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Earthís cousin, wet and warm

New Delhi, April 25: Astronomers have discovered the first planet outside the solar system that could have liquid water and liveable temperatures —- a potential new spot to look for extraterrestrial life.

The planet, orbiting a red dwarf star named Gliese 581, is the first that appears to lie in a habitable zone — the region around a star where water is in its liquid form — a team of European astronomers has said.

The astronomers, who announced their finding today after submitting it to the journal Astronomy and Astrophysics, have estimated that the average temperature on the planet — which is about five times more massive than Earth — would be 0 to 40 degrees Celsius.

Like other exoplanets — orbiting other stars — the planet around Gliese 581 is too small and too far for direct observations or temperature measurements. The researchers detected its presence indirectly by tracking tiny changes in the velocity of its parent star in the constellation Libra.

“This planet will most probably be a very important target of future space missions dedicated to the search for extra-terrestrial life,” said Xavier Delfosse, a team member at Grenoble University in France.

“On the treasure map of the universe, one would be tempted to mark this planet with an X,” Delfosse said.

Gliese 581 is among the 100 stars closest to the solar system — only 20 light years away from Sun, but smaller and colder. Although astronomers have detected more than 220 exoplanets, most are gas giants that resemble Jupiter or Saturn.

Smaller planets are harder to find, but computer simulations of the formation of planets show that small planets are more likely to be rocky and hold water.

“Calculations show that small planets turn out to be rocky like Mars or Earth,” Xavier Bonfils, a team member at Lisbon University, Portugal, told The Telegraph. “During formation, small planets may accumulate water which — if still present — would be liquid in the temperature range we’ve estimated.”

The planet is so close to its star that it takes only 13 days to complete an orbit. The astronomers estimated the temperature using star-planet distance and assuming that the reflecting power of the planet is the same as that of Venus in the solar system.

“This finding suggests that planets may be fairly common in our galaxy,” Thierry Forveille, a Grenoble astronomer said. “Red dwarf stars of this type are the most common in our galaxy,” he said. Among the 100 stars closest to Sun, nearly 80 are red dwarfs.

The scientists now plan to look for transits of the planet across the face of its star. “There is a small chance that such a transit will occur, but if it does, we could measure the size of the planet,” said Forveille. “Our current size estimations are based on computer models — not measurements.”

The scientists detected the Super Earth by observing the change in the velocity of the parent star typically caused by orbiting planets with the help of an instrument called HARPS located in an observatory perched high in the mountains of Chile.

The velocity change method has been used to detect at least 200 of the 220 planets discovered outside the solar system. “HARPS is the most precise instrument in the world to measure the velocity change and has helped detect 11 of the 13 planets with masses within 20 Earth masses,” Stephane Udry, an astronomer at Geneva Observatory, said.

Two years ago, Udry and his colleagues had detected a planet with the mass of Neptune around the same star. Their new finding adds two planets — the five Earth mass planet in the habitable zone and an eight Earth mass planet further away.

Astronomers cautioned that the estimation of temperature on the planet was based on unproven assumptions and that the idea of liquid water is mere speculation.

“The atmosphere of a planet is absolutely crucial to determine its surface temperature and we know nothing about its atmosphere,” Eugene Chiang at the University of California, Berkeley, told The Telegraph.

The standard way to study the atmosphere would be to examine the feeble emissions of light from a planet. “But we don’t have technology yet to study emissions from so far away,” Chiang said. “But when technology becomes available — perhaps in about 20 years — this planet would be one of our first targets.”

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