Thursday, September 13, 2007

Extreme Exoplanet

About 5 billion years from now, the Sun will begin to exhaust its primary source of energy -- the hydrogen near the center that fuels nuclear fusion. The remnant helium core will begin to contract, and the surrounding shell of hydrogen will start to burn hotter. Our star will slowly bloat to 100 times its former size, becoming a "red giant" and possibly destroying some of the inner planets of our solar system. An unsettled question among astronomers is: could the Earth make it through such a catastrophic event?

New evidence in this longstanding debate surfaced this week, when an Italian-led group of scientists announced the discovery of the first planet around a distant star that appears to have survived this phase of stellar evolution. "The future of the Earth is to die with the Sun boiling up the oceans, but the hot rock will survive", said Don Kurtz, one of the coauthors of the investigation. The study focused on a star called V391 Pegasi, which is estimated to be about 10 billion years old and appears to have already gone through the red giant phase. This particular star is special: the light coming from its surface exhibits very regular pulsations, a kind of continuous "star-quake" that provides a window into its interior structure. The astronomers have been monitoring these pulsations for more than 6 years, and they noticed something interesting.

As expected, the period of the pulsations are increasing slightly over time -- a consequence of the continuing evolution of the star, which slowly modifies the interior conditions that determine the regularity of the pulsations. But in addition to the expected changes, the pulsations sometimes arrived a few seconds sooner or a few seconds later than predicted, in a pattern that suggests the star is wobbling around in space from the gravitational pull of an orbiting planet. This is the same technique that led to the discovery of the very first planets detected outside of our solar system in 1992, which were orbiting a distant pulsar. As it turns out, the planet around V391 Pegasi is close enough to the star right now that its orbit was probably the same size as the Earth's orbit around the Sun at the time the star became a red giant. The fact that the planet is still there suggests that close planets can, at least in some circumstances, survive this phase of stellar evolution.

Of course, more research is needed to determine whether or not V391 Pegasi represents a typical outcome for close planets. But, as Kurtz succinctly stated, "It is psychologically interesting to think that the Earth will survive." Even if all that's left is a hot rock.