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.