Water sign on far-off planet

An illustration of exoplanet HR8799c with its host star in the distance. Picture by Dunlap Institute for Astronomy and Astrophysics

New Delhi, March 14: Astronomers have analysed in exquisite detail the direct light from a planet outside the solar system and spotted the most convincing evidence yet for water on an extrasolar planet.

Four astronomers using a telescope in Hawaii have extracted unprecedented details of the atmosphere of a giant planet discovered five years ago orbiting a star about 130 light years from the Sun.

Their observations of the planet around the star, named HR8799, have also yielded clues that its planets were formed through the same mechanism that gave rise to the planets of the solar system.

“This is not the first detection of water on an exoplanet but the level of detail we can see is unprecedented,” Quinn Konopacky, a team member at the University of Toronto, told The Telegraph.

Konopacky and her colleagues have detected spectral signatures of water vapour and carbon monoxide in the light from one of the star’s planets, called HR8799c. Their findings appeared today in the US journal Science.

The planet HR8799c is a gas giant about seven times larger than Jupiter, and its host star is young — only 30 million years old in contrast to the Sun’s age, estimated to be about 4.6 billion years.

“Finding direct signatures of molecules from exoplanets has been difficult — that’s what makes this exciting,” said Sujan Sengupta, a scientist at the Indian Institute of Astrophysics, Bangalore.

Sengupta was not associated with this study but had theoretically predicted that the four planets around HR8799 would have high rotational spin speeds. In line with this forecast, Sengupta said, the US-Canadian team has inferred a minimum rotational speed of about 40km per second.

Planet hunters have since the mid-1990s catalogued more than 800 planets, but almost all have been detected indirectly through subtle changes in the movements or the light of their host stars.

The researchers used the Keck Telescope in Hawaii to study HR8799, whose family of four planets looks, as Konopacky said, in some ways like a scaled-up version of the solar system.

“The HR8799 planets line up very nicely with where Jupiter, Saturn, Uranus and Neptune are located (relative to the Sun),” Travis Barman, a team member at Lowell Observatory in the US, told this newspaper.

Astronomers believe that the temperatures and pressures on HR8799c have led to the hydrogen, carbon and oxygen already present in the disk that formed the planets to combine and form water and carbon monoxide.

The planet has no solid surface and is estimated to have a surface temperature of more than 800 degrees Celsius. The heat and luminosity of the planet springs from the young age of this planetary system.

“The planet will simply cool with time — this cooling may eventually change the composition of the gas as it is partially controlled by temperature,” Barman said.

The scientists say the planets around HR8799 seem to have formed through core-accretion, a process in which a large enough core gravitationally pulls in gas from the disk leading to the formation of a gas giant.

But the large masses of the four planets also pose a challenge to conventional theories of planetary formation.

“Traditional models of the core-accretion process have had difficulty in making a planet as large as HR8799c so far from its host star,” Konopacky said.