SEATTLE, WASHINGTON--It's a question almost as old as humanity: How unique is Earth? Astronomers don't know the answer yet, but they suspect there may be many Earth-like planets out there. The first detection of a habitable planet circling another star is probably just a matter of time, they say. And that neighborhood might be very different from our own solar system, according to astronomers here at the 209th meeting of the American Astronomical Society (AAS). "It's an exciting time," says Wesley Traub, chief scientist of NASA's Navigator Program that aims to find and characterize worlds where life could thrive. "This is no longer the stuff of supermarket tabloids."
Planet hunting has become a big industry in astronomy. At the AAS meeting, a number of sessions, poster presentations, and press conferences were devoted to exoplanetary systems. Yesterday, for example, graduate student Ellyn Baines of Georgia State University in Atlanta presented the first direct measurement of an exoplanet's diameter: a giant 185,000 kilometers across, making it 30 percent larger than Jupiter.
However, most planetary systems found to date around sun-like stars aren't much like our own. Their gas giants have migrated into extremely small orbits, ending up as so-called "hot Jupiters" (ScienceNOW, 8 September 2006), while most other planets move in egg-shaped paths, unfit for life because of the huge temperature swings.
That's why astronomers have broadened their search to include red dwarf stars--the most common stars in the Milky Way. A recently launched European satellite named COROT (Science, 15 December 2006) may be the first one to find red dwarf satellites, which--in order to have liquid water on their surface--would need to orbit close to their relatively cool mother stars.
Studies led by astronomer Edward Guinan of Villanova University in Pennsylvania show that red dwarfs are indeed suitable as hosts for habitable planets. But according to Guinan, who presented the theoretical results on Sunday, such close-in planets always keep one side to their star due to tidal effects and would therefore need relatively thick atmospheres to distribute heat evenly across the globe. Moreover, he says, they would need a strong magnetic field to protect living organisms from the energetic ultraviolet radiation and x-rays known to be produced by red dwarf stars.
Even in the weird systems of sun-like stars with hot Jupiters, Earth analogs may be common, says theoretical astrophysicist Avi Mandell of Pennsylvania State University in State College. Mandell and his colleagues ran computer simulations of the formation of Earth-like planets under the influence of migrating giants. They found that a migrator leaves enough dust in its wake for the subsequent formation of a rocky ball like Earth. Moreover, says Mandell, after giant planets migrate inward, there is no longer a gravitational barrier blocking ice-rich comets from the outer regions of the disk. Such comets could collide with habitable zone-planets, supplying them with water.
Still, detecting the signatures of life on any of these worlds will not be possible until the early 2020s, when NASA's proposed Terrestrial Planet Finder (TPF) is scheduled to launch. "If TPF would see oxygen and nitrous oxide in a planet's atmosphere, that would be a very strong indication of life," says Traub. Meanwhile, although Earth may not be unique, the lay-out of our solar system, with its neat circular orbits and its outlying giant planets, has not been seen anywhere else so far.