An enormous halo of dust that circles Saturn and dwarfs its more familiar rings has been discovered by astronomers.
The ring, the largest in the solar system, surrounds the planet from a distance of 8m miles, about 50 times further out than the others.
It is also unusual in that it wraps the planet at an angle of roughly 27 degrees, while the planet's other rings surround the equator.
Anne Verbiscer, research scientist at the University of Virginia, said that if the ring were visible from Earth, it would be twice the size of the full moon.
"There's nothing else like it in the solar system. You could fit a billion Earths inside it," she said.
While Saturn's regular rings are only metres thick, the new one is around 1.55m miles thick.
Verbsicer, who led the team, said the ring appeared to be made up of debris that breaks off one of Saturn's moons, Phoebe, as it orbits the planet.
It was spotted from infrared images taken from Nasa's Spitzer space telescope.
Every cubic kilometre of the ring contains around 20 tiny particles of dust, making it so tenuous, it would be invisible from the inside. The study is published in the journal Nature.
The discovery may explain a long-standing mystery surrounding another of Saturn's moons, Iapetus.
In the 17th century, the astronomer Giovanni Cassini noticed Iapetus had one dark side and one lighter side, but had no idea why.
Verbiscer said one face of the moon may be pelted by dust from the newly discovered ring, making it darker. Debris on the surface of Iapetus has a similar composition to material known to exist on Phoebe.