Kuiper Belt object QuaoarFAJARDO, Puerto Rico—It smacked of a cunning plan. The organizers of last week's planets conference put one of the best talks in the very last session of the very last day. Most scientists had either left for the airport or the beach. I almost didn't make it myself—the room and time got switched at the last minute.

If the speaker, Wesley Fraser of the California Institute of Technology, is right, planetary scientists are going to have to rethink the Kuiper belt—the vast band of smallish planets that orbit beyond Neptune. Using the Hubble Space Telescope, Fraser and his colleague Mike Brown observed one of the largest of these objects, Quaoar, and its little moonlet, Weywoot, to refine estimates of its size and mass. They found that Quaoar is smaller than previously thought, only 900 kilometers in diameter. Consequently, it must be denser—about four grams per cubic centimeter. This makes it by far the densest Kuiper belt object (KBO). It outdoes even a fairly dense asteroid such as Vesta.

Planetary scientists typically explain dense KBOs by imagining that they started off larger and less dense; they then collided with one another, stripping off lighter material such as ice and leaving behind mostly rock. Yet by the new estimate Quaoar is denser even than rock, so even completely stripping its ice wouldn't be enough. Moreover, astronomers detect ice on its surface. It makes no sense. "It's confusing, to say the least," Fraser says.

That's only half of it. Most KBO moonlets have circular orbits. That is thought to be a consequence of coalescing from collisional debris: debris rings naturally settle into a circular shape. Yet Weywoot's orbit is distinctly oblong. "We have to completely rewrite the book," Fraser concludes.

One radical idea is that Quaoar is a refugee from the asteroid belt between Mars and Jupiter. Asteroids tend to be denser than KBOs, and Jupiter might conceivably flick one to the farthest reaches of the solar system. But the hapless exile would tend to wind up on a highly elliptical orbit around the sun, whereas Quaoar's orbit is nearly circular.

Erik Asphaug of the University of California, Santa Cruz, offered another explanation based on work he presented earlier in the week. Perhaps Quaoar collided with a much bigger body—something approaching the proportions of Mars. Like a Mini blindsided by a Hummer, Quaoar would have gotten seriously banged up by such a collision, maybe enough to give it an anomalously high density. If so, Quaoar might be living proof that substantial planets used to orbit in the distant solar system and may still lurk out there.

Clearly this is one of those "more data are needed" situations, and Fraser has applied for additional telescope time. One thing is sure, though. People typically call KBOs icy bodies, like giant comets. But they can also be rocky, like small Earths.


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Photo of Quaoar from the Hubble Space Telescope: NASA and M. Brown (Caltech)