Quaoar, a large object in the outer solar system, is mostly made of rock, unlike its icy neighbours (Illustration: NASA/G. Bacon/STScI)

A giant rock is walking among the “dirty iceballs” in the outer solar system, a new study suggests. Researchers say it may have journeyed there from the asteroid belt near Mars, or it may have been the victim of a cosmic crash that blasted away its once-icy exterior.

Quaoar was discovered in 2002 in the Kuiper belt, a ring of icy bodies beyond Neptune. At about 900 kilometres across, or 40 per cent as wide as Pluto, it is not the biggest denizen of the belt, but researchers now say it may be the densest.

Wesley Fraser and Michael Brown of Caltech confirmed its size by studying archival images from the Hubble Space Telescope. They also used Hubble images to study the motion of its moon, Weywot, which allowed them to calculate Quaoar’s mass.

Combining the size and mass revealed Quaoar’s density to be between 2.9 and 5.5 grams per cubic centimetre. That is much higher than that of other Kuiper belt residents like Pluto, which has a density of about 2.0 grams per cubic centimetre.


Quaoar’s high density suggests it is made almost entirely of rock, unlike its neighbours, which are a mixture of ice and rock, the researchers conclude. They say the rocky world may be a refugee from the asteroid belt between Mars and Jupiter, thrown outwards early in the solar system’s history, when the orbits of the giant planets are thought to have shifted.

‘Far-fetched’ scenario

Previously, other researchers have suggested that the same upheaval threw some Kuiper belt objects into the asteroid belt, so the new study suggests the migration may have been a two-way street.

But Renu Malhotra of the University of Arizona in Tucson says that hurling Quaoar from the asteroid belt to the Kuiper belt would have left it with an elongated orbit, making it hard to explain why Quaoar is on a nearly circular orbit today.

“I think that’s pretty far-fetched,” she says.

She favours the other possible explanation that Fraser and Brown suggest – that a collision with another Kuiper belt object blasted off most of Quaoar’s ice, leaving behind only its dense, rocky core.

“That kind of thing seems a lot more possible to me,” she says, noting that there were probably far more objects in the early Kuiper belt than there are today, making collisions more common in the past.

Journal reference: Astrophysical Journal Letters (in press)