I, for one, wouldn’t want to be around to witness what happens when the Earth is faced with an ever expanding Sun that has exhausted its hydrogen fuel. Conventional wisdom has it that the planet will likely be engulfed by what will then become a red giant. Certainly Mercury and Venus will, and the Earth’s orbit is close enough that it may meet the same fate. But it’s intriguing to learn that other outcomes are possible.
Thus news out of Iowa State that the planet known as V 391 Pegasi b has evidently survived just such an encounter with its own star. Larger than Jupiter, the distant world in the constellation Pegasus was once situated at roughly the same distance from its parent that the Earth is from the Sun. That distance has changed over time as the star lost its outer regions in the helium flash, the onset of helium fusion that is produced as hydrogen is exhausted and contraction heats the stellar core.
Image: An artist’s conception of V 391 Pegasi b as it survives the red giant expansion of its dying sun. Credit: HELAS, the European Helio- and Asteroseismology Network.
Now located at some 1.7 AU from V 391 Pegasi, the doughty planet is still there, testimony to the durability of planetary systems, and perhaps an indication that the Earth of the distant future might survive such a catastrophe. Steve Kawaler (Iowa State), a member of the research team working on this project, puts it this way:
“The exciting thing about finding a planet around this star is that it indicates that planetary systems can survive the giant phase and the helium flash of their parent star. It bodes well for the survival of our own Earth in the distant future. Before V 391 Pegasi lost its outer regions at the helium flash, the planet orbited the star at about the same distance that the Earth orbits our sun.”
Of course, what would be left on the surface of an Earth-class survivor scarcely bears contemplating. In any case, no one can say for sure whether the Earth will escape engulfment like the inner planets. Its orbit should widen as the Sun loses mass even as tidal forces drag the planet inward. We seem to be in an ambiguous zone about which too little is known to feel confidence in the outcome. Mario Livio (Space Telescope Science Institute) is quoted in this New York Times story: “Earth’s fate is actually the most uncertain because it is at the border line between being engulfed and surviving.”
But the new work does show that planets in orbits closer than 2 AU can survive the red giant phase. V 391 Pegasi’s maximum radius is thought to have reached 0.7 AU, a close brush indeed with the planet in question. Earth’s fate won’t be decided for five billion years. The paper is Silvotti et al., “A giant planet orbiting the ‘extreme horizontal branch’ star V 391 Pegasi,” Nature 449 (13 September 2007), pp. 189-191 (abstract).