The Corot satellite strikes again with another fascinating planet discovery. This time, the newly discovered gas giant planet may have an interior that closely resembles those of Jupiter and Saturn in our own Solar System.

Very few planets are temperate enough to allow the presence of liquid water, but the newly discovered Corot-9b is one of them. It was found on 16 May 2008 and orbits its star every 95.274 days, a little longer than Mercury takes to go round the Sun.

More than 400 exoplanets have been discovered so far and 70 of them have been found by the 'transit' method. A transit is a kind of eclipse and occurs when a celestial body passes in front of its host star and blocks some but not all of the star's light.

This temporarily dims the apparent brightness of the star and enables the planet's mass, diameter, density and temperature to be deduced. The time between similar transits gives the orbital period of the planet.

Corot-9b 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 kilometres.

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 -23C and 157C.

Corot-9b 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 Astrof�sica de Canarias, whose paper on the discovery is published in Nature this week.

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 tantalising 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? ...

earlier related report
A Temperate Exoplanet Called CoRoT-9b
Bonn, Germany (SPX) Mar 19 - CoRoT-9b, a Jupiter-sized exoplanet thats orbits its star every 95 days, is the latest discovery of the CoRoT satellite, a project in which the German Aerospace Center (DLR) is a participant. "This exoplanet stands out by virtue of its 'normality'. It is a very close approximation of the planets in our own solar system," says Professor Heike Rauer from the DLR Institute of Planetary Research in Berlin, who manages the German contribution to CoRoT.

CoRoT-9b lies far away from our Solar System, some 1500 light-years from Earth, and orbits a star in the constellation of the Serpent. From the duration of its orbit, it would appear that the distance between planet and star is roughly the same as the distance separating Mercury from our Sun.

CoRoT-9b is therefore an entirely normal planet - presumably a gaseous planet with relatively moderate temperatures anywhere from -20 to +160 Celsius, depending on whether or not it is shrouded in a highly reflective cloud layer.

The differences between its day and night sides are probably only slight. CoRoT-9b is therefore substantially different from the class of 'hot Jupiters', which orbit their central star about every three days.

A planet with a short orbital period is located very close to its star and is therefore exposed to powerful stellar radiation. It is from this that the names of the planetary classes 'hot Jupiters' and 'hot Neptunes' are derived.

Waiting for the eclipse
With this discovery of a transiting exoplanet with a long orbital period (long-period planet), CoRoT has accomplished another mission objective. The mission's first such objective was accomplished with the discovery of a rocky exoplanet, CoRoT-7b, which was announced in February last year.

CoRoT has now discovered a total of eight planets and has also tracked down a 'brown dwarf' star. With the planetary transit technique, the space telescope observes several thousand stars over a period of 150 days. Whenever the orbital path of a planet causes it to traverse the line of sight between the telescope and the central star, it darkens the image of that star slightly for several hours. CoRoT measures this reduction in brightness.

For example, a planet like Earth darkens the Sun by a factor of one ten-thousandth during such a transit, and does so just once a year. Since each star is also subject to fluctuations in its luminosity, the search for 'transit events' of this kind is a long and laborious process.

The measurements that culminated in the discovery of this new planet were carried out in the summer of 2007, during a 150-day observation period. The task was made particularly difficult by the planet's wide orbit: The greater the orbital radius of a planet, the lesser the likelihood of the orbit to carry it across the line of sight from the telescope to the star.

The discovery of CoRoT-9b has proved that the planetary transit technique is also useful for discovering exoplanets of this kind. "Once you have observed an exoplanet in transit, you can determine its radius.

This is one of the fundamental parameters of a planet, one that can only be measured directly on transiting planets. Not only that, but transiting planets also provide an opportunity to learn something about their atmospheres. This is the key to the search for Earth-like planets on which life as we know it may be possible," says Professor Rauer. The discovery of CoRoT-9b has been published in an article in the journal Nature.

On the trail of extrasolar planets
To date, we are aware of the existence of more than 400 planets outside our Solar System. Transit events can be observed in about 70 of these. With a transit measurement, it is possible to derive the orbital period, the orbital inclination and the radius of a planet.

If this method is supplemented with other observation techniques - for example, the radial speed method - the mass and therefore the density of the exoplanet can be determined. This enables scientists to distinguish between gaseous and rocky planets.

Subsequent measurements of CoRoT-9b were carried out at the Teide Observatory on Tenerife, while radial velocity measurements were carried out using the high-resolution HARPS spectrometer on the European Southern Observatory (ESO) 3.6-metre telescope in Chile.

The CoRoT mission is led by the French Space Agency (Centre National d'Etudes Spatiales; CNES) and includes researchers from ESA and other research institutes from Austria, Belgium, Brazil, Germany and Spain.