Since the first discovery of a planet orbiting a star similar to our Sun (51 Pegasus) by Michel Mayor and Didier Queloz in 1995 at the Observatory of Haute-Provence, the search for “new worlds” has exploded in Europe and the United States. The catalog of these extrasolar planets, regularly updated, now numbers in the thousands. Several exoplanets can indeed orbit around a star (or several), as is the case in our solar system.
How were all these planets discovered?
Since most of them are invisible to our current telescopes, their presence is detected only indirectly. This can be either by measuring the periodic small perturbations of the speed of the star induced by the planet which orbits around it, method used on the ground by very precise spectrographs like the High Accuracy Radial velocity Planet Searcher (HARPS), or by a tiny variation of the brightness of the star when a planet passes before it (method of transits). This requires high precision photometry achieved in space by satellites.
These two methods have respectively allowed two thirds and a third of the discoveries made to date. Only a very small number of planets (1 to 2%) have been identified by gravitational lens or direct imaging techniques, such as around the famous beta-Pictoris star.
Methods of velocities or transits, which favor the discovery of massive planets close to their star, have detected planets around about one out of five stars. For its part, the gravitational lens method detects planets far from their star. The extrapolation of all these discoveries suggests that there are more planets than stars in our galaxy, the Milky Way. Their number could therefore exceed 200 billion!
How many of them are habitable and inhabited?
This is the big question. Most of the exoplanets inventoried over the past two decades are gas giants like Jupiter (300 times the mass of the Earth), easier to detect because of their size, but totally inhospitable. To find traces of life, astronomers are actively looking for rocky planets, much smaller and therefore much less convenient to highlight.
The famed “habitable” zone
So far 60 “super-earths”, in other words planets whose mass is up to ten times that of our good old Earth, have been identified. But the distance that separates them from their star is very variable. However, for a planet to be conducive to the emergence of life, it must be able to shelter liquid water, so that its surface temperature must be between 0 and 100 °C. This constraint defines a “habitable” zone, which for a star of the brightness of the Sun, corresponds to the Earth-Sun distance of plus or minus 15%.
But the planets discovered so far are very different from those of our Solar System: their orbits, often very eccentric, are not always circular. As a result, the distance to their star (and their temperature) varies a lot. A certain number of exoplanets have even been detected around a binary star system, like the famous Tatooine, whose name refers to the Star Wars film epic. Binary star systems are very numerous in the Galaxy.
A recent statistical study of the currently known planets revealed that the super-earths located in the habitable zone are very frequent (40%) around the red dwarf stars, which represent 80% of the stars of the Milky Way. The number of potentially habitable super-earths would thus be in the tens of billions in the Galaxy. There would be a hundred in the vicinity of the Sun, less than 30 light-years away from us!
In the absence of a real knowledge of the mechanism of emergence of life and especially of its probability of appearance in the universe, this “astronomical” number of planets is not enough, however, to conclude, with certainty, that there are other forms of life other than ours elsewhere in the universe.