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Paris - Astronomers said on Wednesday they had found the first-ever ringed planet beyond our Solar System, a super-world with a girdle of halos 200 times bigger than Saturn's.
Called J1407b, the giant has a disk of 30-odd rings which is so vast that, had it been around Saturn, it would have dominated our night sky, the proud discoverers said.
"It'd be huge! You'd see the rings and the gaps in the rings quite easily from Earth," Matthew Kenworthy of the Leiden Observatory in The Netherlands told AFP.
"It'd be several times the size of the full Moon."
Kenworthy and Eric Mamajec of New York's University of Rochester trawled through a database of millions of stars photographed by telescopes around the world in an exoplanet search project called SuperWASP.
Exoplanets, worlds beyond our own Solar System, are observed from Earth through changes in the brightness of their central star.
The light is partly blocked, generally for a few hours, when the planet passes between Earth and its star.
In the case of star J1407, the astronomers observed an unusual and active light display that lasted two months.
Data on the star was gathered between 2005 and 2008, and "right slap-bang in the middle of the year 2007 the light curve goes crazy. It suddenly starts flickering and twinkling.
"It was a very strange-looking thing that nobody had ever seen before," Kenworthy said.
At first the team was puzzled.
But they ultimately concluded that the only explanation was a vast disk of spaced rings around a planet that had moved between Earth and J1407, blocking the starlight in bursts.
"This is the first time somebody's seen such a giant ring system outside the Solar System - any ring structure at all around what we think is a planet," Kenworthy said.
The research has been accepted for publication by the Astrophysical Journal, he added.
The rings begin at a distance of about 30 million kilometres from the planet and stretch out to a distance of 90 million kilometres.
And they are probably made of dust, as planet J1407b is too hot - about 1 000°C to 2 000°C - to support ice rings like those orbiting Saturn.
Kenworthy said the planet itself was probably about 10-40 times the mass of Jupiter, the biggest planet of our star system.
J1407 and its planet are about 16 million years ago, which makes it an infant in terms of stellar age. The Sun and Earth are some 4.5 billion years old.
Kenworthy said the find provided the first direct evidence for theories about planetary ring formation.
It is widely held that big clouds of gas and dust collapse to form stars orbited by a thick disk of debris.
This later collects into planets, themselves orbited by rings of debris that clump together over a few million years to form moons.
"If our hypothesis is true and it [J1407b] really is a ring system, and we think it is the best explanation, then this is the first direct evidence of that process," Kenworthy said.
Data on the ringed exoplanet have been out there since 2007, he added.
"What we think happened is that other astronomers may have looked at it but they thought there was a problem with the camera because it was so weird.
"It required a new way of thinking to say: 'Oh hold on a minute, this could be a giant ring system.' That is such an incredible idea.... it's completely nuts.
"We think other astronomers, if they'd seen it, they would have gone: 'Oh that's too crazy', and moved on to next thing."
