Some of the
comets that make their way to Earth's neighborhood from the frigid outer
reaches of the solar system likely follow a different route than previously
thought, new modeling suggests.
The study's
findings, detailed in the July 31 issue of the journal Science, are good news
for our planet (especially in light of Jupiter's
recent impact): Comets from this region should rarely cross Earth's orbit,
and so aren't a collision
concern.
In turn
these rare encounters mean that these comets are unlikely to be the causes of
past mass extinction events.
Oort
cloud origins
So-called long-period
comets (those with highly elongated orbits that take them hundreds or
thousands of years to circle the sun) were long thought to come from the outer
region of the Oort Cloud.
The Oort
Cloud is a remnant of the nebula from which the solar system formed some 4.5
billion years ago. It encircles the solar system from a point about 93 billion
miles from the sun (1,000 times the distance from Earth to the sun) and extends
to about three light-years away (a light-year being the distance it takes light to travel in one year, about 5.9 trillion miles).
The Oort Cloud
is thought to contain billions of comets, most of which are far too small and
distant to be seen even with powerful telescopes.
But
gravitational nudges from a passing star can send the comets on a path to the
inner solar system, where astronomers can finally get a glimpse of these
long-exiled bodies.
There are
about 3,200 known long-period comets (the most well-known of which was Comet
Hale-Bopp, which was visible for much of 1996 and 1997).
Scientists
thought that very few of these comets came from the inner Oort Cloud, and that
they only did so when a passing star made a particularly close fly-by, setting
off a comet shower in events that play out over millions of years.
"It
was thought the long-period comets we see just tell us about the outer Oort
Cloud," said lead author of the study Nathan Kaib, a graduate student of
the University of Washington in Seattle.
Kaib's work
suggests this isn't the case.
Innies,
not outies
Scientists
had thought that most of the comets coming from the inner Oort Cloud would be
ejected from the solar system by gravitational interactions with Saturn and
Jupiter, which act like body guards for the inner solar system planets.
"They
cut down on the number of bodies reaching Earth-crossing orbits," Kaib
said.
But after
running computer models of the evolution of comet clouds for 1.2 billion years,
Kaib and his colleague found that comets from the inner Oort Cloud could slip
past the protective barrier of Jupiter and Saturn and reach an Earth-crossing
orbit.
The new
modeling suggests that a substantial portion of observable long-period comets
actually come from the inner, not the outer, Oort Cloud.
Unlikely
impactors
While the
actual number of comets in the inner Oort Cloud is unknown, Kaib and his
colleague were able to make an estimate of the highest possible number of
comets in the region.
With this
maximum, they could further estimate the number of comets likely to have struck
Earth during the last 500 million years. They determined that it should be no
more than two or three comets, which would have been part of the most powerful
comet shower in that time span.
Three major
impacts are known to have occurred nearly simultaneously (within a million or
so years of each other) at around the same time as a mass
extinction event about 40 million years ago. If that relatively minor
extinction event was caused by a shower of inner Oort Cloud comets, it was
likely the most intense comet shower since the fossil record began, and so it
is unlikely that inner Oort Cloud comets were responsible for other extinction
events (though other impactors may still be the culprits).
"That
tells you that the most powerful comet showers caused minor extinctions and
other showers should have been less severe, so comet showers are probably not
likely causes of mass extinction events," Kaib said.
So while
some comets slip through the Jupiter-Saturn barrier, most don't, and those that
do aren't likely to hit Earth. Some might hit Jupiter and Saturn themselves
though.
Whether or
not Jupiter's bruise last week was caused by an impacting long-period comet
isn't known for sure, but "it's certainly a possibility," Kaib told
SPACE.com.
Kaib's work
was funded by the National Science Foundation and NASA.
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