Setting SETI's Sights II: Abodes for Life?

With the latest discovery of a "Super-Earth" around a dim, red star 15 light years from Earth, SETI scientists have been pondering the implications for their search for intelligence on other worlds. "This planet answers an ancient question," said Geoffrey Marcy, professor of astronomy at the University of California, Berkeley, and leader of the team that discovered the planet, which is seven to eight times the mass of Earth. "Over 2,000 years ago, the Greek philosophers Aristotle and Epicurus argued about whether there were other Earth-like planets. Now, for the first time, we have evidence for a rocky planet around a normal star." Team member Paul Butler of the Carnegie Institution of Washington emphasized the similarity between this most recently detected planet, located around an M star called Gliese 876, and our own world. "This is the smallest extrasolar planet yet detected and the first of a new class of rocky terrestrial planets," he explained. "It's like Earth's bigger cousin."

For astronomers pondering the possibility of life outside our solar system, the discovery is especially promising due to the sheer number of M stars in our galaxy. "The overwhelming majority of stars are M dwarfs--hundreds of billions in our galaxy alone. This suggests that there could be enormous numbers of planetary habitats capable of sustaining life," said Seth Shostak, Senior Astronomer at the SETI Institute.

But the mere existence of rocky planets isn't enough to ensure the evolution of life. One critical requirement, according to Shostak, is having enough time for life to get underway and then develop into something interesting. "Unlike Sun-like stars, which burn for 10 billion years and then die, M dwarfs live much longer -- as long as 100 billion years," he noted. "So if such stellar runts can occasionally spawn life, the majority of that life will be far older than the biology of our own planet. The most ancient, and potentially most interesting life might be found in the neighborhoods of M stars."

Long-lived planets may be especially important for the evolution of life, given the devastating effects of periodic asteroid and meteor impacts. For example, many scientists believe that the massive asteroid that hit Mexico's Yucatan Peninsula 65 million years ago was responsible for the wholesale extinction of dinosaurs. That catastrophe opened the way for the proliferation of mammals on Earth, eventually resulting in humankind. But on other worlds, such chance events might have obliterated an even greater variety of complex life, perhaps effectively stopping the evolution of intelligence--at least on planets with only modest lifetimes.

Given the longevity of M stars, however, complex life on worlds circling such stars might get a second chance. "If evolution happens at a very slow pace, or if many times evolution gets started and gets truncated, because of some extinction events," explained Jill Tarter, Director of SETI Research at the SETI Institute, "planets around M stars may get more than one chance, and they may be able to accommodate a slower evolutionary mode and still end up with telescope builders."

For life to evolve on another world, time alone isn't enough. The planet must circle its star within a "habitable zone," orbiting close enough to gather the life-giving light from its star, but far enough away to avoid the scorching temperatures that would obliterate life.

Because M stars are so dim compared to stars like the Sun, an M star's habitable zone is quite close to the star itself. Simply put, the planets around M stars need to lie in orbits that circle close to the stars if they are to have any chance for gathering enough energy to bear life.

But the tight orbits that would be needed to host life around an M star come at a cost. When a planet orbits its star so closely, one side of the planet always faces the star, while the other side is always hidden. The same phenomenon, called "tidal locking," is evident when we look at the Moon, which always has its same side facing the Earth. The result? When a planet is tidally locked in orbit around its star, the temperatures on the sunny side would be scorching, while the dark side would be a frozen wasteland. "On the star-facing side, you pump a lot of energy in and heat up the atmospheric gas, and in the shadow on the other side it's dark and cold," said Tarter. As a consequence, the difference in temperatures whips up "enormous wind velocities." Or so scientists have thought until recently.

But that may all be changing. "New models indicate that perhaps you can, with greenhouse gases, get a less dramatic energy distribution--that you can in fact circulate the energy that's being put in on one side from the star without totally tearing apart the atmosphere," explained Tarter.

"The planet's mass could easily hold onto an atmosphere," noted Gregory Laughlin, speaking about the newly discovered planet around Gliese 876. "It would still be considered a rocky planet, probably with an iron core and a silicon mantle. It could even have a dense steamy water layer," said Laughlin, an assistant professor of astronomy at UC Santa Cruz and a member of the discovery team. Along with 40 other scientists, he will be attending a workshop at the SETI Institute from July 18-20, 2005, with the mandate to consider whether M stars might provide suitable conditions to sustain life on circling planets--an idea previously dismissed because of tidal locking and the intense radiation that life on closely orbiting worlds would have to endure. In Butler's view, the latest planet detection is likely to be the first of many similar discoveries. "So far we find almost no Jupiter-mass planets among the M dwarf stars we've been observing," he noted, "which suggests that, instead, there is going to be a large population of smaller mass planets." And depending on the results of next month's meeting at the SETI Institute, this may result in a much long list of target stars for the search for civilizations beyond Earth.