Rocky planets not much bigger than Earth could be detected by a space telescope called COROT set to launch on 27 December. The mission is expected to provide a better understanding of planets smaller than Saturn, of which only a small number of examples are known so far.

The vast majority of the more than 200 extrasolar planets found to date have been detected from the ground by watching for the slight gravitational tug they exert on their parent stars, called the radial velocity technique.

Most of these planets are similar in mass to Jupiter or even heavier, because these 'gas giants' are the easiest to detect. But the new telescope, called COnvection ROtation and planetary Transits (COROT), will be able to detect much smaller planets.

The satellite will use its 27-centimetre telescope to search for dips of light due to planets passing in front of their parent stars in events called transits. It will monitor different patches of the sky that each span the width of about six Full Moons, staring at each for 150 days at a time. Watch an animation of the COROT mission.

The mission is capable of detecting tiny drops in light of only 300 parts per million, which is good enough to detect planets as small as two or three times the size of Earth, says COROT team member Marc Ollivier of the Université Paris-Sud in Orsay, France.

Planet stats

Planets this small have too little gravity to collect much gas during their formation, so are expected to be largely composed of rock. They would have about five to 10 times the mass of Earth, Ollivier says. Astronomers refer to these relatively small, rocky objects as 'telluric' planets.

Astronomers estimate that about 10 to 20% of Sun-like stars are orbited by giant planets, Ollivier says.

But only a handful of relatively small planets have been found, so it is not clear how common they are (see 'Naked super-Earth' revealed by microlensing). COROT will help pin down the proportion of stars orbited by the small, rocky worlds. "We'll have information about the formation rate of such objects," Ollivier told New Scientist.

COROT will need to observe two transits of a given planet to be sure it is real, Ollivier says, and three transits to pin down its orbital period. This means the planets detected by COROT will be very hot and closer to their parent stars than Mercury is to the Sun, he says.

COROT will be investigating unexplored territory, so it might also find planets unlike anything that has been imagined. "We're ready to see unexpected things," Ollivier says.

False alarm

In addition to searching for planets, COROT will study giant sound waves that cause stars like the Sun to vibrate. By looking for the characteristic brightness variations that the waves produce, scientists will be able to work out the details of stellar structure.

The COROT mission is led by France's Centre National d'Etudes Spatiales (CNES) with participation from the European Space Agency (ESA). The spacecraft is set to launch on 27 December at 1423 GMT from the Baikonur cosmodrome in Kazakhstan. It will be put in a circular orbit 900 kilometres above Earth.

The launch was originally scheduled for 21 December, but was delayed because of a suspected fuel leak in the upper stage of its Soyuz launch rocket. It turned out to be a false alarm caused by a glitch in the system that monitors the rocket's vital signs from the ground.

COROT will start its scientific observing campaign around the end of January 2007, after mission managers have tested its instruments to make sure everything is working properly. The entire mission is scheduled to last 2.5 years.

NASA is launching a similar mission called Kepler in 2008, which will be able to find transiting planets as small as Earth.