A new paper published in the Astronomical Journal suggests that the unusual tilt of some Kuiper Belt objects could be explained by a Mars-sized planet lurking in the outer solar system. To be clear, this hypothetical planet-sized mass is not the same thing as the hypothesized Planet 9 that we’ve discussed at several points over the past year. It’s difficult to find a single diagram that shows all of these areas in proper relation to each other, so we’ll have to split this up a bit:

The image above shows the hypothetical Planet 9 as well as a group of Trans-Neptunian Objects. None of these are considered to be Kuiper Belt objects, all of which sit considerably closer to the sun. Sedna, for example, has an orbit that takes it from 76 AU at perihelion (its closest position to the sun) to 936 AU at aphelion (its farthest point from the sun). Pluto, which is now classified as the largest Kuiper Belt object, has a perihelion of 29.7 AU and an aphelion of 49.3 AU.

The Kuiper Belt can be seen here, as the large cloud of objects that exist near and beyond the orbit of Pluto (shown in yellow). As you can see, this large cloud appears to generally exist in the same plane as the rest of the solar system. The invariable plane of the solar system is defined as the plane passing through the center of mass of a planetary system. It is perpendicular to the angular momentum of the solar system.

In our system’s case, Jupiter is responsible for most of the invariable plane (its measured value is within 0.5° of Jupiter’s orbital plane). Most of the objects in the Kuiper Belt are similarly inclined towards the invariable plane of the solar system, but a few of the most distant objects aren’t. (Pluto’s own moderately inclined orbit should be considered a special case and is influenced-by and in alignment with Neptune).

What this new paper, by Kat Volk and Renu Malhotra of the University of Arizona’s Lunar and Planetary Science Library, hypothesizes is that the 8-degree tilt by a distant group of Kuiper Belt Objects (KBOs) away from the invariable plane could be the result of a Mars-sized object that exists in the far reaches of the Kuiper Belt.

“The most likely explanation for our results is that there is some unseen mass,” Volk told The Space Reporter. “According to our calculations, something as massive as Mars would be needed to cause the warp we measured.”

We’ve searched the outer solar system for large planets, but something Mars-sized could still be hiding in the outskirts of the Kuiper Belt. As with Planet 9, more observational data and modeling are needed.

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