Earth Might Survive Sun’s Explosion
By DENNIS OVERBYEWhat happens to planets when their stars age and die?
That’s not an academic question. About five billion years from now, astronomers say, the Sun will run out of hydrogen fuel and swell temporarily more than 100 times in diameter into a so-called red giant, swallowing Mercury and Venus and dooming life on Earth, but perhaps not Earth itself.
Astronomers are announcing that they have discovered a planet that seems to have survived the puffing up of its home star, suggesting there is some hope that Earth could survive the aging and swelling of the Sun.
The newly discovered planet is a gas giant at least three times as massive as Jupiter. It orbits about 150 million miles from a faint star in the constellation Pegasus known as V 391 Pegasi. But before that star blew up as a red giant sometime in the past and lost half its mass, the planet must have been about as far from its star as the Earth is to the Sun — about 90 million miles — the astronomers led by Roberto Silvotti of the Observatorio Astronomico di Capodimonte in Naples, Italy, calculated.
Dr. Silvotti said that the results showed that a planet at the Earth’s distance “can survive” the red giant and he hoped the discovery would spur searches for more like it. “With some statistics and new detailed models we will be able to say something more even to the destiny of our Earth (which, as we all know, has much more urgent problems by the way),” he said in an e-mail message.
He and his colleagues report their results in Nature on Thursday.
In an accompanying commentary in Nature, Jonathan Fortney of NASA’s Ames Research Center at Moffett Field in Sunnyvale, Calif., wrote, “This system allows us to start examining what will happen to planets around stars such as our own Sun as they too evolve and grow old.”
The star V 391Pegasi is about 4,500 light years away and is now about half as massive as the Sun, burning helium into carbon. It will eventually sigh off another shell of gas and settle into eternal senescence as a “white dwarf.” Meanwhile, the star’s pulsations cause it to brighten and dim every 6 minutes.
After studying the star for seven years, Dr. Silvotti and his colleagues were able to discern subtle modulations in the 6-minute cycle, suggesting that it was being tugged to and fro over a three-year period by a massive planet. “Essentially the observers are using the star as a clock, as if it were a G.P.S. satellite moving around the planet,” explained Fred Rasio, of Northwestern University, who was not involved in the research.
This is not the first time that a pulsing star has been used as such a clock. In 1992, astronomers using the same technique detected a pair of planets (or their corpses) circling the pulsar PSR1257+12.
And , today, X-ray astronomers from the Goddard Space Flight Center in Greenbelt, Md. and the Massachusetts Institute of Technology announced they had detected the remains of a star that had been whittled by radiation down to planetary mass circling a pulsar in the constellation Sagittarius. Those systems have likely endured supernova explosions.
The Pegasus planet has had to survive relatively less lethal conditions, although it must have had a bumpy ride over its estimated 10 billion years of existence. Alan Boss, of the Carnegie Institution of Washington, said “stellar evolution can be a wild ride for a planet that is trying to survive, especially inner planets like Earth.”
When our own Sun begins to graduate from a hydrogen-burning “main sequence” star to a red giant, two effects will compete to determine the Earth’s fate, the astronomers explain. On one hand, as the Sun blows off mass in order to conserve angular momentum, the Earth will retreat to a more distant, safer orbit. But at the same time tidal forces between the Earth and the expanding star will try to drag the planet inward where it could be engulfed. The latter effect, in particular, is difficult to compute.
As a result, said Mario Livio of the Space Telescope Science Institute, of the inner planets, “the Earth’s fate is actually the most uncertain because it is at the border line between being engulfed and surviving.”
A particularly dangerous time for Earth, Dr. Silvotti said, would be at the end of the red giant phase when the Sun’s helium ignites in an explosive flash. In the case of V 391 Pegasi, that explosion sent a large fraction of the star’s mass flying outward.
“This is another reason why the survival of a planet in a relatively close orbit is not trivial,” he said.
Quoted from http://www.nytimes.com/2007/09/12/science/space/12cnd-planet.html?ei=5090&en=2e3c30bc2beed102&ex=1347249600&adxnnl=1&partner=rssuserland&emc=rss&pagewanted=print&adxnnlx=1189688477-s+T6k7PXPQeI1iKDt2Y1rg:
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