Image: Sun and exoplanets
This illustration shows the relative sizes of the Sun, the object COROT-exo-3b and Jupiter. COROT-exo-3b has 20 times the mass of Jupiter.
updated 10/6/2008 6:05:06 PM ET 2008-10-06T22:05:06

Astronomers have sighted a very dense planet-sized object that orbits its parent star in just four days and six hours.

The object, COROT-exo-3b, fits into the category of a failed star known as a brown dwarf, but the team that made the discovery has not ruled out the possibility that it is a planet. Brown dwarfs are failed stars. They burn lithium but are not massive enough to generate the thermonuclear fusion of hydrogen and helium that powers real stars. Planets do none of that.

"It has puzzled us; we're not sure where to draw the boundary between planets and brown dwarfs," said Hans Deeg, an astronomer at the Instituto de Astrofisica de Canarias in the Canary Islands, Spain.

The object has a mass 20 times greater than that of Jupiter, but is roughly the same size. It falls outside the range of planets and stars discovered to date, with the largest planets having 12-Jupiter-mass and the smallest stars 70-Jupiter-mass.

If astronomers confirm the object as a planet, it would weigh in as the most massive and densest planet found so far. A full study will be detailed in the journal Astronomy and Astrophysics.

"COROT-exo-3b might turn out to be a rare object found by sheer luck", said Francois Bouchy, an astronomer at the Institut d'Astrophysique in Paris. "But it might just be a member of a new-found family of very massive planets that encircle stars more massive than our sun. We're now beginning to think that the more massive the star, the more massive the planet."

Ground-based telescopes around the world helped pinpoint the object, including observatories in France, Chile, Germany, Hawaii, Israel and Spain's Canary Islands.

The hunt for exoplanets has intensified over recent years, with astronomers usually finding such objects indirectly by observing their gravitational influence on parent stars. Another team showcased what might be the first direct image of an exoplanet around a sun-like star in September.

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