AN invisible star may be circling the Sun and causing deadly comets to bombard the Earth, scientists said yesterday.

The brown dwarf - up to five times the size of Jupiter - could be to blame for mass extinctions that occur here every 26 million years.

The star - nicknamed Nemesis by Nasa scientists - would be invisible as it only emits infrared light and is incredibly distant.

Nemesis is believed to orbit our solar system at 25,000 times the distance of the Earth to the Sun.

As it spins through the galaxy, its gravitational pull drags icy bodies out of the Oort Cloud - a vast sphere of rock and dust twice as far away as Nemesis.

These "snowballs" are thrown towards Earth as comets, causing devastation similar to the asteroid that wiped out the dinosaurs 65 million years ago.

Now Nasa boffins believe they will be able to find Nemesis using a new heat-seeking telescope that began scanning the skies in January.

The Wide-Field Infrared Survey Explorer - expected to find a thousand brown dwarfs within 25 light years of the Sun - has already sent back a photo of a comet possibly dislodged from the Oort Cloud.

Scientists' first clue to the existence of Nemesis was the bizarre orbit of a dwarf planet called Sedna.

Boffins believe its unusual, 12,000-year-long oval orbit could be explained by a massive celestial body.

Mike Brown, who discovered Sedna in 2003, said: "Sedna is a very odd object - it shouldn't be there.

"The only way to get on an eccentric orbit is to have some giant body kick you - so what is out there?"

Professor John Matese, of the University of Louisiana at Lafayette, said most comets come from the same part of the Oort Cloud.

He added: "There is statistically significant evidence that this concentration of comets could be caused by a companion to the Sun."