After a decade of searching, astronomers have found a second dwarf-like planet far beyond Pluto and its Kuiper Belt cousins, a presumed no-man's land that may turn out to be anything but.
How Sedna, which was discovered in 2003, and its newly found neighbor, designated 2012 VP 2113 by the Minor Planet Center, came to settle in orbits so far from the sun is a mystery.
Sedna comes no closer than about 76 times as far from the sun as Earth, or 76 astronomical units. The most distant leg of its 11,400-year orbit is about 1,000 astronomical units.
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Newly found VP 2113's closest approach to the sun is about 80 astronomical units and its greatest distance is 452 astronomical units. The small world is roughly 280 miles (450 kilometers) wide, less than half the estimated diameter of Sedna.
Neither body can be explained by the present structure of the solar system, with its four rocky planets, four outer gas giant planets, disk of small icy Kuiper belt objects beyond Neptune and the comet-rich spherical Oort Cloud located some 10,000 times farther from the sun than Earth.
"Something else earlier on in the history of the solar system had to put them on these orbits," astronomer Megan Schwamb, with Academia Sinica in Taipei, Taiwan, wrote in an email to Discovery News.
One theory is that a sister star to the sun, jointly incubated in a long-disbanded stellar nursery, gravitationally nudged some of the bodies in the Oort Cloud inward, creating a new and stable orbital subdivision.
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Option two is that another planet at least as big as Earth got booted out of the solar system, taking with it a clutch of Kuiper Belt objects into far more distant orbits.
That renegade planet could have left the solar system – or may still be in orbit today.
Astronomers suspect several Earth-mass sized planets formed in the giant planets region.
"What happened to those objects – if they collided with the giant planets and eventually became a part of the giant planets, or if they got ejected – is not known," astronomer Scott Sheppard, with the Carnegie Institution of Washington DC, told Discovery News.
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Some computer models show it was another giant planet that got expelled from the solar system, pulling bodies as it went.
"The bigger the object, the more planets it could pull out to this region," Sheppard said.
The search for other Sedna-like objects continues. Sheppard and colleague Chad Trujillo with the Gemini Observatory in Hawaii, are working this week to try to confirm six more potential objects discovered last year.
Several observations over a period of at least a year are needed to detect the slight motion of the small, dim targets against background stars.
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"It's like looking out your car window while you're driving on the highway. Things that are very close move very quickly with respect to the background things and things that are more distant, like mountains, move slowly. The technique is to basically to look for objects that move with respect to the background stars, but move very slowly. The slower they move, the farther away they are," Trujillo said.
Ultimately, scientists suspect the population of Sednas could outnumber their Kuiper Belt cousins, making them the second largest collection of bodies after the planets that are in stable orbits around the sun.
The research appears in this week's Nature.