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Long odds on a long shadow

Astronomers take a gamble on Earth-like planet.

Astronomers may make a major step forward in studying planets around other stars as early as next week — but the odds are strongly against them.

Last week's announcement by Stéphane Udry and his colleagues at the Geneva Observatory that they had found a planet in the 'habitable zone' of the red dwarf Gliese 581 caused a great deal of excitement (S. Udry et al. Astron. Astrophys.; in the press). Its low mass (about five times that of Earth) and its position in the habitable zone, where temperatures are compatible with water being present as a liquid, mean that Gliese 581 c, as the planet is called, could be more Earth-like than anything else yet seen beyond the Solar System.

Planets in the habitable zones of red dwarfs could be strange new Earths. Credit: L. R. COOK

However, finding out whether Gliese 581 c is a larger version of Earth or a smaller version of Neptune — a much less homely prospect — means measuring its density. The only way this can be done is by observing the planet cross the face of Gliese 581 and thus working out its radius. Unfortunately, such a transit will be visible from our Solar System only if we happen to be sitting in the plane of the planet's orbit. The chances of that being the case are a daunting 50 to 1 against.

Nevertheless, any odds are worth taking when it's the only game in town. On 26 April, just three days after the planet's existence was revealed, Dimitar Sasselov of Harvard University began using the small Canadian space telescope MOST (Microvariability and Oscillations of Stars) to observe Gliese 581. Sasselov predicts that, if the geometry is right, his team should see the star dim on 7 May as the planet passes in front of it.

“If Gliese 581 c transits then the doors are open” says David Charbonneau, another Harvard exoplanet hunter. A transit would not only supply data about density, it would open up the possibility of follow-on observations that might reveal clues to the contents of the planet's atmosphere. A definitive failure to see a transit, on the other hand, would close off almost all lines of future observation with current technologies. An exception is listening out for evidence of artificial radio transmissions. The SETI Institute of Mountain View, California, kept an ear out for signals from Gliese 581 in 1995 and 1997. “We didn't find any signal,” says Seth Shostak, a SETI Institute astronomer. But the institute's next project — the Allen telescope — will nevertheless make looking at the Gliese 581 system a priority.

Without transits, the wild card of SETI is the only way forward on this particular planet. But Charbonneau is still uplifted by the discovery. He draws the inference from Gliese 581 c that there are more such low-mass planets around dwarf stars, and indeed Udry says he already has candidates for which he needs just a bit more data. It's not unreasonable to hope that within a few years enough data will have been found to make it likely that at least one will be observable in transit.

“One day we'll be lucky,” Sasselov agrees. “And hopefully,” he adds, “it'll be next week.”


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Sanderson, K. Long odds on a long shadow. Nature 447, 7 (2007).

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