Exoplanet Says Cheese

13 September 2004 (All day)

Red alert. The faint spot close to the brighter brown dwarf star may be the very first real image of an exoplanet.

It's the Holy Grail of planet hunters: an image of a planet outside our own solar system. Almost 140 exoplanets have been found in the past 7 years by indirect means, and there have been a few false alarms about actual sightings. But now, astronomers may have the grail within reach. In a paper accepted for publication in Astronomy & Astrophysics, a team presents what may be the first photo of a world orbiting another star.

If confirmed, the photo will grace the pages of astronomy textbooks for many years. It was taken by a European-American team led by Gaël Chauvin of the European Southern Observatory (ESO) and shows the faint image of a brown dwarf star about 230 light-years from Earth. Very close to the star is an even more inconspicuous red speck that would never have been seen were it not for the extreme sensitivity of the NAOS-CONICA (NACO) instrument at ESO's Very Large Telescope on Cerro Paranal in northern Chile and the adaptive optics used to cancel out the blurring effect of Earth's atmosphere. Chauvin and his colleagues even managed to obtain a spectrum of the exoplanet candidate, revealing water vapor in its atmosphere--a sure indication it's not a remote background star.

The brown dwarf, officially known as 2MASS WJ1207334-393254, is part of a very young group of stars. If the red object is indeed at the same distance as the brown dwarf (and has the same young age of just 8 million years), calculations show it to be about five times more massive than Jupiter. Its orbit would be larger than Pluto's.

Ray Jayawardhana of the University of Toronto stresses that there's a remote chance, perhaps 1 in 10 million, that the "exoplanet" is actually a very old and dim brown dwarf star in the foreground. The distance "definitely has to be confirmed," he says, most likely by using the Hubble Space Telescope to check if the two objects share the same motion through space.

There's an intriguing twist, too: Jayawardhana and others have shown that young brown dwarfs generally do not have massive protoplanetary disks of gas and dust, which means that if the new object is indeed a planet, it may not have formed the same way planets in our solar system did.

Related sites
Paper by Gaël Chauvin's team (pdf)
ESO's NACO instrument, used in the discovery
ESO's Very Large Telescope

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