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See Inside December 2010

Black Plants and Twilight Zones: New Evidence Prompts Rethinking of Extraterrestrial Life

Discoveries of distant planets are challenging theorists to think deeply about extraterrestrial life



Ron Miller

Astronomers have long searched for a planet that could harbor life outside our solar system. When reports came in earlier this fall of the not too hot, not too cold exoplanet Gliese 581g, it was like the answer to a dream. “If it’s confirmed, I think it’s definitely the planet we’ve been waiting for, for a long time,” says Rory Barnes, an astrobiologist at the University of Washington who wasn’t involved in the research.

The wait may continue for a while. Soon after University of California, Santa Cruz, astronomer Steven Vogt and his collaborators reported the “Goldilocks” exoplanet, a rival Swiss group said it could not find evidence for Gliese 581g in its own data set. Confirming the new find, based on 11 years of subtle and indirect telescope-based measurements, could require several more years.

The tantalizing data, though, have already galvanized astronomers to step up their research on the conditions necessary for extraterrestrial life. The possibility that Gliese 581g may exist, they say, has added a new urgency for more sophisticated supercomputer models of life on other Earth-size planets.

Scientists, theoretical astrophysicists among them, combine astronomical observations with what they know about life on Earth to build simulations of exoplanet environments. Amid a recent surge of detected planets, realistic models could provide critical guidance for future missions seeking out signs of life in the universe. Recently Gliese 581g has become a focal point for this research. Its nearly circular orbit around a red dwarf star would position it at the optimal distance for temperatures permitting liquid water on the surface—an essential feature for life. The red dwarf, though, emits only 1 percent of the light from our sun. Photosynthetic organisms on the planet would likely absorb as much of the weaker starlight as possible, making them appear black, according to modeling by Nancy Kiang of the NASA Goddard Institute for Space Studies in New York City and collaborators at the University of Washington–based Virtual Planetary Laboratory.

Preliminary calculations also support the idea that one side of Gliese 581g always faces its star and roasts in temperatures up to 64 degrees Celsius, whereas the planet’s dark side sees relentless North Pole–like winters. This positioning, still a matter of debate, might leave a more livable zone awash in a “perpetual sunset,” as Vogt calls it. If such a hypothesis proves correct, Kiang says the specific wavelengths of light reaching each longitude could even prompt a rainbowlike gradient of plant colors with pigments adapted to absorb the light streaming across the surface.

Beyond energizing theorists, Gliese 581g has whet astronomers’ appetites for what many expect to be hundreds of similar discoveries outside our solar system. “Either we’ve been very lucky and we won’t find another one again for a long time,” Vogt says, “or there’s a lot of them out there.”   

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