Alien-hunting scientists have had an eventful year, and they're about to get busier. In just the past few months, life-friendly soil and ice turned up on Mars, astronomers bagged a trio of Earth-like planets in a distant star system, and scientists looking closer to home reported that certain hardy microbes thrive below Earth's ocean floor—a big clue that life may exist on planets that at first glance appear inhospitable.
None of the findings shout, "Here be aliens!" but each report has stoked optimism among astrobiologists that they will discover life beyond Earth. Some leading stargazers, in fact, suspect we're now on the verge of learning that we're not alone—and that genesis wasn't a unique event.
In space, "everywhere we look, we see the same processes that we think led to the origin of life on Earth," says John Rummel, NASA's senior scientist for astrobiology.
In the coming months, two new tools will greatly expand astrobiologists' capacity to hear and see other promising signs of life. Later this summer, the nonprofit SETI Institute, named with the acronym for the search for extraterrestrial intelligence, will begin listening for alien broadcasts on the new $50 million Allen Telescope Array. A spread of 42 radio dishes in California's Cascade Mountains, the array is the first such facility built specifically to listen for E.T. "We're looking for life that's clever enough to hold up its side of the conversation," says Seth Shostak, a senior astronomer at the SETI Institute. The array, half funded by Microsoft mogul Paul Allen, will search for alien signals at a clip "hundreds to thousands times faster" than current SETI projects, says Shostak.
Meanwhile, NASA's $550 million Kepler space telescope is being readied to launch next February. Kepler is the first telescope designed to glimpse Earth-like worlds orbiting distant stars. Though astronomers have had trouble detecting them directly, they suspect that such planets are commonplace throughout the galaxy. A recent survey of 200 potential solar systems suggests that perhaps a third of all sunlike stars possess Earth-like planets, says Christophe Lovis of the Geneva Observatory in Switzerland, whose team conducted the analysis.
Since 1995, astronomers have found some 300 planets outside our solar system. But most are giant balls of heated gas that revolve tightly around searing suns. Such "hot Jupiters" lack the solid orliquid real estate—not to mention the climate—needed to sustain life. In June, however, the Geneva team announced finding three presumably rocky "super-Earths" orbiting a star similar to the sun. Unfortunately, those planets, which range from four to nine times the mass of ours, whiz around their star in just four to 20 days. Their fast orbits reflect short distances between the planets and their star, meaning they're too hot to support water-based life.
Transit time. To detect distant planets—the newfound super-Earths are some 42 light-years away—ground-based astronomers infer their presence from the slight wobble each body's gravity gives the closest star. That technique, while good at finding large planets blazingly close to their suns, is not sensitive enough to spot smaller, rocky worlds in the "Goldilocks zone," where the temperature is just right for liquid water.
In contrast, Kepler's 5-foot-wide, unblinking eye can spot Earth-size planets as they pass in front of their stars—a phenomenon known as a transit. When a transit occurs, the brightness of the mother star drops by a few parts per million, just enough for Kepler's 95-megapixel camera to notice. And if Kepler discovers Earth-like planets, says Rummel, then the odds of finding life, perhaps intelligent life, could soar.
Even before the space telescope takes flight, scientists expect to learn more about the habitability of our nearest planetary neighbor. In May, the Phoenix probe touched down near the Martian north pole, the latest in a string of NASA missions to investigate whether the Red Planet ever sustained life. The little lander promptly discovered ice just inches beneath the surface, adding to the evidence that the entire northern plain of the planet is a dust-covered ice sheet. Then, in late June, the probe tested a soil sample and found "the nutrients to support life," reports Samuel Kounaves of Tufts University, who led the soil experiment. The dirt was slightly alkaline, which on Earth is good for growing asparagus and turnips. "There is nothing about it that would preclude life," Kounaves says. "In fact, it seems very friendly."
Thriving below. The lander isn't capable of actually detecting life, and in any case, cosmic rays have undoubtedly sterilized Mars's surface, says Carol Stoker, who works on the Phoenix project at the NASA Ames Research Center. But future missions may drill into the dirt. "Microbes could be getting along nicely under the surface," she says.
Recent discoveries on Earth bolster the possibility that life might thrive under the Martian soil and in other seemingly inhospitable locales. Scientists are stocking an ever-expanding zoo of "extremophiles"—single-celled organisms that survive the harshest of conditions. These microbes live in boiling heat or icy cold, in nail-dissolving acid pools or even deep underground, divorced from all sunlight. The subterranean ones, discovered in 2006 by a NASA-funded team in a South African gold mine, survive on energy derived from natural radioactivity in rock. This May, scientists from Cardiff University in Wales added to the extremophile menagerie when they found microbes teeming in sediments collected more than a mile beneath the North Atlantic seafloor. The extremophile discoveries "really expand our understanding of what is a habitable environment," says Carl Pilcher, director of the NASA Astrobiology Institute, which funds such work.
In addition to looking for microbes on Mars, NASA plans future missions that "follow the water" to other promising outposts in our solar system, namely Europa and Titan. The former, a moon of Jupiter, harbors a vast liquid-water ocean under a miles-thick carapace of ice. Titan, a satellite of Saturn shrouded in a dense atmosphere, may also harbor liquid water under its surface. "The potential for something interesting going on under the surface of Titan is probably as high as it is on Mars," says Rummel.
But, like Phoenix, all of NASA's near-future missions share a common shortcoming: They can find environments conducive to life but not detect life itself. "We're trying for a series of base hits rather than going for a big, risky home run," says Rummel.
That makes SETI the only project with grand-slam potential. Astronomer Shostak boldly predicts that SETI will hear from a real E.T. within 20 years. "A lot of my colleagues don't like that prediction. They think it's going too far," Shostak says. But he's convinced that "we're going to find out, one way or another, that biology is not a miracle."