BioEd Online

A European mission to scan the skies for signs of other worlds is set to launch next week. The six-year mission will also measure ripples from stellar cores to find out what distant stars are made of.

The French-led campaign is due to set off on 27 December from Baikonur, Kazakhstan. COROT, which stands for Convection Rotation and planetary Transits, has already been delayed twice because of problems with the Soyuz-Fregat launch vehicle, but mission staff are now confident that everything will go according to plan.

The COROT craft will be placed in a circular orbit that passes over Earth's poles, and within two months it should begin its first 150-day stare into space. During that time it will be scouring thousands of stars outside our Solar System for signs of orbiting planets (exoplanets). The plan is to spot these planets as they pass in front of — or 'transit' — their stars.

By measuring how much light is blotted out, astronomers can deduce the size of each planet. They can also work out the planet's mass by observing its effect on its parent star, allowing them to calculate the planet's density — and thus whether it is made of rock or gas.

Planetary physics

So far, 209 extrasolar planets have been found. With information gathered by COROT’s telescope, this number is set to rise rapidly. Over the next six years 120,000 stars will be scanned, says Malcolm Fridlund, a scientist on the project, which is run by the French space agency CNES, with help from the European Space Agency (ESA) and other partners in Europe and Brazil.

“COROT has the great opportunity to detect the first Earth-size planets around normal stars similar to the Sun,” says Geoffrey Marcy of the University of California, Berkeley. Marcy is arguably the most successful planet-hunter to date — his team has spotted 121 of the 209 known exoplanets. COROT’s 150-day scan time means it is most likely to spot planets with shorter, smaller orbits than Earth’s, so is unlikely to find a true Earth analogue. But it will nonetheless provide useful statistics about the number of rocky planets that are out there.

“With COROT, we will hopefully begin to map out the mass–radius relation for close-in planets in detail, which will tell us a lot of about the internal physics and composition of extrasolar planets,” says Scott Gaudi, an extrasolar scientist at Ohio State University in Columbus.

Dual objectives

Besides searching for Earth-like planets, COROT will measure ripples on the surface of the stars and study how these change the wavelength of the stars’ light. The ripples seen on a star's surface are caused by acoustical waves originating deep inside a star, so this information can give details about its inner workings. “We can retrieve information about the stellar density and temperature profile and determine how the star rotates,” says Fridlund. “COROT is a mission with two objectives, and will be doing both of them in beautiful harmony.”

COROT is the first of an international battery of planned studies of distant stars. Following closely behind is NASA’s Kepler mission, due to launch in late 2008, which will stare at the same place for four years, and may therefore spy an Earth-like planet in transit. After that are planned ESA’s Darwin mission in 2015, and NASA’s Terrestrial Planet Finder, though the funding and timing on these missions are uncertain.

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