"A thin atmosphere made of nitrogen and oxygen has allowed life to thrive on Earth. But nature is full of surprises. Many exoplanets discovered by the Kepler mission are enveloped by thick, hydrogen-rich atmospheres that are probably incompatible with life as we know it," said Ian Crossfield, the University of Arizona astronomer who led the study.

"Most planets we have found to date are scorched. This system is the closest star with lukewarm transiting planets," added University of California Berkeley graduate student Erik Petigura. Petigura discovered the planets on January 6 while conducting a computer analysis of the Kepler data NASA has made available to astronomers.

"There is a very real possibility that the outermost planet is rocky like Earth, which means this planet could have the right temperature to support liquid water oceans," he noted. After Petigura found the planets in the Kepler light curves, the team quickly employed telescopes in Chile, Hawaii and California to characterise the star's mass, radius, temperature and age. The star's proximity means it is bright enough for astronomers to study the planets' atmospheres to determine whether they are like the Earth's atmosphere and possibly conducive to life.

The next step will be observations with other telescopes, including the Hubble Space Telescope, to take the spectroscopic fingerprint of the molecules in the planetary atmospheres."If these warm, nearly Earth-size planets have puffy, hydrogen-rich atmospheres, Hubble will see the telltale signal," Petigura said.

The paper has been submitted to Astrophysical Journal.

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