Scientist finds medium sized Kuiper belt object less dense than water

(Phys.org) —Michael Brown, a planetary scientist with California Institute of Technology, has found a medium sized object in the Kuiper belt (dubbed 2002 UX25) that doesn't appear to conform to theories of how such objects came to exist. In his paper to be published in Astrophysical Journal Letters, Brown notes that the mid-sized object appears to be less dense than it should be if it followed conventional thinking that suggests the larger the objects are in the belt, the more dense they should get.

The Kuiper belt, is of course, a group of rock-like objects (comets, dwarf planets, etc.) orbiting the sun that lie farther out than Neptune. Such Kuiper belt objects (KBOs) are believed to have formed in ways similar to the way planets did, i.e. due to accretion of material over time. Conventional theory suggests that small KBOs are less dense than water because of their porous nature—large KBOs grew more dense as they grew larger due to gravity causing them to compact. If the theory is correct medium size KBOs should have medium density. But this new KBO that Brown has found doesn't conform to the theory at all, instead, its density is roughly the same as smaller KBOs, suggesting that it's not size that determines KBO density, but something else. And right now, Brown notes, nobody knows what that something else might be.
2002 UX25 has a diameter of roughly 650 kilometers, putting it squarely in the mid-size KBO category, and it, like other KBOs, is believed to exist in very nearly the same state it's held since the formation of the solar system. It's in studying such objects that scientists learn more about how everything in our solar system came to be the way it is. Until now, most scientists agreed that KBOs of a size smaller than 350 kilometers across had a density less than that of water, whereas bigger ones had a greater density. That theory might have to be changed however as 2002 UX25 is the first medium sized KBO to have its density measured and it clearly doesn't conform.

The discovery of 2002 UX25's density properties has already led to new theories, Brown notes, with some suggesting that scientists have been wrong to assume that KBOs and the planets formed at the same time. Instead, they suggest, that it's possible that KBOs came first and afterwards as the planets were forming, eddies formed causing KBOs to knock into one another breaking them into different sized pieces.

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