COROT and the new chapter of planetary searches
European Space Agency
14 November 2006
The launch of COROT on 21 December 2006 is a long awaited event in the
quest to find planets beyond our Solar System. Searching from above the
Earth’s atmosphere, COROT - the CNES project with ESA participation -
will be the first space mission specifically dedicated to the search
COROT is expected to greatly enlarge the number of known exoplanets
during its two-year mission and provide the first detection of rocky
planets, perhaps just a few times the mass of the Earth.
“COROT could detect so many planets of this new type, together with
plenty of the old type that astronomers will be able to make
studies of them,” says Malcolm Fridlund, ESA’s Project Scientist for
This will allow astronomers to more accurately predict the number and
type of planets that will be found around other stars.
The world of astronomy changed forever on 6 October 1995, when Michel
Mayor and Didier Queloz of the Geneva Observatory announced the first
discovery of a planet around a star similar to the Sun. As well as
celebration, there was surprise because 51 Pegasi b, as the planet
became known, was half the mass of Jupiter and orbiting much closer to
its parent star than expected. Whereas Mercury orbits the Sun at 57.9
million kilometres in 88 days, 51 Pegasi b shoots around its orbit in
just 4.23 days. This indicated that the planet was just 7.8 million
kilometres from its star.
An American team led by Geoff Marcy, San Francisco State University,
Paul Butler, University of California, Berkeley, soon discovered other
planets around other stars. They too were large, Jupiter-like planets
extremely close orbits.
These planets had not been seen directly. Instead, their presence had
been inferred by the gravitational pull they exerted on their parent
star. The astronomers had measured the wobble of the star through
changes in its light, and used this data to calculate the orbit and
minimum mass of the planet.
COROT relies on a new way of detecting planets. As tens of thousands of
people witnessed on 8 June 2004, a planet moving across the face of the
star creates a noticeable silhouette. On that day, onlookers watched
black dot of Venus slip across the Sun’s bright surface.
COROT is designed to detect such transits of extrasolar planets across
the faces of their parent stars. It will monitor the brightness of
stars, looking for the slight drop in light caused by the passage of
planet. Because this relies on the chance alignment of the star and the
planet with Earth, a large number of stars must be monitored to make
certain of seeing enough events. COROT will monitor hundreds of
thousands of stars. “The first target field is towards the galactic
centre. Then the spacecraft will turn towards Orion,” says Fridlund.
COROT will be the first extrasolar planet search mission capable of
seeing the smaller, rocky worlds; although they will have to be in
orbits around their stars. COROT also opens the way for the future. Two
years later, in October 2008, NASA will launch Kepler, a space
with a 0.95 metre mirror. Kepler works the same way as COROT, looking
for planetary transits, and is expected to find the first Earth-sized
planets in similar orbits to our world.
Following Kepler, a new technique will be needed. The problem is that
the larger the telescope’s mirror, the smaller its field of view
becomes. So building a larger telescope to reach fainter stars means
restricting the area of sky it looks at. Although seeing fainter stars
brings gains, the field of view shrinks, leaving fewer stars in total
Claude Catala, Observatoire de Paris-Meudon, amongst others has
a method that may solve this problem. Instead of a larger space
telescope with a smaller field of view, the new proposal uses hundreds
of 10-cm telescopes in parallel.
Each telescope is smaller than most amateur astronomers use on Earth
each has a wide field of view, 30 degrees across. That’s about 60 times
the width of the full Moon. Because they are so small, each tiny
telescope is incapable of collecting enough light on its own to make a
useable image. However, a computer on the proposed spacecraft would
combine the faint images recorded by each tiny telescope. This would
give enough information to detect transits. Thus, the future of
detecting planetary transits may be to launch a spacecraft that
hundreds of mini-telescopes.
After this, the next big leap in planetary searches is likely to be the
isolation of reflected light from a planet. This would allow the
planet’s atmosphere to be chemically analysed. In the case of an
Earth-like world, the analysis could reveal signs of life. ESA is
currently developing the necessary technology to make such a mission
possible. They are developing it under the name of Darwin, to be
possibly launched after 2020.
For more information
Malcolm Fridlund, ESA COROT Project Scientist
Email: Malcolm.fridlund @ esa.int