Is there life on Mars or Saturn

Are there any habitable planets?

Our blue planet is something very special: It is the only celestial body in the solar system that has liquid water on its surface. And liquid water is an important requirement for the existence of life. No wonder then that the earth is also the only planet in the solar system that is overflowing with life.

“Habitable zone” or “ecosphere” is what astrobiologists call the area around a star in which the radiation is strong enough to thaw ice, but not so strong that all the water evaporates. On a planet that orbits its central star in this area, there can be liquid water. Exactly how big the ecosphere of the sun is is, however, controversial. For some researchers the earth alone, for others Venus and Mars as well, lie within the habitable area. Indeed, it is conceivable that there were seas on young Venus. And Mars, too, could have been a friendlier world with rivers and lakes in the past. Primitive life could also have originated on our two neighboring planets and maybe even - in the clouds of Venus or deep in the Martian soil - have survived to this day.

And what about other stars? Astronomers have now tracked down almost 350 planets outside of our solar system - and the number is growing steadily. First of all, due to the methods used, the researchers came across giant planets in extremely narrow orbits. But thanks to new methods and instruments, they are increasingly finding smaller planets on orbits further out. The so far - as of April 2009 - smallest planet Gliese 581e has 1.9 times the mass of the earth and orbits the star Gliese 581, 20.5 light years away, on a very narrow orbit and therefore not in the habitable zone.

Gliese 581 has three other planets, and one of them, Gliese 581d, traces its orbit 0.22 times the distance between the earth and the sun. Since Gliese 581 is a cool dwarf star, the orbit is exactly in the ecosphere. However, this planet has seven times the mass of the earth. Astronomers suspect that planets of this size have a considerably larger proportion of water than Earth-like planets - Gliese 581d could therefore be covered by an ocean hundreds of kilometers deep.

Life on moons

Ice-covered: Jupiter's moon Europa

Another example of a planet in the habitable zone around a star is HD 28185b. This celestial body has about six times the mass of Jupiter and orbits a sun-like star at a fairly precise distance from the earth to the sun. The giant planet itself is certainly not life-friendly. But in our solar system a large group of moons accompanies the great planets Jupiter and Saturn. If this is typical for such gas giants, then HD 28185b could also have a multitude of moons. And many of the great moons of Jupiter and Saturn have thick armor of frozen water on their surface. In the ecosphere this water would be liquid and the deep seas on such moons could perhaps offer friendly conditions for life to emerge.

The moons of giant planets could even offer living conditions outside of the ecosphere. The strong tidal forces of the large planet literally knead the interior of a moon and thus heat it up from the inside. So there could be oceans beneath the ice armor of the moons. There are indications of such hidden oceans in our solar system at Jupiter's moons Europa, Ganymede and Callisto, as well as at Saturn's moon Titan. So perhaps these moons are the most promising candidates to find life in our solar system.

However, all these considerations on the possible habitability of other celestial bodies are based on life forms that are similar to earthly life - that are based on liquid water as the solvent and carbon as the central building block. So far, the earth is the only living world that we know. To draw such far-reaching inferences from a single example could well prove to be a mistake. Perhaps there are living beings who use other liquids as solvents - or who can do without solvents at all. Perhaps a biology is also possible that is not based on carbon but, for example, on silicon. For such alien life forms, planets that appear completely hostile to life could of course also be habitable.