Famous JFCs include comet Wild 2, which was encountered by NASA's Stardust spacecraft in January 2004, and comet Shoemaker Levy-9, which broke up and collided with Jupiter in July 1994.
(Related news: "Stardust's Comet Clues Reveal Early Solar System" [December 15, 2006].)
The Shoemaker Levy-9 impact was so violent that it left scars on Jupiter visible from Earth for a year (see photo).
A popular theory first put forth by the late Carnegie Institute astronomer George Wetherill in 1994 says that Jupiter protects Earth from these types of violent cometary impacts.
The massive gas giant slings oncoming bodies away through the force of its gravity, the theory goes.
In their 2000 book Rare Earth: Why Complex Life is so Uncommon in the Universe, astronomer Donald Brownlee and paleontologist Peter Ward support this theory.
They assert that without the long, peaceful periods offered by Jupiter's shield, intelligent life on Earth would never have been able to take hold.
But the new research points out that Wetherill's work focused on so-called Long Period Comets.
These comets are thought to originate very far away in a hypothetical region dubbed the Oort Cloud, and they whiz toward Earth with little warning.
Comet Hale Bopp, for example, came from the Oort Cloud and was discovered less than two years before it arrived.
JFCs, on the other hand, can spend thousands of years orbiting the outer planets before being knocked out of place and heading for Earth—giving astronomers plenty of time to see them coming.
Overall, the new study says, Earth is no more or less vulnerable to JFCs with Jupiter on the scene.
But if a Saturn-size planet took its place, the rate of impact would be significantly higher.
"Most impacts were recorded when Jupiter was [modeled to be] one-fourth its current mass, so just slightly smaller than Saturn," study co-author Horner said.
Astronomers believe Earth is most vulnerable to impacts from so-called Near Earth Asteroids, which come in from the asteroid belt between Mars and Jupiter.
The real question, Rare Earth co-author Brownlee said, is how the impact rates of these closer bodies would differ without Jupiter.
"Without Jupiter," he said, "the asteroids would probably have formed a planet, and there would be no source of inner solar system bodies to pound on Earth."
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