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<-   The first lessons   ->

We can already learn some interesting lessons from these discoveries.

The proportion of stars hosting at least one planet

At least 5 % of the observed stars have a planetary companion. This proportion will increase as observation techniques become more able to detect less massive planets that are more distant from their stars.

Planet-star distances

All the giant planets that have been discovered to date are much closer (up to 100 times) to their stars than Jupiter is to the Sun. It has been a big surprise, since theory predicted that a giant planet should form at a distance of at least 5 AU from the star. This disparity is now explained with the "migration" phenomenon : once a planet starts to form relatively far from its star in the protoplanetary disk, a gravitational interaction between the disk and the forming planet occurs, which slows the planet down. So the planet moves closer to its star (migrates) until the interaction stops. These planets, being very close to their stars, have a high temperature (up to 1200 K). That's why they are called "hot Jupiters".

Orbit eccentricities

Another surprise has been the observation that the majority of the planetary orbits are fairly eccentric, contrary to the almost circular orbits in the Solar System. To date, this phenomenon remains relatively unexplained. However, one plausible explanation is that if two planets migrate, they do it at different rates. Therefore they must encounter each other, and the net effect of this encounter is to evict one of the planets from the system. From the laws of celestial mechanics, we can then show that the remaining planet has an elliptical orbit.

Parent star metallicity

A last observation is that the stars, which have one or several planets, also have more heavy elements (carbon, oxygen, iron,...) than the average. This correlation is not well explained : this could be due to the fact that the star and its planets come from a molecular cloud rich in heavy elements, or to the fact that the star has been enriched in heavy elements because of the destruction of a planet.