18 march 2010

Cool Jupiter exoplanet found

Science

Cool Jupiter-like exoplanet CoRoT-9b

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, a team including Oxford University scientists report.

This finding, by a team of 60 astronomers, is published today in Nature.

The Jupiter-sized planet, named ‘CoRoT-9b,’ is orbiting a star similar to the Sun in the constellation Serpens Cauda, at a distance of 1500 light-years from the Earth.

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.

‘Over the past decade, densities could be measured only for hot planets orbiting very close to their host stars,’ said Dr Suzanne Aigrain of Oxford University, a co-author of the paper. ‘Their evolution is completely dominated by the huge amount of radiation they receive from their host stars. CoRoT-9b is much cooler, and provides us with a clean, isolated test of our theories of how giant planets evolve.’

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.’

Co-author Claire Moutou, from the Laboratoire d’Astrophysique de Marseille, added: ‘CoRoT-9b also has great potential for future studies concerning its physical characteristics and atmosphere.’

‘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.

Dr Aigrain leads a team of UK researchers at the Universities of Oxford, Exeter and St Andrews who participate in the CoRoT exoplanet program. Their research is supported by the Science and Technology Facilities Research Council (STFC).

A report of the research, ‘A transiting giant planet with a temperature between 250 K and 430 K’, is published in the 18 March edition of Nature.