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Planet Nine
An artist’s conception shows Planet Nine with the sun in the far background. (Credit: R. Hurt / IPAC / Caltech)

Astronomers haven’t yet seen Planet Nine, the theoretical world that some say lies far beyond Pluto’s orbit, but they’re seeing more phenomena that could be explained by its existence.

Today, astronomers laid out evidence that the undiscovered planet may be responsible for twisting the main plane of our solar system about 6 degrees off-kilter with respect to the sun.

In a paper accepted for publication in the Astrophysical Journal, astronomers say Planet Nine’s gravitational influence could disrupt planetary orbits to account for that much of a tilt.

Based on a close analysis of the movements of celestial objects in the Kuiper Belt, a broad ring of icy material at the solar system’s edge, astronomers proposed in January that an unseen planet exists at an average distance about 20 times farther away from the sun than Neptune.

Their calculations suggested that the planet, which they nicknamed Planet Nine, should be 10 times bigger than Earth and inclined somewhere around 30 degrees from the ecliptic plane.

“Because Planet Nine is so massive and has an orbit tilted compared to the other planets, the solar system has no choice but to slowly twist out of alignment,” Caltech’s Elizabeth Bailey, the study’s lead author, said in a news release.


A different team of researchers reported that the extremely eccentric orbits of four objects in the Kuiper Belt with the longest known orbital periods were best explained as the result of Planet Nine’s gravitational influence.

“We analyzed the data of these most distant Kuiper Belt objects and noticed something peculiar, suggesting they were in some kind of resonances with an unseen planet,” the University of Arizona’s Renu Malhotra said in a news release.

Such resonances would help keep the Kuiper Belt objects in stable orbits.

Malhotra and her colleagues stressed that their findings, published in the Astrophysical Journal Letters, shouldn’t be regarded as proof that Planet Nine exists. But they said their data could help point to areas of the sky where the planet might be found.

Both studies were presented in Pasadena, Calif., during this week’s joint meeting of the American Astronomical Society’s Division for Planetary Sciences and the European Planetary Science Congress.

Meanwhile, the quest to spot Planet Nine continues. Caltech’s Mike Brown, who participated in the discovery of several dwarf planets as well as the calculations for the Planet Nine hypothesis, says it may take three years or more to complete the sky survey.

Elizabeth Bailey, Konstantin Batygin and Mike Brown are the authors of “Solar Obliquity Induced by Planet Nine.” Renu Malhotra, Kathryn Volk and Xianyu Wang are the authors of “Corralling a Distant Planet with Extreme Resonant Kuiper Belt Objects.”

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