It's a tantalizing possibility.

Rogue One

For over a century, researchers have been puzzled by strange gravitational forces seemingly pulling on the orbits of Neptune and Uranus in the distant reaches of our solar system.

While some have suggested that a ninth planet lurking in the Kuiper Belt — often dubbed "Planet X" — could account for the strange behavior, others have pushed back against these theories.

But now, an international team of researchers has come up with a new explanation: such a planet-sized object could instead be lurking in the Oort cloud, a spherical region surrounding the solar system that is suspected to be home to a collection of icy objects stretching several times farther from the Sun than the Kuiper Belt.

Alley Oort

In a new paper accepted for publication in the journal MNRAS Letters, the team suggests that larger pieces of debris could've clumped together to form a planet-sized object billions of years ago that could've been sent hurtling out into the Oort cloud.

Alternatively, an exoplanet from another system could've gotten stuck in the solar system's Oort cloud, according to the researchers, the more likely outcome.

"Dynamical instabilities among giant planets are thought to be nearly ubiquitous, and culminate in the ejection of one or more planets into interstellar space," the paper reads. "We find that a fraction of planets that would otherwise have been ejected are instead trapped on very wide orbits analogous to those of Oort cloud comets."

Lurker in Darkness

But, as the scientists are the first to admit, neither theory is enormously likely: they estimate an 0.5 percent chance that the Sun could've sent a planet out to the Oort cloud, and a seven percent chance that it could have captured a Neptune-like planet after it was shot out of its home star system.

It's a tantalizing theory that's bound to draw scrutiny. Could there be more planets lurking in the dark, mingling with icy comets? As NASA points out, no mission has yet been sent to explore the Oort Cloud — though there's still a chance Voyager 1 and Voyager 2 may eventually pop in for a visit.

More on Planet X: Rogue World: The Solar System's Mysterious Ninth Planet

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