Transiting exoplanet discovered

By Dr David Whitehouse
BBC News Online science editor

In a crowded place, the new planet's star

German astronomers have found a third star which has a planet, somewhat larger than Jupiter, moving in front of it - as seen from Earth.

This special alignment is useful to scientists because details of the orientation of the planet's orbit allow important parameters to be determined, such as the planet's mass.

The chances of an exoplanet passing in front of its central star as seen from the Earth are small. However, because of the scientific value of such events, astronomers have been busy searching for stars that experience small dips in brightness that might be explained by these exoplanetary transits.

The new exoplanet has the shortest orbital period found so far. Being so close to its star, only 3.5 million kilometres (2.1 million miles), the hemisphere that faces the star must be extremely hot, meaning the gas-giant planet is almost certainly losing its atmosphere.

Chance alignment

So-called exoplanets - planets in star systems outside our own - come in different sizes and move in a variety of orbits at different distances from their central stars. Some are nearly circular; others quite elongated.

Some are five to 10 times more massive than the largest planet in our Solar System, Jupiter. The lightest exoplanets known are about half as massive as Saturn, i.e. about 50 times more massive than the Earth.

Last year, a list of 59 such possible cases of stars with transiting planets was announced by the Optical Gravitational Lensing Experiment (Ogle).

The latest star, Ogle-TR-3, is very similar to our Sun, with a temperature of about 5,800 Celsius (6100 Kelvin).

Although there are still uncertainties, the researchers provisionally deduce a mass for the planet of about one half of that of Jupiter. The density is found to be about one-quarter of that of water, or one-fifth of that of Jupiter, meaning that the planet is quite big for its mass.

The orbital period, 28 hours 33 minutes, is the shortest known for any exoplanet and the distance between the star and the planet is correspondingly small, only 3.5 million kilometres.

This means that the temperature on the side of the planet facing the star must be very high, of the order of 2,000 C, suggesting that the planet must be losing its atmosphere by evaporation.

The astronomers suggest that it might be possible to observe this exoplanet directly because of its comparatively strong infrared radiation. An attempt will be made soon.

As only the third exoplanet found this way, the new object confirms the impression that a considerable number of stars may possess giant planets in close orbits around them.

Since such planets cannot form so close to their parent star, they must have migrated inwards to the current orbit from a much larger, initial distance. It is not known how this happens.