A metal planet was discovered around by the trail of gas it left around the star SDSS J122859.93+104032.9. Image Credit: University of Warwick/Mark Garlick


The remains of a heavy metal planet are in orbit around a dead star – a white dwarf over 400 light years away - it was recently discovered.

It’s thought the planet, whose discovery was announced on 4 April 2019, was once a larger rocky world, but it was torn apart when the star reached the end of its life.

When stars like the Sun begin to run out of fuel, they enter into a red giant phase, ballooning in size before cooling and shrinking down to form a white dwarf.

During this process, any rocky worlds in the inner planetary system are destroyed, leaving a disc of debris around the remaining white dwarf.

Astronomers studied one such white dwarf and debris disc using the Grand Telescopio Canarias in La Palma.

“The star would have originally been about two solar masses, but now the white dwarf is only 70 per cent of the mass of our Sun.

It is also very small – roughly the size of the Earth – and this makes the star, and in general all white dwarfs, extremely dense,” says Christopher Manser from the University of Warwick, who lead the study.

The team were attempting to track iron, magnesium, silicon and oxygen – four of the major elements in forming terrestrial planets.

They found there was a ring of gas which appears to be created by a solid body moving through the disc, streaming gas behind it like a comet’s tail.

The body could be anything from a kilometre to a few hundred kilometres in size, comparable to the largest asteroids in our Solar System.

What is particularly surprising is how close to the star the fragment is, taking just two days to complete an orbit.

“The white dwarf’s gravity is so strong – about 100,000 times that of Earth’s – that a typical asteroid would be ripped apart by the gravitational forces if it passes too close to the white dwarf,” says Manser.

It’s likely that the original rocky planet started further out in the planetary system, but drifted inwards as the star began to cool after the red giant phase.


The planetary fragment’s composition seems to suggest it is the remains of a planet’s iron core, and so astronomers will continue to study it as a window into a part of a planet that is usually buried thousands of kilometres beneath the surface.


Elizabeth Pearson
Ezzy PearsonScience journalist

Ezzy Pearson is the Features Editor of BBC Sky at Night Magazine. Her first book about the history of robotic planetary landers is out now from The History Press.