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| Nature 424, 866 (21 August 2003); doi:10.1038/424866b |
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QUIRIN SCHIERMEIER
[MUNICH] Kevin
Costner's 1995 film Waterworld might have flopped at the box office,
but researchers think that real water worlds — Earth-sized planets
predominantly covered by oceans — are more likely than land-covered
planets to host life. Simple assumptions about the likely distribution of planets in the
Milky Way suggest that many water worlds exist in our Galaxy, but elude
existing methods of detection. "There could be as many as one billion
stellar systems with potentially habitable zones," says Siegfried
Franck, a geophysicist at the Potsdam Institute for Climate Impact
Research in Germany. To try to pin down the locations of planets that might host life,
Franck and Manfred Cuntz, an astrophyicist at the University of Texas
in Arlington, used a mathematical model to locate the 'habitable zone'
of 47 UMa, a Sun-like star some 45 light years away. The pair devised
equations coupling stellar age and luminosity, distance from the star,
and planetary climate, to determine the chance of habitable planets
existing near 47 UMa. They also calculated geodynamic constraints on
the biospheres of planets that could have formed there. (S. Franck et al. Int. J. Astrobiol. 2, 35–39; 2003). Earth-like planets in stable orbits in habitable zones are the most
likely places to harbour life. "Earth would have a slight chance of
being habitable in the 47 UMa system," says Franck, "but a water world
almost entirely covered by oceans would have a better chance."
The 47 UMa system intrigues experts because the star has roughly the
same mass, age and spectrum as the Sun. Moreover, it hosts two giant
gas planets, analogous to Jupiter and Saturn. It is thought that such
large planets help to shelter Earth from bombardment by comets and
asteroids. "Studies like this help to publicize the notion of habitable zones,"
says Jim Kasting, an atmospheric scientist at Pennsylvania State
University. But he warns that 'models of early planetary evolution are
not particularly well constrained' and may not provide a reliable
pointer to where inhabitable planets can be found. NASA plans to launch two space-based telescopes, perhaps by 2013,
dedicated to the pursuit of Earth-like planets, and to the analysis of
their atmospheric composition. "Then the whole thing will get really
exciting," says Kasting.