Sedna, the most distant planetoid ever seen in the Solar System, probably got kicked into its orbit when a star swept past the Sun more than four billion years ago, suggest the first detailed calculations of the object鈥檚 origins.
The research supports the leading theory of Sedna鈥檚 origins but also leaves open more outlandish possibilities.
The planetoid, about three-quarters the size of Pluto, was discovered in November 2003. It takes about 12,000 years to traverse an elongated orbit that stretches from 74 to 900 times the distance from the Sun to the Earth. And its journey around the Sun is thought to take Sedna from its present location in the shadowy Kuiper Belt out towards the Oort Cloud at the Solar System鈥檚 outer edges.
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The Kuiper Belt is a mysterious band of rock and ice leftover from the birth of the Solar System, which lies beyond Neptune. The remote Oort Cloud forms a spherical shell of icy bodies around the Solar System and its edges lie many thousands of times Pluto鈥檚 distance from the Sun.
Sedna鈥檚 orbit is so extreme researchers say it could not have formed simply from the gravitational kicks of the giant planets, which are responsible for the eccentric orbits of the comets and Pluto.
鈥淚f this thing was scattered out by a planet, something else had to change the orbit, something we don鈥檛 see,鈥� says study co-author Hal Levison, an astronomer at the Southwest Research Institute in Boulder, Colorado. 鈥淭hat鈥檚 why Sedna and 2000 CR 105 [the next most-distant object] are so cool. They tell us something was different back when they formed.鈥�
Cluster of stars
Levison and colleague Alessandro Morbidelli of the Observatoire de la Cote d鈥橝zur in Nice, France, used computer simulations to study five different scenarios for how Sedna and 2000 CR 105 got their orbits.
The most likely scenario supports one of the theories put forward by Sedna鈥檚 discoverers. They believe the Sun was born in a cluster of stars, and that one or more of those siblings passed by the Sun in the stars鈥� first 100 million years.
The new study recreates Sedna鈥檚 orbit using this scenario. 鈥淚 still strongly favour that hypothesis,鈥� Sedna鈥檚 co-discoverer Michael Brown of the California Institute of Technology in Pasadena told 快猫短视频.
But the new study discounts Brown and his colleagues鈥� other main theory 鈥� that a planet lying at about 75 times the Sun-Earth distance is responsible for Sedna鈥檚 orbit. 鈥淚t鈥檚 still a possibility, but we haven鈥檛 found anything there so we don鈥檛 believe it so much these days,鈥� Brown concedes.
Brown dwarf
The study also quashes other theories, including the hypothesis that Neptune and Uranus, thought to have been in more eccentric orbits in the past, could have pushed Sedna and other bodies outward. Those planets are not massive enough to have done the job in their short eccentric phases, Levison says.
But the researchers thought up another improbable scenario that managed to explain Sedna鈥檚 orbit remarkably well. Sedna could have been born around a brown dwarf about 20 times less massive than the Sun and captured by our Solar System when the brown dwarf approached.
鈥淲hat鈥檚 striking about this idea is how efficient it is,鈥� says Levison, whose calculations suggest about half of the material orbiting the dwarf would have gone into orbit around the Sun. 鈥淓ven if it鈥檚 wrong it鈥檚 a cool idea.鈥�
鈥淚t just seems implausible, but that doesn鈥檛 mean it鈥檚 not true,鈥� agrees Brown.
The study is scheduled for publication in November 2004 in the Astronomical Journal.