Published online 6 October 2010 | Nature | doi:10.1038/news.2010.522

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Diversity of objects and frequency of pairs not easily accounted for by standard models of planet formation.

The rate of Kuiper belt objects found in loosely bound couples makes it difficult to explain how Neptune could have flung these remote objects into ther present orbits without also splitting them up. Alex Parker, University of Victoria

The cold and shadowy fringe of the solar system known as the Kuiper belt is generating increasing debate among scientists as data accumulates on the growing population of objects discovered there. Now, two new studies of Kuiper belt objects presented 5 October at a meeting of the American Astronomical Society's Division for Planetary Sciences in Pasadena, California, may reveal a crucial hole a prevailing model of the solar system's early history.

One of the scientists challenging established theory is Michael Brown of the California Institute of Technology in Pasadena. Rather than growing incrementally from small precursors, as has been conventionally believed, he argues the largest Kuiper belt objects formed in a series of collisions between objects of roughly equal size — a process Brown describes as "pyramidal growth".

Evidence for this, Brown says, comes from recent discoveries that large Kuiper belt objects, which can reach diameters in excess of 2,000 kilometres, have widely disparate densities. Some seem to be comprised almost entirely of rock, with densities as high as 3.0 grams per cubic centimetre. Others have densities so low they appear to be almost entirely water ice punctuated with void spaces. Pluto, the best-known Kuiper belt object, lies midway between these extremes, with a density of about 2.0 grams per cubic centimetre.

If all these bodies accreted from multitudes of small precursors, Brown says, they should all have densities representing the average composition of the protoplanetary nebula in which they formed. "To get something that large, you would have had to accrete from a very large swath of the outer solar system," he said. "You would think they would be some of the most uniformly composed objects in the solar system." Instead, there "is about as big a variation as you can get."

Brown believes that this wide variation is a sign that the biggest Kuiper belt objects were produced not from gradual accretion, but from a small number of collisions among large objects, beginning with ones on the order of 500 kilometres in diameter. In each of the collisions, most of the mass stuck together to form a new, larger object, with rest blowing off into space - the amount varying with the size and power of each impact.

Because the blown-away material is primarily ice, this means that some large Kuiper belt objects could have been built from a small number of really big collisions among increasingly ice-depleted bodies, while others might have been formed from smaller, less powerful collisions that allowed more ice to remain. "You can have different combinations," he said.

Neptune not guilty

Brown's argument that current theory is lacking was bolstered by a study presented by Alex Parker, a graduate student in astronomy at the University of Victoria in British Columbia, Canada.

Parker examined binary objects in the cold classical Kuiper belt, a region on the edge of the Solar System, 6 to 7 billion kilometres from the Sun.

Traditional theory finds it hard to explain how these objects formed there because, that far out, their accretion would have been too slow to have been finished during the known life of the solar system. Thus, in a theory known as the Nice model (for Nice, France, where it was first proposed), scientists suggested that these objects formed closer to the Sun, where faster growth was possible. Then they were flung outward by dramatic shifts in the orbits of the outer planets, most importantly Neptune.

But there's one problem with that theory, according to Parker. About 30 percent of these objects are binaries. "The most famous are Pluto and Charon, but there are many others," he says.

Painstaking studies of their slow-motion orbits, in which they can take 4 to 17 years to complete a circuit of each other, reveal that many are so far apart that they are very loosely bound — loosely enough that any interaction with Neptune would have sent them flying in different directions.

"So if the Kuiper belt was subjected to this violent event, these [binary] systems should have been destroyed," Parker says.

Stephen Tegler of Northern Arizona University in Flagstaff agrees that Parker's finding forces theoreticians back to the drawing board. "We have got to come up with a way to tweak the Nice model, or they formed in situ," he says.

But, Tegler notes, "It's a brand new idea and it's up to the rest of us to check this out — to confirm or refute it."

Big hunks falling

One reason for caution is that for Kuiper objects to have emerged in situ would require them to have formed more quickly than traditional theory predicts.

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It's here that Brown's and Parker's findings support each other. That's because one way Kuiper belt objects might have formed in situ, Parker says, is under an emerging model of planet formation in which turbulences and vortexes in the protoplanetary nebula allow many tiny particles to coalesce extremely rapidly into big ones.

Brown refers to this as "big hunks falling out of the nebula" and thinks it's a way in which large building blocks might have been formed as starting material for his 'pyramidal growth' model. He cautions, though, that the new theory is in its infancy. "No one has really put all the pieces together yet," he says. But, he adds, "We could well be completely rewriting how planets form."