Halley's comet, which lights up Earth's sky every 75 years
with its glowing tail, is a bit of a scientific mystery.
So far theories have been at a loss to explain how it
acquired its extremely unusual backwards orbit, but the recent discovery of
another odd comet orbiting farther out in the solar system may shed light on
Halley's origins.
The newly-discovered comet 2008 KV42 circles the sun at a
tilt of 104 degrees compared to the main plane in which most of the planets and
asteroids travel. The newfound oddball also orbits in reverse compared to
almost everything else. Scientists think it might represent an intermediate
point between comets like Halley's and their progenitors in the far and totally
uncharted reaches of the solar system.
"The big mystery's been, how do you get comets like Halley's comet?"
said astronomer JJ Kavelaars of the National Research Council of Canada, who
worked on the team that discovered KV42. "It's one of the most famous
comets known and we have no dynamic explanation for how it got into its orbit,
and how it got to be there. Now we've found an object that could provide a
source for a Halley-type comet."
KV42 was discovered at about 32 times the distance from the
Earth to the sun, and the closest it swings to the sun is about where Uranus
is. Researchers suggest the comet may have originated out in the distant Oort
cloud of objects theorized to swarm in a sphere around the solar system
almost 1 light-year out from the sun, or roughly a quarter of the way to the
next nearest star.
At some point, they think, gravitational interactions (such
as with Uranus or Neptune, or even a nudge from another star in the galaxy
passing nearby the sun long ago) could have kicked the comet from its perch in
the lower level of the Oort cloud, about a tenth of a light-year away, down to
where it is now, orbiting beyond Neptune in a region known as the Kuiper Belt,
where several other normally-orbiting rocky objects have been found.
This would explain how it got such a bizarre backward, or
retrograde, and tilted orbit. If it originated in the spherical Oort cloud, the
comet could develop an orbit with any tilt at all, as opposed to most comets,
which originate in the plane of the solar system, so must have an orbit in line
with the planets.
Eventually, researchers predict KV42 could get kicked down even
closer to the sun by interactions with the outer planets and the solar system's
heavyweight, Jupiter. Ultimately, both Halley's and KV42 are likely to end up
slingshot out of the solar system altogether from gravitational interactions
with the planets.
Until now, physical models haven't been able to come up with
an explanation for a comet like Halley's, but the researchers hope the
discovery of KV42 will allow physicists to plug a new half-way point into their
models to derive a reasonable evolutionary path for these objects.
"An orbit like this provides the road sign that says
there must be a source of objects which could very plausibly be something
related to the Oort cloud, which is feeding in slowly over the age of the solar
system," research team member Brett Gladman of the University of British
Columbia told SPACE.com. "It's a pointer hopefully to a coherent model."
The researchers presented their discovery at the 10th
"Asteroids, Comets and Meteors" meeting in Baltimore in July 2008.
They first observed KV42 with the Canada-France-Hawaii Telescope in Hawaii, and
made follow-up observations on the MMT telescope in Arizona, Cerro Tololo
Inter-American Observatory (CTIO) 4-metre telescope in Chile, and Gemini South
telescope of Canada's Gemini Observatory, also in Chile.
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