© Julian Baum
Earth experienced an especially heavy bombardment of asteroids and comets early in the solar system's history.
Contrary to prevailing wisdom, Jupiter does not protect Earth from comet strikes. In fact, Earth would suffer fewer impacts without the influence of Jupiter's gravity, a new study says. It could have implications for determining which solar systems are most hospitable to life.

A 1994 study showed that replacing Jupiter with a much smaller planet like Uranus or Neptune would lead to 1000 times as many long-period comets hitting Earth. This led to speculation that complex life would have a hard time developing in solar systems without a Jupiter-like planet because of more intense bombardment by comets.

But a new study by Jonathan Horner and Barrie Jones of Open University in Milton Keynes, UK, shows that if there were no planet at all in Jupiter's orbit, Earth would actually be safer from impacts.

The contradictory results arise because Jupiter affects comets in two different, competing ways. Its gravity helps pull comets into the inner solar system, where they have a chance of hitting Earth, but can also clear away Earth-threatening comets by ejecting them from the solar system altogether, via a gravitational slingshot effect.

Tripled impacts

According to the new study, the worst scenario for Earth is when Jupiter is replaced by a planet with about the mass of Saturn. "[Such a planet] is fairly capable of putting things into an Earth-crossing orbit, but still has some difficulty ejecting them, so they will stay on an Earth-crossing orbit for a much longer time," Horner told New Scientist. The projected result was more than three times as many impacts as in the real solar system.

So both the new study and the one from 1994 suggest that a smaller planet in Jupiter's orbit would leave Earth worse off, although they disagree about how much worse.

That may be because they differ on the source of the comets they examine. Horner looked at objects coming from the Kuiper belt, a region just beyond Neptune's orbit where many dormant comets reside. The previous study, meanwhile, looked at the Oort cloud, a vast collection of dormant comets extending hundreds of times further from the Sun.

Ultimately, knowing what kinds of solar systems are safest from bombardment could help in the search for alien life. But, despite the latest work, it is still unclear where we should be looking.

Asteroid threat

Alessandro Morbidelli of Nice Observatory in France, who studies solar system dynamics, says neither Horner's analyis nor the earlier study included the most important source of impacts - the asteroid belt. About 95% of the impacts on Earth are due to asteroids, he says.

He suspects that a smaller planet in place of Jupiter may lead to fewer asteroid impacts. "Given that near-Earth asteroids dominate the impact rate, decreasing asteroid impacts might cause a decrease in the overall bombardment rate of the Earth," he told New Scientist.

Horner and Jones plan to extend their study to include asteroids, but Morbidelli says there are even more factors to examine. "You can imagine solar systems where a much more massive and broader asteroid belt is preserved - it would be difficult to live in that solar system," he says. "You can imagine giant planets migrating and destroying an asteroid belt. There are so many factors it is difficult to handle them all."

The results were presented Friday at the European Planetary Science Congress 2007 in Potsdam, Germany.