter in what is considered the roughly “right place” in relation to its sun, along with an eccentrically orbiting and even larger Jupiter in the inner solar system region where rocky planets are supposed to live. As it turns out, Butler said, having our solar system as a model “can be worse than having a sample of zero because it leads you down one road and you don't imagine the others.” But because of research like Butler's, the field of planet hunting has abandoned its previous assumptions and now is working hard to make sense of the new reality that solar systems structured like our own are a distinct minority.

None of these discoveries were, or are, particularly good in terms of the search for extraterrestrial life. But they're not the final story at all; rather, they're scientific waystations along the path to detecting the Earthlike planets virtually all astronomers believe exist, and an introduction to the kinds of solar systems to avoid if finding habitable zones and distant biology is your goal.

For instance, a consensus exists within the astronomy community that to have any chance of supporting life, a solar system needs a huge Jupiter- or Saturn-sized planet (300 and 100 times more massive than Earth, respectively) in roughly the locations where they sit in our solar system. That's because the gravitational force of the giant planets serves to pull in and destroy asteroids and other celestial bodies that might otherwise head into the “habitable” zone and smash the small rocky planets to bits. This is why in astronomical circles Jupiter is often called our protective “big brother” or “big bouncer.” But if Jupiters and Saturns in many other solar systems are close in to their suns, or otherwise in what is considered the wrong place, then they can offer no protection at all. The question of eccentric orbits is perhaps even more unsettling. A planet that swings very close to and then very far from its sun will almost assuredly experience temperature swings too extreme to support life. We know that living things can exist in very hot and very cold environments, but the same organisms probably can't exist in both. In addition, the gravitational force of a large planet with a strongly eccentric orbit would most likely kick any smaller planets out of their solar system and into space. Nothing personal—it's just gravitational physics at work. “Solar systems with really eccentric orbits are about the worst place to look for life,” Butler says.

So while Butler and his colleagues continue their two-decade search to detect and characterize extrasolar planets, the new and most important questions in the field have changed and become quite a bit more complicated and ambitious. With Miles Davis and John Coltrane as the backdrop to his thinking, Butler described the two goals that he hoped to help achieve before his time as a peripatetic, globe-hopping astronomer comes
to an end. Like his stargazing colleagues everywhere, Butler speaks in a language that can often seem mysterious and impregnable—throwing out references to laws of physics, categories of stars and planets, and modes of measurement that are foreign to the uninitiated. The concepts behind them, however, usually make a pleasing, even elegant kind of sense:

“Overall, what we're trying to find is solar system analogues because we'd like to know how common or rare the architecture of our solar system is. What are the systems that have Jupiters and Saturns beyond four or five AUs [astronomical units, or the distance from the sun to the Earth]? What are the systems in circular orbits? Those are the signposts for us—systems with noneccentric orbits and with big brothers to shield the smaller inner planets. That's where you'll find habitable zones with the potential for life. When we find them we want to go back and stare at them hard and look for the Earths and other inner planets [that] should be there.” He said it would probably take another ten years of planet hunting to get a good representative census of solar system architectures, and that the percentage of systems like ours might be as low as 5 to 15 percent—a perhaps disappointing number until you recall that there are trillions of trillions of stars out there.

“Second, we need to know about habitable planets, how common or rare they might be. Right now our best guess is that rocky planets like Earth and Mars in zones where life could theoretically exist are present in most solar systems, but we really don't know and could be all wrong. Finding them will be hard because of the ways we look for planets, and so it may seem that any clear understanding is way off.” But people are making progress, he said, and right now his team can find planets only five times the size of Earth in habitable zones—that is, positions in relation to their suns where they are likely to be rocky planets with liquid water—around M dwarf stars. M dwarf stars, or red dwarfs, are the most prevalent in the sky. They are about half the size of our sun and produce far less energy, making it theoretically possible for habitable planets to orbit in close, where current planet-hunting technology can better detect them.