Red Giant Sun May Not Destroy Earth

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Most experts think that the sun will enter its red giant phase in about five billion years.

Researchers have long argued whether Earth and the other inner planets in our solar system will be destroyed at this point or just get pushed out to more distant orbits.

(Related: "Solar System's Fate Predicted by Nearby White Dwarf?" [December 21, 2006].)

In a 2001 study in the journal Icarus, Kacper Rybicki of the Polish Academy of Sciences in Warsaw and co-authors proposed one scenario.

Mercury and Venus would evaporate, the team said, while Mars would remain intact and be pushed away.

The sun will actually expand and contract several times during its red giant phase. If those expansions are brief enough, Earth just might survive.

Scientists know two main forces will affect the outcome, but neither is well understood.

One is the mass loss of the sun, which happens during the bloating phase. This expansion tends to push planets out, and since the star is less massive, its gravity is weaker and the planet can settle into a more distant orbit.

The other force is a tidal friction, which draws planets into the star.

"For the Earth, the delicate balance between these two processes leads to a rather uncertain fate," said Frederic A. Rasio, a professor of physics and astronomy at Northwestern University, who was not involved in the study.

Silvotti, author of the new study, echoed that uncertainty, adding that finding V391 Pegasi b is a good first step to understanding what might happen.

But whatever happens to Earth in the future, Silvotti holds no hope that wildlife will be around to witness the sun's changes.

"And concerning [the fate of] human life in particular," he said, "I am afraid we have much more urgent problems than this."

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