How big are blue dwarf stars

Astronomers discover an enigmatic object - too heavy for a planet, but too light for a star

Planet in front of star

Marseille (France) - The French Corot satellite observatory has discovered a celestial body as large as Jupiter but containing more than twenty times the mass of this planet. The object orbits in a very tight orbit every 4 days and 6 hours a star that is a little larger than our sun. The discovery raises the question of where the boundary between large planets and small stars runs, according to the team of discoverers in a report that will soon be published in the journal "Astronomy and Astrophysics".

"It was a surprise for us to find such a massive object in such a narrow orbit," explains Magali Deleuil from the Laboratoire d'Astrophysique de Marseille, who leads the discovery team. "The object is really unique and we are still debating what its true nature may be." The Corot satellite observatory is specifically used to search for planets near other stars. To do this, the satellite monitors the brightness of a large number of stars. If a planet moves past a star on its orbit, the brightness of the star is slightly reduced - and Corot can demonstrate this attenuation.

Giant planets in narrow orbits are nothing unusual among the more than 300 known planets in other stars. But so far the maximum mass of such companions was 12 times the mass of Jupiter. On the other hand, the mass of the smallest known dwarf stars - called brown dwarfs by astronomers - is 70 times that of Jupiter. With Corot-exo-3b, the catalog name of the new object, the researchers have for the first time tracked down a celestial body in the mass gap between giant planets and dwarf stars.

It remains to be seen whether Corot will find more properties in this void. "Maybe we were just lucky and celestial bodies like Corot-exo-3b are extremely rare," says team member Francois Bouchy. On the other hand, it could also be that it is normal for more massive stars to have more massive planets as well. Only further observations can show exactly where the mass boundary between stars and planets runs - and whether there is even a sharp boundary between these astronomical object classes.