Red Giant Sun May Not Destroy Earth

September 14, 2007

The first glimpse of a planet that survived its star's red giant phase is offering a glimmer of hope that Earth might make it past our sun's eventual expansion.

The newfound planet, dubbed V391 Pegasi b, is much larger than Earth but likely orbited its star as closely as our planet orbits the sun (explore a virtual solar system).

When the aging star mushroomed into a red giant about a hundred times its previous size, V391 Pegasi b was pushed out to an orbit nearly twice as far away.

"After this finding, we now know that planets with an orbital distance similar to the Earth can survive the red giant expansion of their parent stars," said lead author Roberto Silvotti of the National Institute of Astrophysics in Napoli, Italy.

"But this does not automatically mean that even the Earth, much smaller and much more vulnerable [than V391 Pegasi b], will survive" our sun's expansion billions of years from now, he said.

Silvotti and colleagues present their research in this week's issue of the journal Nature.

Opposing Forces

At an age of ten billion years, V391 Pegasi b is one of the oldest known planets. The planet and its host star are in the constellation Pegasus, about 4,500 light-years from Earth.

The red giant star is six times hotter than our sun and its surface gravity is ten times greater.

But when it was a middle-aged star, "it had a mass similar to that of the sun, and stably fused hydrogen into helium for billions of years," writes Jonathan Fortney in a review article also appearing in Nature.

Fortney is a scientist at the NASA Ames Research Center in Moffett Field, California, who was not involved in the new paper.

Stars like the sun become red giants when the hydrogen in their cores burns fully into helium. At that point they expand, becoming large enough to engulf any inner planets.

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