spaceSpace and Physics

Mid-Sized Planetary Body in Kuiper Belt Could Float In Water

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Lisa Winter

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139 Mid-Sized Planetary Body in Kuiper Belt Could Float In Water
NASA/JPL-Caltech/T. Pyle (SSC)

Planetary scientist Michael Brown from CalTech has measured the density of a planetary body in the Kuiper Belt that is over 400 miles (650 km) wide - and he found that it has a lower density of than water. This is the largest rock discovered in the solar system which would be capable of floating. Brown published the results in The Astrophysical Journal Letters, which appeared online this week.

The body is known as 2002 UX25 and may shake up current theories about how planetary bodies are formed, as it is the first mid-sized body measured for density. Traditionally, it has been believed that planetary bodies were formed by bits of dust whipping around the sun in its infancy. Eventually, the dust particles slammed into one another to form larger rocks. It has been thought that all of the rocky planets formed this way, along with all of the planetary bodies in the Kuiper Belt at the edge of the solar system.


However, if that model was entirely correct then all of the bodies in the Kuiper Belt should have roughly the same density. In fact, smaller bodies are reported to be less dense than water. This was thought to have been reconciled by the idea that the larger rocks have a greater gravitational pull and bring the dust in closer and tighter. Since 2002 UX25 is mid-sized and has a density more similar to a small body instead of one halfway between the small and large as would be expected, there might be something else going on.

Because 2002 UX25 only has a density of about 0.8 grams per cubic centimeter, it is assumed that it contains a great deal of ice. This allowed other planetary scientists to propose a new theory of planetary formation: the solar system was made from the outside in. As the dust flew around creating small rocks, it collected a lot of ice. Larger rocks that had been hit many times had some of the ice knocked out of them, creating a more dense, larger rock. The largest of the proto-planets were drawn inward toward the sun, while the icy remnants remained out at the Kuiper Belt. 

It’s an interesting thought, but several other 2002 UX25-sized planetary bodies will need to be measured before strong assertions of a new theory for formation can be made.


spaceSpace and Physics
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  • kuiper belt,

  • 2002 UX25,

  • planetary formation,

  • density