Other Earths

In 1961, the American scientist Frank Drake suggested a simple formula for calculating the number of technologically advanced civilisations in the galaxy. But the Drake equation contains a number of parameters that are very difficult to estimate, such as the number of Earth-like planets around other stars and the percentage of these planets that are likely to evolve life.

Now Siegfried Franck and a team of German climate researchers from the Potsdam Institute for Climate Impact Research near Berlin have set new limits on these parameters. Franck's team calculated how many planets lie in a star's habitable zone--the region where the temperature allows photosynthesis. And they hit on a figure of half a million "Gaias", as they call extrasolar terrestrial planets with a globally acting biosphere "Because we also allowed for the fact that the habitable zone of any star will migrate and shrink as the star evolves, our number is much lower than earlier estimates," says Franck.

Even so their estimate might still be conservative. Based on theoretical arguments, the team assumed that only one per cent of all stars in the Milky Way is accompanied by Earth-like planets. They also assumed that life will form and evolve on only one per cent of all habitable planets. "Some people believe this factor to be hundred per cent," says Franck.

Although many variables remain utterly uncertain, Franck's work is useful in setting an upper bound on the number of Gaias, says Alan Boss of the Carnegie Institution of Washington, DC. "If Earths turn out to be commonplace, then they will not have overestimated the number of life-bearing planets greatly, but if Earths turn out to be rarer, then their estimate may be too high by several powers of ten."

But other researchers are sceptical. "It is a very good paper," says Donald Brownlee, an astronomer at the University of Washington in Seattle, "but we have no way to know if the estimate is good or not. We have no data."

Other astronomers think the conclusions of Franck's team are premature. "We don't know enough to predict habitability," says planet hunter Geoffrey Marcy of the University of California in Berkeley. And there are other unknowns Franck has not taken into account. For instance, as Brownlee and others have suggested, a star's position in the Milky Way may play an important role in the development and sustenance of Earth-like planets and life.

The only way to find out is to observe extrasolar Earth-like planets for real. "That may actually happen within the next several years if NASA decides to fly the Kepler mission," says William Cochran of the University of Texas in Austin. Kepler should reveal how common terrestrial planets really are, and provide astronomers with statistical data on their orbits and sizes.

© Govert Schilling

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