Star 'gnome' is nuclear surprise

A comparison between our Sun, Jupiter and the stellar "gnome"

A shining star has been located that is not much bigger than Jupiter, the biggest planet in our Solar System.

The discovery is fascinating, say scientists, because it shows how small an object can be and still trigger the nuclear reactions for sunshine.

The existence of the star, known as OGLE-TR-122B, was confirmed by the Very Large Telescope (VLT) in Chile.

Sited in the Carina constellation, the stellar "gnome" was seen to pass in front of a much bigger companion star.

As it did so, it dimmed the companion's light received at the VLT, a facility run by the European Southern Observatory organisation (ESO).

In a volume

It is not known precisely how big an object needs to be to shine. At some point a gas body will become so massive that the gravitational forces pulling material into its core will initiate fusion reactions - just like those at the core of our Sun that give us light.

What is interesting is that although OGLE-TR-122B is a mere 16% larger than Jupiter, it is actually 96 times more massive.

"Imagine that you add 95 times its own mass to Jupiter and nevertheless end up with a star that is only slightly larger," suggests Claudio Melo, from ESO and member of the team of astronomers who made the study.

"The object just shrinks to make room for the additional matter, becoming more and more dense."

Indeed, the density of OGLE-TR-122B is more than 50 times greater than that of our own Sun.

Setting limits

The stellar gnome shines, but not very brightly - especially when compared with the energy released by its large companion star.

"This result shows the existence of stars that look strikingly like planets, even from close by," said Frederic Pont, of the Geneva Observatory, Switzerland.

"Isn't it strange to imagine that even if we were to receive images from a future space probe approaching such an object at close range, it wouldn't be easy to discern whether it is a star or a planet?"

What is remarkable, however, is that OGLE-TR-122B is actually smaller than some of the planets discovered recently outside our Solar System.

Details of the study on OGLE-TR-122B will appear in a forthcoming edition of the journal Astronomy & Astrophysics.