India clue to planet hunt

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By G.S. MUDUR in Delhi
  • Published 22.01.06

New Delhi, Jan. 22: Two Indian astronomers have proposed a new method to look for Earth-like planets outside the solar system and expand the catalogue of the so-called extrasolar planets.

Over the past 15 years, international planet-hunters have detected nearly 160 planets outside the solar system, orbiting other stars in the galaxy.

But most of them are gas giants that resemble the solar system’s Jupiter or Saturn.

Astronomers typically look for changes in starlight or gravitational effects or the transit of a planet across the surface of its parent star, but all of these existing observation techniques have so far led to detection of only large planets.

Astronomers Sujan Sengupta and Malay Maiti at the Indian Institute of Astrophysics in Bangalore have now proposed that even small planets with atmospheres might alter a property called polarisation in the light from their parent stars.

“The advantage of this proposed method is that it can detect a planet irrespective of its size and distance from its star,” said Sengupta. “So it could be used to detect even small Earth-sized planets.” A research paper by Sengupta and Maiti proposing the use of polarisation signatures of starlight to detect planets is scheduled to appear in a forthcoming issue of The Astrophysical Journal.

This is not the first time that polarisation has been suggested as a tool to study extrasolar planets. Three years ago, Sara Seager at the Institute of Advanced Study in Princeton and Steven Saar at Harvard in the US had suggested that polarisation observations might be used to study the atmospheres of some of the large extrasolar planets that have been discovered.

Normal light from a star is not polarised. But the light from a parent star that is scattered by the atmosphere of its planet gets polarised. “As a planet rotates and goes around its parent star, the incoming polarisation signature will vary with time. A highly sensitive polarimeter coupled to a telescope could pick up such time variations and confirm the presence of a planet around a star,” said Sengupta.

The polarisation technique will only be able to find planets with atmospheres. Rocky planets with no atmosphere would not polarise light and would thus remain hidden, Sengupta said.