Every time I think there is definitely no planet X, they pull me back in.

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FEATURE
PLANETARY SCIENCE
A distant planet may lurk far beyond Neptune
If Planet X exists, it may be anywhere from 250 to 1,000 times as far from the sun as Earth

THE LOST WORLD A hidden planet (foreground) might lurk in the uncharted realms of the solar system, leaving clues to its existence only in the orbits of dwarf planets or of chunks of ice in the outer edges of the Kuiper belt.


Magazine issue: Vol. 186 No. 11, November 29, 2014
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Out beyond Neptune, the solar system resembles the deep ocean: dark, remote and largely unexplored. To an Earth-bound observer, even the brightest objects, such as Pluto, are 4,000 times as faint as what the human eye can see. An undiscovered planet could easily lurk out there unnoticed, a possible fossil from a time when the giant planets jockeyed for position 4 billion years ago, scattering planets and asteroids in their wake. But even the largest telescopes would struggle to find such a faint spot of light. Most likely, the clues would be entangled in the distorted orbits of faraway ice boulders tumbling around the sun.

Astronomers Chad Trujillo and Scott Sheppard provided a hint about how such a world might reveal itself last March when they announced the discovery of a 450-kilometer-wide dwarf planet just outside the Kuiper belt — the icy debris field past Neptune (SN: 5/3/14, p. 16).

Their find, designated 2012 VP113, is on a course that loops around the sun in a vastly elongated orbit far from the known planets. It has thousands of neighbors but shares its odd trajectory only with Sedna, another dwarf planet, discovered in 2003.

“They’re kind of in a no-man’s-land,” says Sheppard, of the Carnegie Institution for Science in Washington, D.C. “These objects couldn’t get out there with what we currently know.”

Something had to drag the two dwarf planets from their original, smaller orbits. Except nothing is close or massive enough to take the credit. At least, nothing astronomers are aware of.

The discovery of 2012 VP113 confirmed that Sedna is not a fluke but is possibly the first of a large population of icy bodies distinct from others in the rest of the solar system. So Trujillo and Sheppard continued to poke around the Kuiper belt, and the mystery deepened. They noticed that beyond 150 astronomical units (150 times the distance from the sun to the Earth), 10 previously discovered objects, along with Sedna and 2012 VP113, follow orbits that appear strangely bunched up.

“That immediately piqued our interest,” says Sheppard. Could an unseen planet, a Planet X, be holding the orbits of all these far-out bodies in place?

“The idea’s not crazy,” says David Jewitt, a planetary scientist at the University of California, Los Angeles. “But I think the evidence is slim.” The trail of bread crumbs leading to an undiscovered planet is sparse: just 12 chunks of ice lead the way. But it’s enough to get some researchers wondering about a ninth (or 10th, depending on your attitude regarding Pluto) planet roaming the outer solar system and how it might have arrived there.