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OUR galaxy may contain hundreds of thousands of life-bearing planets,
according to a new estimate. This could make the nearest inhabited twin of
planet Earth as close as a few hundred light years away.

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鈥檚 team first calculated how many planets lie in a
star鈥檚 habitable zone鈥攖he region where the temperature allows
photosynthesis. They arrived at a figure of half a million 鈥淕aias鈥�, as they call
extrasolar terrestrial planets with a globally acting biosphere. 鈥淏ecause 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,鈥�
says Franck.

Their estimate is also conservative in other respects. 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. 鈥淪ome people believe this
factor to be 100 per cent,鈥� says Franck.

Although many variables remain utterly uncertain, Franck鈥檚 work is useful in
setting an upper bound on the number of Gaias, says Alan Boss of the Carnegie
Institution of Washington, DC. 鈥淚f 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 10.鈥�

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

Other astronomers think the conclusions of Franck鈥檚 team are premature. 鈥淲e
don鈥檛 know enough to predict habitability,鈥� 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鈥檚 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. 鈥淭hat may actually happen within the next several years if NASA decides to
fly the Kepler mission,鈥� says William Cochran of the University of Texas in
Austin. Kepler should reveal how common terrestrial planets are, and produce
statistical data on their orbits and sizes.