DOES Pluto have a wayward cousin lurking in the inner solar system? The dwarf planet Ceres – and other icy chunks – may have been born in the same realm as Pluto, but travelled all the way to the asteroid belt between the orbits of Mars and Jupiter. If so, it would be further evidence that a massive upheaval rearranged the early solar system.
At 950 kilometres in diameter, Ceres is by far the largest object in the asteroid belt. And that’s not the only reason it doesn’t quite fit in with many of its companions, according to William McKinnon of Washington University in St Louis, who presented his idea at the Asteroids, Comets, Meteors (ACM) meeting this week in Baltimore, Maryland.
McKinnon points out that Ceres has a low density, which suggests it is 25 to 30 per cent water ice. That’s a high proportion for an asteroid, but closely matches Pluto and other icy objects native to the outer solar system, known as trans-Neptunian objects (TNOs). What’s more, a dip in Ceres’s light spectrum may be a sign of ammonium-rich clay at the surface. This material has never been found in the fragments of asteroids that have fallen to Earth, but fits the expected ammonia-rich composition of a TNO.
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“Ceres has a high proportion of ice to rock for an asteroid, but a close match to Pluto and other objects of the outer solar system”
So if Ceres formed in Pluto’s neighbourhood, how did it end up 2 to 4 billion kilometres away? Some researchers think that the orbits of the planets were once unstable. According to this idea – known as the Nice model – Uranus and Neptune went rampaging through the outer solar system around 3.9 billion years ago (Â鶹´«Ă˝, 28 November 2006, p 40). As a result, many of the icy objects that formed in the outer solar system were pulled inward by the gravity of the two planets, and some ended up joining the rocky asteroids that were born in the asteroid belt. Ceres would simply be the largest of these immigrants. “The odds for this seem low, but it is not inconceivable,” says Bill Bottke of the Southwest Research Institute (SwRI) in Boulder, Colorado.
Bottke and Hal Levison of SwRI led a pair of studies also presented at the ACM meeting, which support the idea of refugees from the outer solar system orbiting in the asteroid belt. They focused on the so-called D and P-type asteroids that comprise 20 per cent of the population in the outer part of the belt. These objects are a dark reddish colour that suggests they are covered in carbon-rich gunk – just the sort of residue that might have been left behind on an icy object that had its outermost layers vaporised in the bright sunlight of the inner solar system. Bottke and Levison’s computer simulations show that the observed number of objects is about right if they are immigrants, though they have assumed many of them broke up after transport.
Thomas McCord of the Bear Fight Center in Winthrop, Washington, who was not involved in any of the three studies, agrees that the asteroid belt probably hosts some small refugees from the outer solar system, but says there is no reason to believe Ceres is a stranger there. Its ice-to-rock ratio matches the expected composition of the raw materials that would have been available at its current position early on, he says. What’s more, objects of its size are expected to have formed in the inner solar system. New measurements of Ceres’s composition by NASA’s Dawn mission, for which McCord is a team member, could help pin down its birthplace when the mission arrives in 2015.
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