A few weeks ago I learned of a curious discovery out beyond Neptune. The object itself, dubbed 2008 KV42, was no bigger than about 30 miles (50 km) across — no threat to Pluto and Eris as the new king of the Kuiper Belt.
What caught my attention was the inclination of its orbit: 104°. In other words, the path it takes around the Sun is tilted up and over so much that the motion is more "backward" than "forward." Astronomers sometimes term this a retrograde orbit.
It's the first known retrograde trans-Neptunian object, but it didn't seem like that big a deal at the time. After all, 17 asteroids are similarly headed the wrong way around the Sun.
But today, at a meeting of the American Astronomical Society's Division for Planetary Sciences, I learned that 2008 KV42 might be more archetype than oddball. Brett Gladman, a member of the discovery team, explained that this find could be the "missing link" in a long-running cometary conundrum. (I also learned that he and his observing buddies have nicknamed their find "Drac," because Dracula and other vampires purportedly could walk on walls.)
Gladman, J. J. Kavelaars, and Jean-Marc Petit found it on May 31st while trolling for just such high-inclination objects using the Canada-France-Hawaii Telescope atop Mauna Kea in Hawaii.
Comets follow one of three general paths as they plunge toward the Sun. Jupiter-family comets glide in close to the major planets' orbital plane. Most eventually encounter Jupiter and become trapped in tighter, short-period paths around the Sun. The thinking goes that they must originate either from the "classical" Kuiper Belt, a loose disk of bodies that lurk from Neptune's orbit out to about 55 astronomical units (a.u.), or from what are termed "scattered-disk objects," which have eccentric but low-inclination orbits.
Nearly isotropic comets, meaning they come screaming in from pretty much any direction, originate in the outer Oort cloud at least 10,000 a.u. from the Sun. Finally, Halley-type comets, named after the most famous ice ball of them all, have orbits that are highly inclined and often retrograde (as Halley's is).
Despite their best efforts, dynamicists have yet to puzzle out how the Halley-types ended up so skew to the rest of the solar system. Computer models that simulate long-term orbital evolution haven't been able identify a source region in either the Kuiper Belt or the more distant Oort cloud.
Gladman has a hunch that 2008 KV42 might provide some clues. Its average distance from the Sun is 32 a.u. and comes its closest near the orbit or Uranus. So does 2002 XV93, which is inclined steeply at 77°. But Uranus doesn't have enough mass to have yanked these so far up, Gladman says. So, for now, he's headed back to the dynamical drawing board.