A fragment of a planet that survived the death of its host star has been discovered by astronomers, offering a hint as to the future of our Solar System, 6 billion years from now.
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.
Believed to have once been part of a larger planet, 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.
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.
"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 hint as to what planets may reside in other solar systems - and a glimpse into the future of our own.
"As stars age they grow into red giants, which 'clean out' much of the inner part of their planetary system," said Dr Christopher Manser.
"In our Solar System, the Sun will expand up to where the Earth currently orbits, and will wipe out Earth, Mercury, and Venus. Mars and beyond will survive and will move further out.
"The general consensus is that 5-6 billion years from now, our Solar System will be a white dwarf in place of the Sun, orbited by Mars, Jupiter, Saturn, the outer planets, as well as asteroids and comets.
"Gravitational interactions are likely to happen in such remnants of planetary systems, meaning the bigger planets can easily nudge the smaller bodies onto an orbit that takes them close to the white dwarf, where they get shredded by its enormous gravity."
The discovery, reported in the journal Science, is the first time that scientists have used spectroscopy to discover a solid body in orbit around a white dwarf.
This method involves using subtle variations in light emitted by the star to identify additional gas that the planetesimal is generating.
The more common method for discovering exoplanets is called the "transit method", and relies on debris passing in front of the star and blocking some of its light.
However, this only works if you're looking at the star from the right angle, which means that each system observed for several hours mostly leads to nothing.
"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."
