BALTIMORE, March 12 (UPI) -- Instruments aboard the Hubble Space Telescope have discovered a huge extrasolar planet so close to its mother star its atmosphere is boiling away into space, researchers said Wednesday.
The planet, which last year became the first beyond our solar system to have its atmosphere detected, circles a star similar to our sun about 150 light-years from Earth in the constellation Pegasus. The planet, about 70 percent the size of Jupiter, orbits a scant 4 million miles from its star. Earth, in comparison, averages about 93 million miles from the sun.
An international team of astronomers followed last year's discovery of the planet's atmosphere with additional studies to probe the upper atmospheric regions. What they found caught them by surprise: a hot and evaporating cloud of atomic hydrogen circling the planet and trailing off into a comet-like tail.
"We were astonished to see that the hydrogen atmosphere of this planet extends over 200,000 kilometers," said Alfred Vidal-Madjar, an astronomer in Paris and lead author of a paper on the discovery appearing in the March 13 issue of the British journal Nature.
Scientists were able to make the measurements because the planet passes in front of its parent star, slightly dimming the background emissions. By using the Hubble's Imaging Spectrograph, which breaks light into component wavelengths, the team was able to measure how much of the planet's atmosphere was filtering the starlight.
Various elements that the light passes through etch their unique chemical fingerprints on the beams and by studying these spectra, scientists were able to detect a huge drop in hydrogen emissions from ultraviolet light filtered by the planet's atmosphere.
The likely explanation is a huge, puffed-up atmosphere that extends to roughly three times the size of the planet itself.
So much of the gas is escaping -- scientists estimate the planet is losing at least 10,000 tons of hydrogen per second -- that the planet eventually will evaporate completely.
"We checked we had the real (hydrogen) signature," said University of Arizona researcher Gilda Ballester, a co-investigator. "A colleague said maybe the star could change its spectra, but we saw the same thing three times," she said, adding the team could not come up with any other explanation for the phenomenon.
"When you think of the physics it's not surprising," she added. "This planet is so close to the star. That's what people had underestimated."
The scientists theorize the planet's outer atmosphere is blown up and heated so much by the parent star it starts to escape the planet's gravity.
"The atmosphere is heated, the hydrogen escapes the planet's gravitational pull and is pushed away by the starlight, fanning out in a large tail behind the planet -- like that of a comet," said Alain Lecavelier des Etangs, with the Institut d'Astrophysique de Paris.
Added Ballester: "In ultraviolet light, this planet looks more like a comet than a planet."
Astronomers said they plan additional studies of the planet's hydrogen "tail."
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(Reported by Irene Brown, UPI Science News, in Cape Canaveral, Fla.)
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(Editors: UPI photos WAX2003031201, WAX2003031202 and WAX2003031203 are available)
