Another mighty planet hunter has bitten the dust, drawing to a close the first era of space-based planet spotting. The COROT mission, which counts among its haul the first known rocky exoplanet, has been officially terminated following an on-board computer failure.
With the likely end of NASA's Kepler mission in May, exoplanet searches will now be done solely by ground-based telescopes until the launch of new planet-spotting spacecraft in 2017.
In November last year a fault in COROT's last remaining computer meant that it stopped receiving data from its 27-centimetre telescope. The satellite remained in contact with Earth, so mission managers had hoped they could bring it back online.
"All the trials have not been successful, so now we have decided to stop the investigation," says Olivier La Marle of French space agency CNES, which operated COROT.
The telescope lasted twice the length of its original three-year mission, so it has definitely been a success, say La Marle. Since its launch in 2006, COROT has discovered more than 30 confirmed planets and about 100 planet candidates. Even so, exoplanet hunters are sad to see it go, says project scientist Malcolm Fridlund of the European Space Agency (ESA). "You're always disappointed. You learn so much from a mission like COROT that you always want to extend it."
COROT is in orbit around Earth, but the expense of sending astronauts on a Hubble-style repair mission would far exceed the cost of launching a new telescope. Instead, the spacecraft will slowly descend and burn up in Earth's atmosphere.
Having planet-hunters both on the ground and in space is crucial for building a more complete picture of exoplanets.
Space-based telescopes can see fainter stars more easily, which is why both Kepler and COROT were good at finding the faint dips in starlight as planets transit, or cross in front of their host stars. Ground-based telescopes, meanwhile, are better at seeing the way stars wobble as they are tugged by the gravity of orbiting planets. Transit measurements give us a planet's size, while wobbles reveal its mass. Together that tells us the planet's density and therefore its likely composition.
Both ESA and NASA are planning to launch their next-generation exoplanet missions in 2017. ESA's Characterising Exoplanets Satellite, or Cheops, will scan known worlds for signs of habitability. Meanwhile, NASA's Transiting Exoplanet Survey Satellite (TESS) will hunt for more star-crossing worlds, with a particular focus on small, rocky planets around nearby stars.
Until these telescopes launch, astronomers will keep busy sifting through the host of archived data from COROT and Kepler. The missions have also inspired a number of new ground-based exoplanet observatories to spring up, says La Marle.
"There is a lot more to be done," says Fridlund. "It is a new field – barely 20 years old. We're now starting to recognise there is an enormous diversity of planets out there to be explored."
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