July 30, 2009 -- If a comet collision was among your worries, you can rest easy. Most mass extinctions on Earth were not caused by comets striking the planet, according to a new study.
The study, which took a new look at a comet-rich region of space called the Oort cloud, also found that there is less stuff in that region than scientists suspected and that many of the comets we see actually come from a part of the cloud previously thought unable to produce visible comets.
Compared to the news about extinctions, those findings are most interesting to planetary scientists because they help fill in some blanks about the formation of our solar system. Still, it never hurts to know what the world is up against when it comes to threats from outer space.
"Even the most common comet showers were probably not strong enough to trigger mass extinctions," said Nathan Kaib, an astronomy graduate student at the University of Washington, Seattle, whose study was published online today by the journal Science. "There have got to be other ways to kill off life on Earth."
Kaib's research started as an attempt to determine how much stuff there is in the Oort cloud, a conglomeration of comets that circles the solar system, starting at about 1,000 times the distance from the Earth to the sun and stretching to 20,000 times the Earth-sun distance.
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The cloud may contain billions of lumps of ice, rock and dust, most of which are too small for us to ever see. But the long, elliptical orbits of some comets occasionally bring them into our line of sight.
Scientists have long thought that visible comets came mostly from the outer ring of the Oort cloud, because Saturn and Jupiter have enough gravity to eject from the solar system any comet that creeps in from the inner portion. Sometimes, those inner comets actually slam into the giant planets, which happened to Jupiter last week.
There are exceptions, however. According to traditional thinking, when another star gets close enough to ours every 100 million years or so, the shifting tug of gravity allows a shower of comets to rain down toward Earth from the inner Oort cloud. For a few million years, the chance of impact on our planet goes up.
Without any direct evidence to go on, scientists estimated that there was a large amount of material in the inner cloud and that comet showers were powerful enough and frequent enough to periodically wipe out many species on Earth.
When Kaib and colleagues modeled the Oort cloud and many of the forces that pull on it, however, they found that, after a billion-year run, a large portion of comets with long orbits that come into Earth's view originated in the inner Oort cloud. That suggests that we see a much larger percentage of inner Oort cloud comets than scientists thought we did, and that there aren't as many comets there as were predicted.
A second run of the model showed that the most powerful comet shower of the last 500 million years might have caused a minor extinction event about 40 million years ago. But collisions could not have been responsible for any major extinction events in that time.
"The occurrence of these comet showers is much less frequent than people previously argued," said Hal Levison, a planetary scientist at the Southwest Research Institute in Boulder. The inner Oort cloud, he added, is "not as massive as people who want to use it as an extinction-generator would argue needs to be there."
Looking forward, the chance of comet impacts in our lifetime just went from none to even less.
"I'm not a big fan of worrying about impacts on the Earth now," Levison said. "Our society has more important things to worry about."
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