Posted: August 18, 2008
An ice-rock minor planet 30 to 60 miles in diameter, discovered two years ago between the orbits of Uranus and Neptune (each being a mean distance of 2.72 and 4.35 billion kilometres from Earth respectively) could be a member of the ‘inner Oort Cloud’. But this is only part of the story as the object, dubbed 2006 SQ372, is currently at perihelion (the point where it’s closest to the Sun) on a highly elliptical orbit that will see it sail right out to nearly 1,600 astronomical units (an astronomical unit, or AU, being the distance between the Earth and Sun — 149,598,000 kilometres). That is 40 times the distance out to Pluto, or 239 billion kilometres. It will return in 22,500 years.
The orbit of 2006 SQ372 (light blue) compared with that of Sedna (red). Inset shows these orbits compared with that of the giant planets (white) and Pluto (green). Image: Nathan Kaib, University of Washington (click for larger image).
2006 SQ372, first discovered by a team from the University of Washington using Sloan Digital Sky Survey data (SDSS), is comparable to Sedna — another minor planet that has a highly elliptical orbit. Sedna’s eccentricity is 0.855 and its perihelion and aphelion positions are 76 and 975 AU respectively. The orbit of 2006 SQ372 has an eccentricity of 0.976 and its perihelion and aphelion distances are 24 and 2,010 AU. The ellipse of 2006 SQ372 is four times longer than it is wide and crosses the orbits of Pluto and Neptune. For comparison, Earth’s orbital eccentricity is 0.0167 and its perihelion and aphelion distances 0.983 and 1.017 AU respectively.
Given the small size of the object in comparison with Sedna (1,000 kilometres), how were the University of Washington team able to find it? Team member Lynne Jones says, “If you can find things that explode, you can find things that move.” This is a clue to the team’s ingenious solution: they adapted algorithms used in the search for supernova explosions and examined images covering an area 1,000 times larger than the full moon. “The only objects close enough to change position from one night to the next are in our own Solar System,” Jones adds. The team, led by Andrew Becker (also of the University of Washington), presented their work today at an international symposium held in Chicago, entitled, ‘The Sloan Digital Sky Survey: Asteroids to Cosmology’.
The Öpik-Oort Cloud (first proposed by Estonian astronomer Ernst Öpik in 1932 and then revived in 1950 by Dutch astronomer Jan Oort) is the idea that a vast cloud of icy-rocky cometary bodies surrounds the whole Solar System. This cloud extends three light years out from the Sun (nearly three-quarters the distance to the nearest star, Proxima Centauri) but is thought to start one light year away. The Oort Cloud is the origin of long period comets, though 2006 SQ372 isn’t a comet, as it never gets close enough to the Sun to form an ionisation tail. Nevertheless, this latest minor planet could be from the very inner edge of the Oort Cloud, and as team member Nathan Kaib says, “It's exciting that we are beginning to verify these predictions. One of our goals is to understand the origin of comets, which are among the most spectacular celestial events. But the deeper goal is to look back into the early history of our Solar System and piece together what was happening when the planets formed.” And concurring with his colleague, Becker says, “There are bound to be many more objects like this waiting to be discovered by the next generation of surveys, which will search to fainter levels and cover more area. In a decade, we should know a lot more about this population than we do now.”
SDSS image showing the position of 2006 SQ372 changing over the course of a few days. Image: Andrew Becker, University of Washington, and the SDSS team (click for larger, animated image).