A fragment of a planet that survived the death of its host star has been discovered by astronomers, offering important clues about the fate of the planets in our own solar system.
The so-called "planetesimal", which is rich in iron and nickel, and at least a kilometre in size, survived a system-wide cataclysm that followed the death of its host star.
Its survival is all the more astonishing as it orbits closer to its star than previously thought possible – going around it once every two hours.
Astronomers from the University of Warwick discovered the planet fragment in a disc of debris formed from destroyed planets, orbiting a white dwarf star 410 light years away.
White dwarfs are the remains of stars like our sun that have burnt all their fuel and shed their outer layers, leaving behind a dense core which slowly cools over time.
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This particular star, known as SDSS J122859.93+104032.9, has shrunk so dramatically that the planetesimal orbits within its sun's original radius.
Evidence suggests that it was once part of a larger body further out in its solar system, and is likely to have been a planet torn apart as the star began its cooling process.
"The planetesimal we have discovered is deep into the gravitational well of the white dwarf, much closer to it than we would expect to find anything still alive," said Professor Boris Gaensicke, co-author from the Department of Physics at the University of Warwick.
"That is only possible because it must be very dense and/or very likely to have internal strength that holds it together, so we propose that it is composed largely of iron and nickel."
Professor Gaensicke went on to suggest that the planetesimal could be a massive fragment of the core of a planet that was at least hundreds of kilometres in diameter.
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"It is only at that point planets begin to differentiate – like oil on water – and have heavier elements sink to form a metallic core," he explained.
The discovery offers a glimpse into the future of our own planetary system, when the sun starts to die.
"In about five billion years, the sun will eventually start to expand into a red giant," said Dr Christopher Manser, in an article for The Conversation.
"At this point, it will engulf Mercury, Venus and most likely Earth – unless we manage to move our planet into a wider orbit, which should be possible in theory.
"However, Mars, the asteroid belt and the rest of the solar system will survive engulfment and continue orbiting it as the sun collapses into a white dwarf."
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He added that it was unlikely any living organisms on planetary or moon fragments could survive this process, "and even if they did, they would struggle to live on in the faint light of a white dwarf."
The discovery, reported in the journal Science, is the first time that scientists have used a technique called spectroscopy to discover a solid body in orbit around a white dwarf.
This method involves splitting the light emitted by the star into its separate colours and looking for a doppler shift, caused by a stream of gas from the orbiting planetesimal.
The more common method for discovering exoplanets is called the "transit method", and relies on debris passing in front of the star and blocking out some of its light.
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However, this only works if you're looking at the star from the right angle, which means that several hours of observation can often produce no results.
"The spectroscopic method we developed in this research can detect close-in planetesimals without the need for a specific alignment," said Dr Manser.
“Learning about the masses of asteroids, or planetary fragments that can reach a white dwarf can tell us something about the planets that we know must be further out in this system, but we currently have no way to detect."