July 11, 2011

2009: discovery of CoRoT-7 b

2009 DISCOVERY OF CoRoT-7 b Update: 07/11/2011
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CoRoT-7 b artist view © ESO/L. Calçada


Since 1995 and the first exoplanet discovered, more than 500 exoplanets followed, among which more than 100 using the transit method.

In February 3rd, 2009, the CoRoT scientific team announced the first probably telluric exoplanet.

The transit method offers a huge benefit providing the diameter of the planets. The star's mass is assessed thanks to the radial speed measurements carried out from the ground.

The planet's density can be thus deduced and then it is possible to identify constraints on the planet's structure (rocky, liquid, gaseous, etc.) and composition establishing a pattern and using analogies. It is impossible to go this far in the exoplanet characterization if it does not transit.

Before CoRoT-7b, the only transiting exoplanets discovered (around 70 planets) were gaseous giant planets. The major achievement of CoRoT is to be the first to discover a transiting Super-Earth: the received light emissions only dropped by 0.03%!

CoRoT-7b is located in the Unicorn constellation, some 46 light years away from Earth. Its host star (CoRoT-7) is a sunlike active star. It had been continuously observed by CoRoT during 140 days in 2008.



Filtering the signal, periodic spurious variations of the star's luminosity, principally resulting from the spots on its surface due to its rotation, could be filtered out.

By studying the three "colours" provided by CoRoT combined with the observations from the ground, the scientists were able to eliminate the possibilities of false detections such as a possible companion star or a binary system in the background.

Finally, radial speed measurements performed in particular with one of the best instruments in this field, HARPS at the VLT, confirmed the presence of a planet called CoRoT-7b and showed the existence of another planet, named CoRoT-7c (which does not transit).

Analysing those data, CoRoT-7b appears to be approximately 1.7 times larger and 5 times more massive than Earth. Its density, between 4 and 8g/cm3, is similar to the Earth's. Since CoRoT-7b is very close from its host star (less than 0.02 AU), its rotation is very likely to be synchronized with its revolution because of the tidal effects. The temperature on its day-face, constantly illuminated, can reach 2,500ºC, whereas it is freezing on its night-face at -240ºC.

Those extreme conditions can be a source of hypothesis about the planet's structure and composition. On one hand, an atmosphere made of hydrogen and helium, like on many other planets, is very unlikely to exist because of the efficient erosion triggered by the UV flux and because of the age of its star - between 1.2 and 2.3 billion years old.

On the other hand, an atmosphere made of aerosol silicates is more likely to exist. The planet could present a rocky structure, principally made of silicates, iron and may be water. CoRoT-7b may have experienced an Earth-like origin (material accretion in a proto-planetary disk) or an origin like a giant icy planet such as Uranus which could have migrated to a closer orbit, losing little by little its gaseous envelope, eroded by the flux of its star.

To know more about CoRoT-7b, a direct characterization of its atmosphere will be necessary: still impossible with the current instruments, it may become reality in the future.

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