[tt] NYT: Scientists' Good News: Earth May Survive Sun's Demise in 5 Billion Years

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I am very relieved to learn this.

Scientists' Good News: Earth May Survive Sun's Demise in 5 Billion Years
http://www.nytimes.com/2007/09/13/science/13planet.html

By DENNIS OVERBYE

There is new hope that Earth, if not the life on it, might survive
an apocalypse five billion years from now.

That is when, scientists 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.

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 planet is a gas giant at least three times as massive as
Jupiter. It orbits about 150 million miles from a faint star in
Pegasus known as V 391 Pegasi. But before that star blew up as a
red giant and lost half its mass, the planet must have been about
as far from its star as Earth is from the Sun -- about 90 million
miles -- according to calculations by an international team of
astronomers led by Roberto Silvotti of the Observatorio Astronomico
di Capodimonte in Naples, Italy.

Dr. Silvotti said the results showed that a planet at Earth's
distance "can survive" a red giant, and he said he hoped the
discovery would prompt more searches.

"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 via
e-mail.

Dr. Silvotti and his colleagues reported their results today in
Nature.

In an accompanying commentary, Jonathan Fortney of NASA's Ames
Research Center in California 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 391 Pegasi is about 4,500 light years from Earth and is
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
six minutes. After studying the star for seven years, Dr. Silvotti
and his colleagues were able to discern subtle modulations in the
six-minute cycle, suggesting that the star 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," said 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 only yesterday, X-ray astronomers from the Goddard
Space Flight Center in Greenbelt, Md., and the Massachusetts
Institute of Technology announced that they had detected the
remains of a star that radiation had whittled down to planetary
mass circling a pulsar in the constellation Sagittarius. Those
systems have probably endured supernova explosions.

The Pegasus planet has had to survive 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, the
astronomers said. As the Sun blows off mass to conserve angular
momentum, Earth will retreat to a more distant, safer orbit. At the
same time, tidal forces between 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, Mario Livio of the Space Telescope Science Institute
said of the inner planets, "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.