14 January 2010

Spectrum of light captured from distant world

Agence France-Presse
Astronomers have made the first direct capture of a spectrum of light from a planet outside the Solar System and are using it to decipher its composition.
Exoplanet spectrum

This montage shows the image and the spectrum of the star and the planet as seen with the NACO adaptive optics instrument on ESO’s Very Large Telescope. Credit: ESO/M. Janson

PARIS: Astronomers have made the first direct capture of a spectrum of light from a planet outside the Solar System and are deciphering its composition.

The light was snared from a giant planet that orbits a bright young star called HR 8799 about 130 light-years from Earth, said the European Southern Observatory (ESO).

“The spectrum of a planet is like a fingerprint. It provides key information about the chemical elements in the planet’s atmosphere,” said Markus Janson, lead author of a study on the find. “With this information, we can better understand how the planet formed and, in the future, we might even be able to find tell-tale signs of the presence of life.”

Chemical fingerprint

HR 8799 has a mass about one-and-a-half times that of the Sun and hosts a planetary system “that resembles a scaled-up model of our own Solar System,” the ESO said. The target was the middle one of three planets – initially spotted in 2008 – that are between seven and 10 times the mass of Jupiter.

The find is important, because hidden within a light spectrum are clues about the relative amounts of different elements in the planet’s atmosphere.

“The features observed in the spectrum are not compatible with current theoretical models,” said co-author Wolfgang Brandner. “We need to take into account a more detailed description of the atmospheric dust clouds, or accept that the atmosphere has a different chemical composition from that previously assumed.”

The result represents a milestone in the search for life elsewhere in the universe, said the ESO. Until now, astronomers have been able to get only an indirect light sample from an exoplanet, as worlds beyond our Solar System are called.

They do this by measuring the spectrum of a star twice – while an orbiting exoplanet passes near to the front of it, and again while the planet is directly behind it. The planet’s spectrum is thus calculated by subtracting one light sample from another.

Complex challenge

But the method can only be used if the orientation of the exoplanet’s orbit is exactly right, and only a tiny fraction of exoplanetary systems fall into this category.

HR 8799 is thousands of times brighter than the planet, which means that sifting out the spectrum was a complex task. “It’s like trying to see what a candle is made of, by observing it from a distance of two kilometres when it’s next to a blindingly bright 300-watt lamp,” said Markus Janson, who led the team who uncovered the spectrum.

The authors of the study, in the Astrophysical Journal, used an infrared detector on ESO’s Very Large Telescope, located in Paranal, Chile. A total of 424 exoplanets have now been spotted since the first, 51 Pegasi b, unofficially called ‘Bellerophon’, was detected in 1995, according to the Extrasolar Planets Encyclopaedia.

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