"Some say the world will end in fire, Some say in
ice," wrote the poet Robert Frost. Astronomers, it turns out, are in the
former camp.
A new calculation predicts that Earth will be swallowed
up by the sun in 7.6 billion years, capping off a longstanding debate over
whether the sun's gravitational pull will have weakened enough for Earth to
escape final destruction or not.
Other theorists have predicted that our planet will fry as
the sun expands in its old age. But the time estimates have varied by a couple
billion years.
"Although people have looked at these problems before,
we would claim this is the best attempt that's been made to date, and probably
the most reliable," said astronomer Robert Smith, emeritus reader at the U.K.'s
University of Sussex, who made the new calculations with astronomer Klaus-Peter
Schroeder of the University of Guanajuato in Mexico. "What we've done is
to refine existing models and to put the best calculations we can at each point
in the model."
If 7.6 billion years doesn't sound like an urgent death
sentence, don't relax yet. Regardless of whether Earth will ultimately be
vaporized, as the sun
heats up, our planet will become too hot to live on before then.
"After a billion years or so you've got an Earth with no
atmosphere, no water and a surface temperature of hundreds of degrees, way
above the boiling point of water," Smith told SPACE.com. "The
Earth will become dry basically. It will become completely impossible for life
of any kind to exist. It's a pretty gloomy forecast."
Nonetheless, scientists are curious about the ultimate fate
of our planet after we are gone (like all previous hominids and more than 99
percent of all species that have lived on Earth, humans will probably go
extinct, and it will likely happen sooner than a billion years).
Smith's earlier studies found that Earth would narrowly
escape being engorged. As the sun ages and expands into a red giant star, it
will shed its outer gaseous layers, thus losing mass and weakening its
gravitational pull. Previous calculations found that this let-up would allow
the Earth's orbit to shift outward, enabling the planet to slip free of the
smoldering sun.
But this scenario doesn't account for tidal forces, and the
drag of the sun's outer layers. As the Earth orbits the sun, its smaller
gravitational pull isn't completely negligible — it actually causes the side of
the sun closest to our planet to hoard more mass and bulge out toward us.
"Just as the Earth is pulling on the sun's bulge, it's
pulling on the Earth, and that causes the Earth to slow in its orbit,"
Smith said. "It will spiral back and finally end up inside the sun."
In addition, the gas that the sun expels will also drag Earth
inward toward its demise.
Smith's previous calculations had ignored these effects.
"We didn't think it mattered, but it turns out it
does," he said. "You might say our previous models had a gap."
There may even be hope for Earth. Some scientists have
proposed a
scheme for down the road to use the gravity of a passing asteroid to budge
Earth out of the way of the sun toward cooler territory, assuming there is life
around at the time that is intelligent enough to engineer this solution.
"It sounds like science fiction, but there's a group of
people who have quite seriously suggested that it might be possible,"
Smith said. "If it's done right, that would just keep the Earth moving
fast enough to keep it out of harm's way. Maybe life could go on for as much as
7 billion years."
Smith's findings have been
accepted for publication in the journal Monthly Notices of the Royal
Astronomical Society.
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