In about five billion years, our Sun will become a red giant as it runs out of fuel, swelling to many hundreds of times its present size and destroying the inner planets. However, new findings suggest the outer planets could survive.

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SYDNEY: The discovery of remnants of a planetary system orbiting a dead star is providing astronomers with a window on the distant future of our own Solar System.

"This white dwarf is an exciting discovery. Our Solar System will look very similar in 5 to 8 billion years, and an astronomer in another solar system may witness what we saw," said Boris Gaensicke of the University of Warwick in England.

The findings, which are published today in the U.S. journal Science, provide evidence that some of the outer planets in our Solar System could survive when our own Sun dies in several billion years.

When a medium-sized star such as our Sun begins to run out of hydrogen to fuel its nuclear fusion furnace, it turns to helium for fuel and swells up to many hundreds of times its normal size, becoming a red giant.

As the star grows, the expanding outer layers engulf and destroy the inner planets. The star eventually loses its outer layers completely and forms a planetary nebula, leaving behind an extremely dense core approximately the size of Earth, called a white dwarf.

While the inner planets would be destroyed by a red giant, one theory suggests that some outer planets could survive, falling away from the dying star as its gravity weakens due to loss of mass from its outer layers.

The discovery of planetary material orbiting a white dwarf suggests that this theory could be right. "[This] is the first direct evidence for the existence of asteroids around a white dwarf, and asteroids are part of planetary material," said Gaensicke.

"We did not directly observe a planet around this specific white dwarf, but the fact that we found an asteroid that has been dislodged from its stable circular orbit is a very strong suggestion that this white dwarf has an invisible planet orbiting around it at a fairly large distance," he said.

A team of researchers analysed the light emitted from a white dwarf named SDSS 1228+1040 to determine the star's composition. Different elements emit distinctive light spectrums, and white dwarfs - the remnants of stars - should be pristine hydrogen.

However the researchers were surprised to find evidence of metals - to astronomers, anything heavier than helium - along with hydrogen in the star's spectrum. The team found calcium, magnesium and iron in the spectrum, hallmarks of a gaseous disc rotating around the dwarf, according to the study.

The material that makes up the disc could not have survived close to the sun during the red giant phase, so the researchers concluded it must have migrated inwards from a stable outer orbit after the formation of the white dwarf.

They suggested a scenario in which one or more outer planets survived the formation of the white dwarf, and later disrupted the orbit of an asteroid so it plummeted towards the sun. Close to the compact star, gravity would have crushed the rocky asteroid into a disc of metal-rich debris, which then fell into the sun, causing it to release the distinctive light spectrum.

According to Gaensicke, when the Sun dies the fate of the Earth is unclear: our planet could be engulfed, or it may narrowly escape as its orbit moves further out. "There is a vast uncertainty, which mainly relates to the poor understanding of mass loss during the late stages of evolution [of a star]," he says.

Even if the Earth is not consumed, however, Earth's water and most of its atmosphere will be boiled away.

Estimates of how much bigger our Sun will grow during the red giant phase range between about 200 and 700 times larger than its current radius.