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ESA to test the smartest technique for detecting extrasolar planets from the ground

March 26, 2002

To see a dim planet around a bright star is like looking for a candle flame next to a searchlight. To solve this problem, scientists have developed the concept of nulling interferometry, one of the smartest methods to date in the search for extrasolar planets. The European Space Agency (ESA) and the European Southern Observatory (ESO) are pooling their expertise to build a new instrument to test this innovative technique from the ground before ESA applies it in space.

Nulling interferometry combines the signal from a number of different telescopes in such a way that the light from the central star is cancelled out, leaving the much fainter planet easier to see. This is possible because light is a wave with peaks and troughs. Usually when combining light from two or more telescopes, a technique called interferometry, the peaks are lined up with one another to boost the signal. In nulling interferometry, however, the
peaks are lined up with the troughs so they cancel out to nothing and the star disappears. Planets in orbit around the star show up, however, because they are offset from the central star and their light takes different paths through the telescope system.

ESA and ESO will build a new instrument called GENIE (Ground-based European Nulling Interferometer Experiment) to perform nulling interferometry using ESO`s Very Large Telescope (VLT), a collection of four 8-metre telescopes in Chile. It will be the biggest investigation of nulling
interferometry to date. "It`s being tested in the lab in a number of places but we can do more," says Malcolm Fridlund, project scientist for the Darwin mission at the European Space Research and Technology Centre, the Netherlands. "We intend to use the world`s largest telescope and the world`s largest interferometer to get very high resolution."

Using GENIE to perfect this technique will provide invaluable information for engineers about how to build the `hub` spacecraft of the Darwin flotilla. Scheduled for launch in the middle of the next decade Darwin
is a collection of six space telescopes and two other spacecraft, which will together search for Earth-like planets around nearby stars. The hub will combine the light from the telescopes.

"If you see the way of getting to Darwin as being outlined by a number of technological milestones this is one of the most important ones," says Malcolm Fridlund.

Once up and running, GENIE will also provide a training ground for astronomers who will later use Darwin. For example, it will allow them to perfect their methods of interpreting Darwin data because, as well as the engineering tests, GENIE will be capable of real science. One of its greatest tasks will be to develop the target list of stars for Darwin to study. As recently discovered by ESA`s Ulysses spaceprobe, the signature of a planetary system is probably a ring of dust surrounding the central star. GENIE will be able to look for these dust rings and make sure that the dust is not so dense that it will mask the planets from view.

GENIE will see failed stars, known as brown dwarfs and, if the instrument performs to expectations, may also see some of the already-discovered giant planets. So far, these worlds have never been seen, only inferred to exist by the effect they have on their parent stars.

From Earth, two things handicap nulling interferometry. Firstly, the atmosphere smears out the starlight so that its cancellation is a hundred times less effective than it will be in space. Secondly, planets are most easily seen using infrared wavelengths because they are warm. So, observing from the surface of Earth, itself a planet emitting infrared radiation, is like peering through fog.

In space, these two problems disappear and Darwin will be able to see smaller, Earth-like worlds. "We have calculated that with Darwin we could see an `Earth` if it were ten light-years away with a few hours of observation time. With the VLT, it would be impossible because of the atmosphere. Even if the atmosphere weren`t there it would take 450 days because of the infrared background released by the Earth. So we have to go into space," says Fridlund.

GENIE is expected to be on-line by 2006.

European Space Agency (ESA)