Bad Astronomy

Buffy the theory slayer

Things just got weirder in the outer solar system.

There’s this big ole chunk of ice out there, with the designation 2004 XR 190 (it’s been nicknamed “Buffy”, which makes me wonder when they’ll start naming these objects after good shows– how about “Leela”, or “Cartman”, or “Stewie”?). It’s probably 500 to 1000 kilometers wide, making it fairly big (though smaller than Pluto). It was discovered during routine observations as part of the Legacy Survey, a project where large telescopes peer at big patches of the sky and look for all sorts of astronomical beasties, including these Kuiper Belt Objects.

What’s weird about this iceball is its orbit. Right now it’s 58 astronomical units from the Sun– 58 times the Earth-Sun distance, translating to about 8.5 billion kilometers, twice the distance of Pluto from the Sun. Now, most objects that far out have really elliptical orbits. They get that way, it’s thought, from getting too close to Neptune, and the planet’s gravity flings them way out into those elongated orbits. Others may have been affected by – get this – a passing star, millions or billions of years ago, whose gravity dropped the comets into weird orbits.

But (sigh) “Buffy’s” orbit isn’t elongated; it’s actually fairly circular. That’s pretty strange! It’s not thought that these objects form that far out, so something has to move them out there. But if that’s true, the orbit should be more elliptical.

And it gets weirder– the orbit is tilted, a lot. Compared to Earth’s orbit (which is pretty much the same plane all the planets orbit in), this guy – I guess it’s a gal given the name – is tilted by 47 degrees! It’s really hard to think of a mechanism that could both circularize the orbit and give it that huge tilt.

It’s possible the answer lies in the way the solar system has changed over hundreds of millions of years. Neptune and Uranus may have formed closer in to the Sun, and by gravitationally affecting comets, they moved out to their present positions. That takes a looooong time, so things probably haven’t changed much in the last eon or two. But things were different a long time ago. Maybe as those slow changes took place, something happened to put 2004 XR 190 (sorry, I can’t use the nickname again) in its circular but highly tilted orbit. Maybe it was a combination of two or more processes.

Whatever happened, it’s challenging our theories of how the solar system got its current configuration. And that’s a pretty good thing– scientists love to be challenged, love to have their hypotheses shown to have holes. That means there are more puzzles left to solve, more observations to make, and more time to think about the way things are and how they got that way.