Survivor: Extrasolar planet escapes stellar attack

September 12, 2007 at 10:08 am

From the sizzling outer atmosphere of a sunlike star to the chilly surroundings of a dark, stellar cinder, extrasolar planets keep turning up in the darndest places.

ENDURANCE. A recently discovered planet has withstood the swelling of its aging parent star, which expanded nearly to 1 astronomical unit, the size of Earth’s orbit (top). The star has since shrunk, and the planet has migrated about 70 percent farther from its parent (bottom). Nature

Now astronomers have found a large planet that survived a special type of onslaught. The planet, more than 3 times as heavy as Jupiter, was at one time orbiting the elderly star V391 Pegasi at Earth’s distance from the sun. When the aging star ballooned to more than 100 times its former diameter, the giant planet drifted farther away, escaping engulfment. Roberto Silvotti of the INAF-Osservatorio Astronomico di Capodimonte in Naples, Italy, and his colleagues describe this sequence of events in the Sept 13 Nature.

Earth will face a similar challenge in about 5 billion years, when our sun swells to become a red giant star. As its outer envelope puffs up, the sun will swallow Mercury and Venus, but Earth’s fate is uncertain, theorists say.

Competing forces muddy the picture. The middle-aged sun will shed mass, and its decreasing gravity will let Earth recede. At the same time, its puffy outer layer could envelop Earth, creating a frictional drag that will draw the planet inward.

“[Understanding] what happens to the planet is not at all easy,” says Silvotti. “Now at least we know that a giant planet at an [Earthlike] distance can survive” its parent star’s red giant phase.

There are some key differences between the sun and V391 Pegasi, which is now a B-type subdwarf. Like other stars of this rare class, it reached its present state by following an unusual evolutionary path, Silvotti notes. The star initially had about the same mass as the sun, and after exhausting the supply of hydrogen at its core it became a bloated red giant. Then, however, it somehow shed its extended envelope, losing half its mass and retaining only a thin skin of atmosphere surrounding its helium core. That mass loss caused its planet to move from an orbit about the size of Earth’s to one as big as that of Mars.

Like the sun, V391 Pegasi will end its life as a still-more-compact, burned-out object called a white dwarf.

It probably would have been harder for V391 Pegasi’s planet to survive had it been orbiting a sunlike star, says Jonathan Fortney of NASA’s Ames Research Center in Mountain View, Calif. “In the normal transition from red giant to white dwarf, a star undergoes thermal pulses with fairly high luminosity, which could lead to the evaporation of the planet.”

Another unusual feature of V391 Pegasi made it relatively easy to detect the planet. The star pulsates. Tiny, periodic changes in the arrival times of the pulses enabled the astronomers to infer that an orbiting, massive planet was pulling the star to and fro.

In 1992, researchers used a similar method to find the first known extrasolar planets. Those planets circle a superdense object called a neutron star, which no one had expected to possess planets.

The discovery of the planet orbiting V391 Pegasi starts to delineate the properties, such as mass and orbit size, that a planet needs to survive the vicissitudes of a star’s old age, says Mario Livio of the Space Telescope Science Institute in Baltimore.