Newly Discovered Cometary Route Sneaks Past Jupiter, but Decreases Risk of Earth Impacts
Written by Anne Minard
Astronomers have used the comet record — including 2001 RX14 (Linear) at left, captured in 2002 by the Sloan Digital Sky Survey — to model a new route for incoming comets that sneaks past Jupiter's gravity.
The pathway might even be the dominant one that delivers Oort Could comets on an Earth-bound trajectory, say the authors of a new study in Science this week — but if that's true, comets only rarely cause extinctions on Earth.
(Image credit: Mike Solontoi/University of Washington)
Scientists have debated how many mass extinction events in Earth's history were triggered by a space body crashing into the planet's surface. Most agree that an asteroid collision 65 million years ago brought an end to the age of dinosaurs, but there is uncertainty about how many other extinctions might have resulted from asteroid or comet collisions with Earth.
In fact, astronomers know the inner solar system has been protected at least to some degree by Saturn and Jupiter, whose gravitational fields can eject comets into interstellar space or sometimes send them crashing into the giant planets. That point was reinforced last week (July 20) when a huge scar appeared on Jupiter's surface, likely evidence of a comet impact.
There are about 3,200 known long-period comets, which can take anywhere from 200 to tens of millions of years to orbit the Sun. Among the best-remembered is Hale-Bopp, which was easily visible to the naked eye for much of 1996 and 1997 and was one of the brightest comets of the 20th century.
It has been believed that nearly all long-period comets that move inside Jupiter to Earth-crossing trajectories originated in the outer Oort Cloud, a remnant of the nebula from which the solar system formed 4.5 billion years ago. It begins about 93 billion miles from the sun (1,000 times Earth's distance from the sun) and stretches to about three light years away (a light year is about 5.9 trillion miles). The Oort Cloud could contain billions of comets, most so small and distant as to never be observed.
The orbits of long-period comets can change when they are nudged by the gravity of a neighboring star as it passes close to the solar system, and it was thought such encounters only affect very distant outer Oort Cloud bodies.
It also was believed that inner Oort Cloud bodies could reach Earth-crossing orbits only during the rare close passage of a star, which would cause a comet shower. But it turns out that even without a star encounter, long-period comets from the inner Oort Cloud can slip past the protective barrier posed by the presence of Jupiter and Saturn and travel a path that crosses Earth's orbit.
In the new research, University of Washington astronomers Nathan Kaib and Thomas Quinn used computer models to simulate the evolution of comet clouds in the solar system for 1.2 billion years. They found that even outside the periods of comet showers, the inner Oort Cloud was a major source of long-period comets that eventually cross Earth's path.
By assuming the inner Oort Cloud as the only source of long-period comets, they were able to estimate the highest possible number of comets in the inner Oort Cloud. The actual number is not known. But by using the maximum number possible, they determined that no more than two or three comets could have struck Earth during what is believed to be the most powerful comet shower of the last 500 million years.
"For the past 25 years, the inner Oort Cloud has been considered a mysterious, unobserved region of the solar system capable of providing bursts of bodies that occasionally wipe out life on Earth," Quinn said. "We have shown that comets already discovered can actually be used to estimate an upper limit on the number of bodies in this reservoir."
With three major impacts taking place nearly simultaneously, it had been proposed that the minor extinction event about 40 million years ago resulted from a comet shower. Kaib and Quinn's research implies that if that relatively minor extinction event was caused by a comet shower, then that was probably the most-intense comet shower since the fossil record began.
"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.
He noted that the work assumes the area surrounding the solar system has remained relatively unchanged for the last 500 million years, but it is unclear whether that is really the case. It is clear, though, that Earth has benefited from having Jupiter and Saturn standing guard like giant catchers mitts, deflecting or absorbing comets that might otherwise strike Earth.
"We show that Jupiter and Saturn are not perfect and some of the comets from the inner Oort Cloud are able to leak through. But most don't," Kaib said.
Source: Science and Eurekalert. The paper appears online today at the Science Express website.
Filed under: Astronomy, Comets
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July 30th, 2009 at 11:45 am
Its nice having a few huge bouncers looking after us! ;o)
July 30th, 2009 at 12:57 pm
Interesting!
"… the outer Oort Cloud, a remnant of the nebula from which the solar system formed …": I thought the Oort cloud was a secondary feature, populated by bodies ejected from the Kuiper Belt? Is 'outer' the key here?
I don't understand very well how giant planets act in this. Are we talking about their very-long-term, very-far-away effects? Because surely, once a comet has been sent on an inbound trajectory, it could only be noticeably deflected if it passes really close to that planet, the probability of which is close to 0 for any given comet. Or else, it might be affected once it is in an inbound orbit, by little nudges at each pass, but then it is an Earth-hazard, until disposed of.
July 31st, 2009 at 3:52 am
the origin of the oort cloud has garnered a fair share of controversy. not much has been agreed on, including even many of the basic facts such as its overall mass and distribution.
of course there is a general idea, but the debate is far from over. there is a probability that over the billions of years at least some material from the kuiper belt that migrated or was tossed out there is still there (due to special circumstances) however most of what is presently there is believed to be primordial in nature. this would be especially so with the "outer" oort cloud, as you pointed out.
as for your question about the gas giants, it would be easier to show you with a diagram but it has very much to do with the orbital plane and mass of the gas giants in question and their relationships in regards to earth's orbit and possible comet paths. maybe some other member(s) of the forum could explain it better than i can.
don't take this article too seriously. UT is cutting edge and they report the experiments as they become known, which is why we all love to read the site so much. this particular experiment was a computer sim and even the people who conducted the experiment admitted that they didn't know some of the variables. it's still quite interesting, but definitely not to be taken as law.
July 31st, 2009 at 5:23 am
Pvt.Pantzov: thanks a lot for this answer!
July 31st, 2009 at 10:07 am
Which btw also applies for extinctions and their causes.
I'm sure some "extinctions" really are substantiated by drop of numbers and/or diversity (and the subsequent explosion of both). But in some cases, such as the fabled "Earth galactic plane drift extinctions" they were found by more modern samples and statistical techniques to vanish.
[As much as the "drift theory" itself vanished when more modern samples and statistical techniques was applied to the actual plane and its arms instead of imagined, constrained, periodicity.]
Also, it is auspicious (or suspicious, take your pick
) that the likely earliest and greatest extinctions that could be responsible for the famous banded ironstone formations (BIF) are solely due to life itself oxygenating the atmosphere. Life as the main cause of extinctions would then IMHO be a likely null hypothesis.
IIRC the theory of paleontologist Robert Bakker is niche change and disease spread when sea and continental masses merge, which timing seems to fit some of the main events.
Actually, it is tested today by way of modern transportation standing in place of actual land and sea mass merging. And indeed such biological events seems to have serious repercussions. Why not, after all often the greatest threats to, and so most important environmental factors for, a population are other populations.