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See Inside September 2008

The Sun Will Eventually Engulf Earth--Maybe

Researchers debate whether Earth will be swallowed by the sun as it expands into a red giant billions of years from now



Lynette Cook Photo Researchers, Inc.

The future looks bright—maybe too bright. The sun is slowly expanding and brightening, and over the next few billion years it will eventually desiccate Earth, leaving it hot, brown and uninhabitable. About 7.6 billion years from now, the sun will reach its maximum size as a red giant: its surface will extend beyond Earth’s orbit today by 20 percent and will shine 3,000 times brighter. In its final stage, the sun will collapse into a white dwarf.

Although scientists agree on the sun’s future, they disagree about what will happen to Earth. Since 1924, when British mathematician James Jeans first considered Earth’s fate during the sun’s red giant phase, a bevy of scientists have reached oscillating conclusions. In some scenarios, our planet escapes vaporization; in the latest analyses, however, it does not.

The answer is not straightforward, because although the sun will expand beyond Earth’s orbit, or one astronomical unit (AU), it will lose mass along the way. As a result, Earth should drift outward as the gravitational tug lessens over time. (At its maximum radius of 1.2 AU, the sun will have lost about one third of its mass, compared with its current heft.) In this way, Earth could escape solar envelopment.

But other factors complicate the analysis. Drag on the planet from the sun’s outermost, tenuous layers will cause Earth to drift inward. Smaller forces from the other planets—all in turn reacting to the same reducing, expanding sun—are even more difficult to account for completely.

Earlier this year two teams reported different kinds of calculations indicating that Earth will be swallowed up by the sun. In a calculation that would thrill any college junior studying classical mechanics, Lorenzo Iorio of Italy’s National Institute of Nuclear Physics used perturbation theory. It simplifies analyses by dropping relatively small factors, thereby making complex equations of motions that describe the interactions between the sun and Earth mathematically manageable. Assuming that the sun’s yearly mass loss (currently about one part in 100 trillion) remains small for the duration of its evolution to the red giant phase, Iorio calculates that Earth will move outward at about three millimeters a year, or only 0.0002 AU by the sun’s red giant phase. But at that point the sun will balloon up, in only a million years, to 1.2 AU in radius, thus vaporizing Earth.

Iorio’s paper, submitted to Astrophysics and Space Science, has not yet been peer-reviewed. Several scientists question whether quantities that Iorio assumes are small will indeed remain small throughout the sun’s evolution.

Even if Iorio got his number crunching wrong, he may have the right answer. In an analysis published in the May Monthly Notices of the Royal Astronomical Society, Klaus-Peter Schröder of the University of Guanajuato in Mexico and Robert Smith of the University of Sussex in England also conclude that Earth is doomed, by using more exact solar models and by considering tidal interactions. As the sun loses mass and expands, its rotation rate must also slow down—physics students learn this relation as the conservation of angular momentum. The slowed rotation causes a tidal bulge on the sun’s surface. The gravity exerted by this bulge pulls Earth inward. With such a consideration, the researchers find that any planet with a present-day orbital radius of less than 1.15 AU will ultimately perish.

Could Earth be saved if someone is still left at home? In a bold piece of astronomical engineering, Don Korycansky of the University of California, Santa Cruz, and his colleagues have proposed nudging Earth with a large asteroid arranged to pass nearby periodically. It could take one billion years to move our planet out to somewhere safe, like the orbit of Mars. Our moon, though, might have to be left behind, and any miscalculation could mean extinction. Needless to say, more study is required.

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