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Astronomers eyeball smallest star yet

Just a bit bigger than Jupiter

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Astronomers searching for extrasolar planets have discovered the smallest star spotted yet - just 16 per cent larger than Jupiter. OGLE-TR-122b was identified during analysis of data gathered during the planet-hunting OGLE project in Chile, New Scientist reports.

The OGLE survey identified almost 200 objects by watching for bodies transiting companion stars. In the case of OGLE-TR-122b, the mini-star passes before Sun-like OGLE-TR-122 once a week, dimming the light reaching Earth from the latter by 1.5 per cent. However, although observers were able to use this dimming to calculate the object's diminutive stature, it was not until it was re-examined by Frederic Pont of Switzerland's Geneva Observatory using the the Very Large Telescope in Chile that its true nature was revealed.

The New Scientist explains: "They used the telescope to study the spectrum of the larger star, which wobbles back and forth because of gravitational tugs from the smaller object. The relatively puny body weighed in at 96 times Jupiter's mass - above the threshold of 75 Jupiter-masses required for a bona fide star, which must also burn hydrogen."

Graphic of OGLE-TR-122b by European Southern Observatory

The discovery has implications for exoplanet hunters, who must when classifying objects take into account both dimming - which determines size - as well as the effects of "wobbles" on neighbouring bodies - which determine mass. Accordingly, Claudio Melo of the European Southern Observatory in Santiago, Chile, issued a note of caution for the organisers of any future space-based planet-hunting operation: "Based only on the light curves, you cannot tell what you are observing. When those space missions fly, we need to do ground-based spectroscopic measurements to confirm the nature of the transiting object."

That aside, the astronomers say this is the first time they have eyeballed a star with a radius comparable to a planet. Small it may be, but it isn't the least massive. That honour goes to a star just 93 times Jupiter's mass, announced early in 2005 by the University of Arizona. But, because that lightweight's transit of a companion object is not visible from Earth, its size is unknown. ®

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