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This planet's a true survivor

red giant

V 391 Pegasi as it was about 100 million years ago, when the star was at its maximum red giant expansion, with its nearby gas giant planet (Image: HELAS, Mark Garlick)

Astronomers have discovered the first known planet to survive the 'red giant' phase of its ageing star, a period when the star expands and engulfs bodies orbiting it.

Stars become red giants when the hydrogen in their core runs out and they bloat, gobbling up nearby planets.

But the discovery of a gas-giant planet three times the size of Jupiter orbiting such a red giant has surprised researchers, who report their findings today in the journal Nature.

"At present, [the] discovery is the only planetary system known to have survived its red-giant phase," writes Dr Jonathan Fortney, a NASA researcher who commented on the findings in the same issue of the journal.

"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."

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

The team found that during its time as a middle-aged star, V 391 Pegasi had a mass similar to our sun.

Then it expanded its radius by more than 100 times when it entered its red-giant phase, something our sun is expected to do in 5 billion years.

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

When our sun, which scientists think is 30% bigger than when it came into being, exhausts all its hydrogen and swells up during its red-giant phase, the Earth will also probably avoid complete destruction for the same reason, Kurtz says.

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

"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."

Tags: science-and-technology, astronomy-space