Giant planet around tiny star 'should not exist'

Astronomers have discovered a giant planet that, they say, should not exist, according to current theories.

The Jupiter-like world is unusually large compared with its host star, contradicting a widely held idea about the way planets form.

The star, which lies 284 trillion km away, is an M-type red dwarf - the most common type in our galaxy.

An international team of astronomers has reported its findings in the journal Science.

"It's exciting because we've wondered for long a time whether giant planets like Jupiter and Saturn can form around such small stars," said Prof Peter Wheatley, from the University of Warwick, UK, who was not involved with the latest study.

"I think the general impression had been that these planets just didn't exist, but we couldn't be sure because small stars are very faint, which makes them difficult to study, even though they are much more common than stars like the Sun," he told BBC News.

Researchers used telescopes in Spain and the US to track gravitational accelerations of the star that might be caused by planets orbiting it.

The red dwarf has a larger mass than its orbiting planet - named GJ 3512b. But the difference in their size is much smaller than it is between, say, the Sun and Jupiter.

The distant star has a mass that's, at most, 270 times larger than the planet. For comparison, the Sun is about 1,050 times more massive than Jupiter.