I read somewhere that 10 percent of the stars have one or more exoplanets. What is the probability that there is some form of life on those planets?

Answer

We know too little about the origin of life to make an accurate calculation of probabilities. Today it can’t be much more than wet finger work.

An assumption that seems pretty solid, and one usually makes, is that a necessary condition for life is the presence of liquid water. That defines a ‘habitable zone’ in which the planet must be in relation to the star: not too close, because then water is only present in the form of vapor, not too far, because then it is all ice. It must therefore be a solid planet like Earth, and not a gas planet like Jupiter, because there is no liquid water on the planet within the habitable zone (but possibly on one of its possible satellites).

Why water? Because it is clear that with us life originated in the sea. Also because a liquid medium makes it possible for molecules to be mobile and thus to form increasingly complex connections with other molecules. And also because water shields the chemistry that happens in it from harmful UV radiation from the star. Finally, water is also very useful for keeping temperature fluctuations within limits.

The exoplanets that are found are mostly giant planets, and therefore gas planets. This does not mean that there are no terrestrial planets in those systems, just that they are harder to find. Indeed, the detectability of planets is always greater if their mass is greater. It turns out (and that was surprising) that many of those giant planets are quite close to their stars, which suggests (but even there is no certainty) that there is little chance of finding terrestrial planets in the habitable zone. The search continues and becomes more and more accurate, so that it appears that in the foreseeable future we will be able to make reasonably meaningful statistical statements about the occurrence of solid planets within the habitable zone.

Today (since yesterday actually) we know of an exoplanet that is pretty clear to be solid and located within its star’s habitable zone. It will have about six times as much mass as the Earth. Furthermore, within our own solar system we know a solid planet within the habitable zone, and on that planet is life. Still rather meager to do statistics. Especially that it is difficult for us to include the Earth in the statistics, because there is some ‘distortion’ there: we can only live on an Earth if there is life there!