This Nature report doesn't say it, but another one in The Guardian ends with the possibility that a very large dark planet is way out there too:
The latest work has already thrown up an intriguing possibility. The angle of the body's orbit and that of Sedna's are strikingly similar, an effect most likely caused by the gravitational tug of another, unseen body. One possibility is a "Super Earth" that traces so large an orbit around the sun that it has never been seen.Great news for hard science fiction writers, I am sure.
"If you took a SuperEarth and put it a few hundred astronomical units out, the gravity could shepherd Sedna and this new object into the orbits they have," said Sheppard. An astronomical unit (AU) is around 150m kilometres, or the mean distance from Earth to the sun.
Earlier this month, Nasa'sWide-Field Infrared Survey Explorer (Wise) reported the results from its search for "Planet X", a hypothesised planet far out in the solar system. It found no evidence for a new planet larger than Saturn within 10,000 AU of the sun. But Saturn is 95 times more massive than Earth, so a smaller Super Earth could go undetected in that region.
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