The discovery of a dwarf planet leads to speculation that another rogue planet exists beyond it.

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Scientists say they've found a pink dwarf planet that travels farther from the sun than anything ever seen. Its discovery has led to tantalizing hints that a separate, unknown planet bigger than the Earth lurks at the fringes of the solar system, exerting a stealthy control over its neighbors.

The new planet is a celestial bonbon, a dainty, rose-colored ice ball far smaller than tiny Pluto. Its detection helps confirm that the outer stretches of the solar system are a crowded place. An analysis of its orbit and the orbits of other heavenly bodies provides evidence for a super-sized planet at least 200 times as far from the sun as the Earth is.

The analysis, if correct, shows "there needs to be another planet out there, which is really amazing," says Scott Kenyon of the Smithsonian Astrophysical Observatory, who was not involved in the new research. "Adding an Earth-mass planet … will change a lot of people's thinking."

The possibility of a new super-Earth arose from scientists' campaign to find objects beyond the orbit of Pluto, in some of the chilliest, most distant reaches of the solar system. The only known body in that no-man's land is the dwarf planet Sedna. Found in 2003, it's twice as far from the sun as any other object detected in the solar system.

A decade later, researchers spotted Sedna's first known neighbor. It's officially called 2012 VP-113, but the "VP" inspired the nickname "Biden." Unlike its namesake, it's petite, measuring roughly 20% as wide as Pluto. Biden travels a cold, remote path, coming no closer to the sun than 7.4 billion miles, vs. Sedna's closest approach of 7.1 billion miles. Calculations show Biden and Sedna must have roughly 900 good-sized undiscovered neighbors, scientists report in this week's Nature.

After the discovery of Sedna, "finding the second object shows there is a big population out there," says study co-author Scott Sheppard of the Carnegie Institution for Science. "Our solar system has a very rich inventory of planetesimals, and we've only just touched the tip of the iceberg here in finding these objects."

Something strange emerged when Sheppard and his colleague, Chadwick Trujillo of the Gemini Observatory, compiled data for the orbits of Sedna, Biden and 10 icy objects that lie close to Pluto. Similarities in the 12 objects' paths around the sun strongly suggest that all 12 are under the spell of a hulking, far-distant planet, Sheppard says.

This planet, if it exists, is up to 10 times as massive as Earth and 200 to 1,000 times as far from the sun. It's so big that it has forced the smaller bodies, which like to keep away from large ones, into unexpected paths. This super-Earth may have begun its life in the central solar system before becoming a "rogue planet," ejected from its birthplace to exile in the outermost solar system. Like some celestial Pied Piper, it may have pulled Sedna and Biden along with it from their homes closer to the sun.

Other scientists disagree on the plausibility of a large planet haunting the distant stretches of the solar system.

"It's a fun possibility to consider, but I would say it's pretty speculative," says Sedna co-discoverer Michael Brown of the California Institute of Technology. He expects researchers will "come up with 100 different, more mundane reasons why this arrangement could be like that."

Perhaps the data could be explained by coincidence, says Harold Levison of the Southwest Research Institute in Colorado. Levison says that although he remains to be convinced, this proposal for a rogue planet is "surprising and almost believable." His reaction to previous reports of rogue planets has always been "b.s.," Levison says. "I've never been seduced by any of those arguments, except this one. … If that data's right, I can't think of any other way to explain it."

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