Water On Jupiter-Like Corot-9b?

The newly discovered gas giant Corot-9b may have an interior that closely resembles those of Jupiter and Saturn in our own Solar System, according to a new paper published today in Nature.

Some evidence also suggest that the exoplanet, discovered last Spring, may also be temperate enough to allow the presence of liquid water.

Corot-9b orbits its star every 95.274 days, a little longer than Mercury takes to go round the Sun. It is the first transiting planet to have both a longer period and a near-circular orbit. Its orbit is slightly elliptical but at closest approach to its parent star it reaches a distance of 54 million kilometers.

Although that is only about the distance of Mercury in our Solar System, it is by far the largest orbit of any transiting planet found so far. Because it orbits a star cooler than our Sun, calculations estimate that Corot-9b's temperature could lie somewhere between -23°C and 157°C.



This is an artist's impression of an exoplanet around a star.

(Photo Credit: ESA (Illustration by AOES Medialab))

The new planet has a radius around 1.05 times that of Jupiter but only 84% of the mass. This leads to a density of 0.90 g/cc, or 68% that of Jupiter. "Corot-9b is the first exoplanet that is definitely similar to a planet in our Solar System," says Hans Deeg, a researcher at the Instituto de Astrofísica de Canarias.

The similarity is caused by the fact that Corot-9b is sufficiently far from its star to prevent tidal forces from heating its interior. Tidal forces are created by the strength of gravity weakening from the front to back of the celestial body. When the difference between the near side and the far side is great, the tidal force can prevent the planet from spinning quickly, forcing it to only show one face to the star. It can also provide heat to the interior of the planet, changing its physical condition.

Based on calculations, neither of these is possible in this case. "Although we don't know, because we can't see the planet directly, there is reason to believe that this planet has a normal day-night cycle," says Malcolm Fridlund, ESA Project Scientist for Corot. It means that lacking a tidal heat source, Corot-9b's interior is likely to have remained similar to the gas giants in our Solar System.

There is also one other tantalizing possibility about this world. Although the planet itself is a gas giant and hence has no solid surface to stand on, what if it possessed a moon like Saturn's Titan? If the temperature were towards the lower end of the estimated range, then any moon would be an ice ball. If it were towards the upper end, it would be rather too hot for liquid water. But what if it were somewhere in the middle?

Citation: Deeg et al., A transiting giant planet with a temperature between 250 K and 430 K', Nature, March 2010, 464, 384-387; doi:10.1038/nature08856