Page last updated at 17:47 GMT, Tuesday, 17 February 2009

UK 'eyes' to hunt for other Earths

By Irene Klotz
Titusville, Florida

The British CCDs have become a leading choice for space missions

The telescope Nasa is preparing for launch next month won't reveal if there is intelligent life in the Universe, but it should at least provide concrete evidence that there are places like Earth for ET to live.

"This mission has been designed very carefully, such that a null result is as important as finding planets," said Michael Bicay, director of science at the US space agency's Ames Research Center in California.

"This will not only have scientific relevance, but philosophical ramifications."

Bicay was speaking at the Astrotech Space Operations plant in Titusville, Florida, where the telescope, named Kepler, is being prepared for launch.

Lift-off aboard an unmanned Delta rocket from the nearby Cape Canaveral Air Force Station is planned for 5 March.

Once Kepler is in orbit, it will have one primary task: pointing to a fist-sized section of the Milky Way Galaxy and keeping still so that its light-collecting electronic devices, known as CCDs, can do their job.

British technology

The custom-made detectors were produced for Nasa by the UK-based firm e2v Technologies.

Kepler has 21 pairs of CCDs for science, plus four in the corners that will be used to keep the observatory properly fixed on its target.

Kepler (Nasa)
Will study more than 100,000 suns
Continuously for 4 to 6+ years
Tuned to see Earth-size planets
Will target the habitable zone
Also see Mars to Jupiter sizes
First results within 3 months

Each pair covers five square degrees of the sky, yielding an array with a view encompassing more than 100 square degrees, or about as much sky as the size of a human hand held at arm's length.

The Hubble Space Telescope, by comparison, can view the amount of sky equal to a grain of sand held at arm's length, and do so for only about 30 minutes at a time.

"These devices were especially challenging, given what was wanted for the telescope and the time in which they wanted them," David Morris, e2v's lead engineer on the project told BBC News.

The CCDs needed to be several orders of magnitude more sensitive than the human eye, and efficiently function in the harsh environment of space. But Morris said the greatest difficulties were in designing the detectors to fit the telescope's large focal plane and to have them integrate seamlessly with its mechanical devices.

The CCDs, for example, are in the front of the telescope, where temperatures will be about minus 85C. Just behind are the electronics that take the signals off the silicon CCDs and temperatures there will be about plus 30C.

"This was a very difficult structure to produce because you don't want it to warp with temperature changes," said John Troeltzsch, the programme manager with Kepler prime contractor, Ball Aerospace.

Benign conditions

Kepler is designed to stay on target continuously, breaking its gaze only to re-position its solar arrays and radio its data to Earth about every three months.

Ground-based computers will comb the signals for any slight, periodic changes in the target stars' light which may be caused by planets temporarily blocking a bit of glare as they orbit in front of their parent stars.

Finding a planet with a similar size and orbit as Earth will take several years.

Mercury transiting the Sun
When planets transit their star, they block out light - like Mercury above

"The more transits you see, the more certain you are that you've got a real signal," said Bicay. "We may get false positives just from the noise in our electronics."

Kepler must monitor thousands of stars simultaneously to improve the odds of catching a planet in transit.

"The chances of observing the dimming effects of a transiting Earth-size planet against the disc of its parent star are about 1 in 200, based on the random orientation of the plane of the orbiting planet with respect to our line of sight," Bicay told BBC News.

"Hence, from a well-selected sample of 100,000 stars, we can anticipate discovering on the order of 500 terrestrial-size planets in orbit around other stars - assuming they exist!"

The target stars are, on average, about 3,000 light-years from Earth.

Scientists have already discovered planets around three of them, but they are large worlds, like most of the extra-solar planets discovered so far, that orbit very close to their parent stars.

Statistical quest

Conditions on these worlds, dubbed "hot Jupiters", are believed to be inhospitable for life.

Kepler is after planets in more habitable zones, places where water could exist in liquid form.

Kepler (Nasa)
The telescope is ready to go

Any prospective targets will be studied by ground-based telescopes for verification.

"There are several astrophysical phenomena that will masquerade as transiting planets and we have to sort those out from the ground," said project scientist Nick Gautier.

The goal, added Kepler project manager Jim Fanson, is to develop statistics about what types of planets form and how frequently Earth-sized planets appear in habitable zones.

"That will allow astronomers to make inferences about how many Earth-like planets there may be," Fanson said.

As sensitive as Kepler is, it will not be able to directly image a planet, which would require detectors capable of picking up relative changes in starlight on the order of one part per billion, as compared to Kepler's 10 to 40 parts per million.

"That's Nasa's ultimate goal," said Bicay, "but we're not there yet."

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