Jupiter may not be Earths protector after all: Study

Astronomers have long thought of Jupiter as being a protective big brother to Earth, but now a preliminary study undertaken by an astromer at the Open University in Milton Keynes suggests otherwise.

Washington, Aug.25 : Astronomers have long thought of Jupiter as being a protective big brother to Earth, but now a preliminary study undertaken by an astromer at the Open University in Milton Keynes suggests otherwise.

According to Jonathan Horner, an astronomer with the university, it was originally thought that giant Jupiter's gravitational pull shielded a smaller Earth from objects in the solar system, and thus helped it to support conditions for human and animal life.

Horner now states that Jupiter's role as guardian is overstated

"It seems that the idea isn't so clear-cut." Horner says, but adds that his findings are still tentative.

The study presented today at the European Planetary Science Congress in Potsdam, Germany, indicate that Earth would be struck - by one class of objects at least - at nearly the same rate, regardless of whether Jupiter was there or not.

Planetary scientist George Wetherill first proposed the idea of Jupiter as a protector of Earth in 1941.

Wetherill showed that the planet's enormous mass - more than 300 times that of the Earth - is enough to catapult comets that might hit Earth out of the Solar System.

Some have also postulated that Jupiter would thin the crowd of dangerous asteroids and other objects, making Earth a more stable home.

Other work has suggested that, in the past, changes in Jupiter's orbit might have actually increased the number of objects on a collision course with earth. Until now, Horner says, little work was done to test either idea.

So Horner and colleague Barrie Jones built several versions of the Solar System on the Open University's computer cluster: one with a Jupiter, one without, and several with a gas giant that was either a quarter, half, or three-quarters of Jupiter's mass.

The system also contained 100,000 centaurs - large, icy bodies from the Solar System's Kuiper belt, within which Pluto lies.

After running their models for 10 million virtual years, Horner and Jones found some striking results. The Earth was about 30 percent more likely to be hit by a centaur in a Solar System with a life-size Jupiter than it was in a Jupiter-less system.

Things looked even worse when there was an intermediate-sized planet in Jupiter's place, according to Horner.

However, a lighter version of Jupiter could help pull the centaurs into the inner solar system, while lacking the gravity to heave them back out.

As a result, a planet with a quarter of Jupiter's mass could increase the chances of a strike on Earth by nearly 500 percent when compared to a system with no planet there.

"It's a good speculative paper," says Mark Bailey, director of the Armagh Observatory in Northern Ireland and an expert on Earth-impacting asteroids.

It fails to take into account Jupiter's ability to deflect Earth-colliding objects from the Oort cloud, a massive cloud of comets that surrounds the Solar System, he adds.

It also does it factor in the most likely source of a hit, according to Alessandro Morbidelli, an astronomer at the Observatoire de la Cote d'Azur, in Nice, France.

The asteroid belt between Mars and Jupiter accounts for all but a small percentage of the Earth-crossing objects in the Solar System, Morbidelli says.

To really understand Jupiter's role as a protector will require a calculation of how the planet influences those many, smaller objects. "That's a much more complicated thing to do," Morbidelli says.

Horner says that he and his colleagues will soon begin work on simulations of both the Oort cloud comets and asteroid belt objects.

ANI