Philipp Salzgeber

Close call.
Jupiter brings comets close to Earth, but it also protects us from them.

Is Jupiter a Bodyguard or Troublemaker?

POTSDAM, GERMANY--If Jupiter were just half its mass, Earth would be a far more dangerous place. But without the giant planet, life would be pretty much the same as it is now. That's the surprising outcome, presented here today at the European Planetary Science Congress, of new computer simulations that show how Jupiter regulates the number of comets impacting our planet. According to physicist Jonathan Horner of the Open University in Milton Keynes, U.K., the common belief that Jupiter acts as a protective shield by deflecting onrushing comets needs some tweaking.

Comets are dirty chunks of ice in the outer regions of the solar system left over from the formation of the planets. A few kilometers in diameter, they can wreak havoc when they collide with Earth. But their elongated orbits can be strongly perturbed by Jupiter's gravity. In fact, the massive planet flings some comets out of the solar system altogether. That's why astronomers suggested in the early 1990s that Jupiter serves as a sort of planetary bodyguard for Earth.

But according to Horner, the matter is not so straightforward. His team's computer simulations confirm that comet impacts would be more numerous if Jupiter were less massive because fewer comets would be flung into outer space. But if there were no Jupiter at all, the researchers found, hardly any comets would whiz dangerously close to Earth. That's because Jupiter also pulls comets out of their icy reservoir in the outer realms of the solar system. As a result, the cometary impact rate on Earth wouldn't change at all if Jupiter were removed. "As soon as I heard about this, I thought, 'Yes, of course!'" says solar system dynamicist Alessandro Morbidelli of the Côte d’Azur Observatory in Nice, France.

However, don't give too much credit to Jupiter. Morbidelli points out that far more asteroids slam into Earth than do comets. Most rocky asteroids orbit the sun between Mars and Jupiter, and they probably respond very differently to changes in the mass of Jupiter. Horner's team next plans to investigate Jupiter's influence on the terrestrial impact rate of asteroids and so-called long-period comets, which arrive from far beyond the solar system.

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