Planet Survivor

In about 5 billion years, our sun will grow into a red giant star. Its outer atmosphere will swallow Mercury and Venus, the two planets closest to it. But what will happen to Earth, the next planet in line? No one is certain.

Recently, scientists found a large planet outside our solar system that survived a similar event. The discovery doesn’t necessarily predict what will happen to Earth, however.

Still, “now at least we know that a giant planet at an [Earthlike] distance can survive” its parent star’s red-giant phase, says Roberto Silvotti of the INAF-Osservatorio Astronomico di Capodimonte in Naples, Italy.

This recently discovered extrasolar planet survived the expansion of its aging parent star. Before the event, the planet’s orbit was about the size of Earth’s (top). After the event, the planet’s orbit was 70 percent bigger—about the size of Mars’ or

Nature

The newly discovered planet is more than 3 times as large as Jupiter, the largest planet in our solar system. It is called an extrasolar planet because it orbits a sun other than ours.

The planet used to orbit its star, called V391 Pegasi, at about the same distance that Earth is from the sun.

V391 Pegasi belongs to a rare class of stars, called B-type subdwarfs. It started out with about as much mass as our sun has now. After burning through all the hydrogen gas in its core, it swelled into a red giant. It grew to about 1 astronomical unit in diameter—the size of Earth’s orbit.

Then, an unusual sequence of events occurred. V391 Pegasi expelled its outer envelope of gas. It lost half its mass and kept just a thin skin of atmosphere around its core, which was made of helium gas.

When the star lost that mass, the planet’s orbit began to expand. Originally, its orbit was about the size of Earth’s. After the shift, its orbit was about as big as Mars’.

When our sun becomes a red giant, it, too, will lose mass and gravity. Its core will collapse. Its outer layers will spread, possibly reaching Earth or beyond.

Because these opposite pressures will happen at the same time, it is hard to predict exactly what will happen next. The fate of V391 Pegasi provides some clues, but there are important differences between that star and ours.

Eventually, V391 Pegasi will become a small, dense, burned-out object called a white dwarf. The same fate awaits our sun.

When the sun turns from red giant to white dwarf, however, the transition will probably be brighter and hotter; making it more likely that Earth will get caught up in the explosion.

The power of such an explosion, says Jonathan Fortney of NASA’s Ames Research Center in Mountain View, Calif., “could lead to the evaporation of the planet.”

The scientists were able to detect V391 Pegasi’s planet thanks to another unusual feature of the star. It pulsates, or throbs, giving off signals with each pulse.

By noting changes in the pattern of pulses, the researchers could tell that the gravity of a large, orbiting planet was pulling the star slightly back and forth. In 1992, astronomers used a similar method to make the first discoveries of extrasolar planets.

With the new discovery, scientists can begin to figure out what properties, including mass and orbit size, are required for a planet to survive the midlife changes of its star.—Emily Sohn

Going Deeper:

Cowen, Ron. 2007. Survivor: Extrasolar planet escapes stellar attack. Science News 172(Sept. 15):163-164. Available at http://www.sciencenews.org/articles/20070915/fob2.asp .

Pegg, J.L. 2007. An Earthlike planet. Science News for Kids (May 2). Available at http://www.sciencenewsforkids.org/articles/20070502/Note2.asp .

Sohn, Emily. 2005. Cousin Earth. Science News for Kids (June 29). Available at http://www.sciencenewsforkids.org/articles/20050629/Note2.asp .

______. 2004. Planet hunters nab three more. Science News for Kids (Sept. 8). Available at http://www.sciencenewsforkids.org/articles/20040908/Note2.asp .