From: Andy Soos, ENN
Published March 18, 2010 04:41 PM

Planets in Other Star Systems

An extrasolar planet, or exoplanet, is a planet outside the Solar System. There are billions of stars in our galaxy and a significant percentage of these stars are likely to have planets orbiting them. There are also planets orbiting brown dwarfs and free floating planets between the stars. As of March 2010, over 400 extrasolar planets have been confirmed.

The CoRoT satellite has discovered the coolest Jupiter like exoplanet so far to pass in front of its host star, enabling detailed studies of the planet as reported by a team from Oxford University.

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Planet search programs have discovered planets orbiting a substantial fraction of the stars they have looked at. However, the total fraction of stars with planets is uncertain because of observational statistical selection effects.

The radial velocity method and the transit method (which between them are responsible for the vast majority of detections) are most sensitive to large planets on small orbits. For that reason, many known exoplanets are "hot Jupiters": planets of roughly Jupiter like mass on very small orbits with periods of only a few days.

The newest discovered exoplanet was found by CoRoT (Convection, Rotation and planetary Transits) which is the first space mission dedicated to exoplanetary research and designed for this purpose. The spacecraft is equipped with a 27 cm diameter afocal telescope and a 4-CCD wide field camera, is built around the PROTEUS spacecraft bus, and operates in a low Earth orbit. Launched in December 2006 the mission had a nominal lifetime of 2.5 years, subsequently extended until 31 March 2013, during which time it will study stellar interiors and search for exoplanets.

The fraction of stars with smaller or more distant planets remains difficult to estimate. Extrapolation does suggest that small planets (of roughly Earth like mass) are more common than giant planets. It also appears that planets on large orbits may be more common than ones on small orbits.

The Jupiter sized planet, named "CoRoT-9b," is orbiting a star similar to the Sun in the constellation Serpens, at a distance of 1500 light years from the Earth. Planets are named in the order of discovery after the star. "b" is the first planet of this system with "c" when found being the second planet to be discovered in this system.

CoRoT-9b has a very slightly eccentric orbit similar to that of Mercury around the Sun and, whilst fairly typical of exoplanets found so far, is special in that it passes in front of its host star once per orbit. These transits can be used to measure the planet’s radius. All previously known transiting planets spend some or all of their time very close to their host star and so are much hotter.

Hans Deeg from the Instituto de Astrofisica de Canarias (IAC), lead author of the paper, said: "This is the first transiting planet with a fairly moderate temperature, between -20 and 150 degrees Celsius. It is extremely valuable because we can measure its density, which reflects its internal structure and composition."

"The planet is mostly made of Hydrogen and Helium," said Tristan Guillot from the Observatoire de la Cote d’Azur, "but it may contain up to 20 Earth masses of high-pressure ices and rocks. It is thus very similar to the Solar System’s giant planets, Jupiter and Saturn. Its density agrees well with theoretical expectations."

The transits of CoRoT-9b were identified by CoRoT following a continuous observation of 150 days in the summer of 2008. The team then used a number of instruments on the ground, including the IAC-80 telescope in Tenerife and the HARPS spectrograph on the ESO 3.6-m telescope in Chile, to confirm that CoRoT-9b was indeed a planet and to measure its mass.

A report of the research, "A Transiting Giant Planet with a Temperature Between 250 K and 430 K", is published in the March 18th edition of Nature.

For further information: http://www.ox.ac.uk/media/news_stories/2010/100318.html or http://en.wikipedia.org/wiki/Extrasolar_planet

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