From gas giants to Super Earths

Hardly 15 years ago nobody was capable of establishing the empirical truth of the existence of planets beyond the solar system. Each astronomer had their own theories and the most diverse hypotheses learnt to co-exist with each other. We had to wait until 1995 before a Swiss team, led by Professor Michel Mayor, indirectly discovered the first exoplanet. The method used is that of radial velocity, in other words measuring the oscillation of a star under the influence of a planet orbiting around it (read also the article An exoplanetary drama: a planet collapses on its star). Whilst it does not enable the planet to be observed directly, it is nonetheless with the aid of this method that most exoplanets have been discovered. The reason why an indirect method needed to be fallen back on is a question of light intensity. A planet, producing hardly any light but only reflecting that of its star, by definition a lot more luminous, is thus much more difficult to observe. It was necessary to develop indirect techniques which enable the presence of a planet to be revealed.

The influence of a planet on its star will obviously depend on its mass and its distance from it. The more massive it is and the closer its orbit takes it to its star, the larger will the latter’s oscillation be, which makes it detectable, and that is the reason why most of the planets discovered to date are gas giants. In 15 years the techniques have been refined, as much for measurements of the variation of radial velocity as for other detection techniques, both direct and indirect. The astrophysicists have progressively discovered smaller planets which orbit at further distances from their stars. Besides the gas giants the planetary bestiary now counts amongst its ranks planets which are hardly a few terrestrial masses in size and are known as ‘Super-Earths.’ ‘We are beginning to discover real analogues of Jupiter,’ rejoices Michaël Gillon. ‘There is more than the refinement of our techniques which explains that we are now discovering planets which orbit at greater distances from their star. But a star sometimes needs to be observed for several years before we are able to deduct the orbit of its planet or planets on the basis of radial velocities. And we have to place things in their context: we discovered the first exoplanet hardly fifteen years ago.’