The discovery of the gas-giant planet three times the size of Jupiter offers a look at the future of our own solar system and what will happen to the Earth when the sun grows old and collapses, the researchers said.

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Scientists found the planet some 4,500 light-years from Earth.

It once orbited its star at the same distance as the Earth is now from the sun – about eight light-minutes – but then drifted away, the researchers said in the journal Nature.

“At present, (the) discovery is the only planetary system known to have survived its red-giant phase,” Jonathan Fortney, a NASA researcher, wrote in Nature, where the international team published its findings.

Sheds light on solar system

“This will shed light not only on our own solar system, in which Mercury, Venus and perhaps Earth will eventually be engulfed by the red-giant Sun but also the diverse array of planetary systems that are our galactic neighbours.”

Scientists have identified some 250 planets orbiting stars other than our sun. Most are detected by indirect measurements such as tiny variations in the wobble of a star.

The team found this planet by chance while studying its parent star V 391 Pegasi, Don Kurtz, astrophysicist at the University of Lancashire who worked on the study, said.

Sound waves in the star cause it to pulsate and vary in brightness every few minutes. By observing these changes, astronomers can measure sound speed to see inside the star.

The team found that during its time as a middle-aged star, V 391 Pegasi had a mass similar to the sun before it expanded its radius by more than 100 times when it entered its red-giant phase – something the sun is expected to do in 5 billion years.

The researchers said the planet stayed intact because the parent star lost mass, reducing its gravitational pull just enough to let the planet drift away a bit.

Earth may avoid annihilation

When the Sun – which scientists think is 30 per cent bigger than when it came into being – exhausts all its hydrogen and swells up during its red-giant phase, the Earth will also likely avoid complete destruction for the same reason, Kurtz said.

“The future of the Earth is to die with the sun boiling up the oceans, but the hot rock will survive,” he said.

“We think the processes that are happening on this planet will be the same as on earth … It is psychologically interesting to think that the earth will survive.”

Red giant planet found