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figures/trajectoire.gif
Example of an elliptical orbit of an exoplanet (in red). The blue curve denotes the orbit of the Earth around the Sun, i.e. an orbit with a radius of 1 AU (astronomical unit).
Copyright : Paris Observatory / UFE.

Exoplanets do not look like the Earth :

...at least, those planets discovered until now!...

  • The first exoplanets that were discovered orbit a pulsar, a "dead" star that turned into a radiowave emitting "lighthouse" at the final stage of its evolution. It is difficult to know what these planets look like. Maybe they have been vitrified by the star explosion. They could also have formed after the explosion of the star.
  • A lot of exoplanets are very massive, made of gas, and very close to their host star (much closer than Mercury is to the Sun).
  • Most of the exoplanets have elongated orbits (eccentric orbits) : they alternately move away and get closer to the star, whereas the Earth remains at an almost constant distance from the Sun.

...at least, for the most part!...

In 2007 for the first time a planet was discovered that could very well ressemble Earth. A system of three exoplanets had been detected with the "European Southern Observatory" around a faint star named "Gliese 581", which is located at a distance of 20.5 light-years in the constellation Libra. The smallest one of the three planets, called "Gliese 581c", has only 1.5 times the diameter of the Earth and a mass that is five times larger. It is possible that this object is a telluric planet, situated inside the "habitable zone" of the system.