Red Giant Sun May Not Destroy Earth

Anne Minard
for National Geographic News
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.

Most experts think that the sun will enter its red giant phase in about five billion years.

Researchers have long argued whether Earth and the other inner planets in our solar system will be destroyed at this point or just get pushed out to more distant orbits.

(Related: "Solar System's Fate Predicted by Nearby White Dwarf?" [December 21, 2006].)

In a 2001 study in the journal Icarus, Kacper Rybicki of the Polish Academy of Sciences in Warsaw and co-authors proposed one scenario.

Mercury and Venus would evaporate, the team said, while Mars would remain intact and be pushed away.

The sun will actually expand and contract several times during its red giant phase. If those expansions are brief enough, Earth just might survive.

Scientists know two main forces will affect the outcome, but neither is well understood.

One is the mass loss of the sun, which happens during the bloating phase. This expansion tends to push planets out, and since the star is less massive, its gravity is weaker and the planet can settle into a more distant orbit.

The other force is a tidal friction, which draws planets into the star.

"For the Earth, the delicate balance between these two processes leads to a rather uncertain fate," said Frederic A. Rasio, a professor of physics and astronomy at Northwestern University, who was not involved in the study.

Silvotti, author of the new study, echoed that uncertainty, adding that finding V391 Pegasi b is a good first step to understanding what might happen.

But whatever happens to Earth in the future, Silvotti holds no hope that wildlife will be around to witness the sun's changes.

"And concerning [the fate of] human life in particular," he said, "I am afraid we have much more urgent problems than this."

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