A planet between the size of Mars and Earth may orbit the vast icy darkness beyond Pluto. This isn’t the hypothetical Planet Nine that would explain the orbits of trans-Neptunian objects (small planets that orbit the sun beyond Neptune’s orbit. Pluto is considered one). This planet would be a new one.

New research from the University of Arizona’s Lunar and Planetary Laboratory took a closer look at the tilt angles of the orbital planes of 600+ Kuiper Belt objects (KBO). UA’s Kat Volk and Renu Malhotra were analyzing what general direction the orbital planes were.

Every icy chunk of rock in the Kuiper Belt orbits a little differently. Malhotra says they act a lot like spinning tops.

“Imagine you have lots and lots of fast-spinning tops, and you give each one a slight nudge,” said Malhotra. “If you then take a snapshot of them, you will find that their spin axes will be at different orientations, but on average, they will be pointing to the local gravitational field of Earth.”

The pair expected the orbital plane of these objects to be flat past 50 AU (1 AU = distance between Earth and the sun). But that’s not what they found. Some KBO orbits were tilted away from this average plane by about eight degrees.

“But going further out from 50 to 80 AU, we found that the average plane actually warps away from the invariable plane,” says Volk. “There is a range of uncertainties for the measured warp, but there is not more than 1 or 2 percent chance that this warp is merely a statistical fluke of the limited observational sample of KBOs.”

One possible explanation points to a planet with a mass close to Mars orbiting 60 AU from the sun. It would have an orbit tilted by about eight degrees to the average plane of the known planets.

The possible planet’s gravity would be enough to nudge the KBOs and explain the different orbital plane. More than one planet can’t be ruled out either.

Fun fact: If this chunk of ice and rock the size of Mars is out there, it wouldn’t be called a planet. The authors call it a planetary mass object in their paper. That’s because the official definition says a planet has to clear its orbit to be designated as such. There’s a push going on right now to make the definition broader to account for possible planets like this one and to get Pluto back to being the ninth planet.

A planet the size of Mars orbiting barely 20 AU past Pluto raises another question. Why haven’t we seen it yet? The same reason why we still find asteroids right before they zoom past Earth. The sky is insanely vast.

Malhotra and Volk also explain the planet could be lurking in the galactic plane. Solar system surveys usually avoid it because the area is jam-packed with stars, making any planet observations difficult.

Volk pegs the chance we haven’t found the possible planet because of these issues at about 30 percent.

An unlikely scenario points the finger at a passing star. “A passing star would draw all the ‘spinning tops’ in one direction,” Malhotra said. “Once the star is gone, all the KBOs will go back to precessing around their previous plane. That would have required an extremely close passage at about 100 AU, and the warp would be erased within 10 million years, so we don’t consider this a likely scenario.”

Like pretty much everything astronomy related, the final answer will have to wait for a bigger telescope. And for once, it’s not the James Webb Space Telescope.

Today, there are about 2,000 observed KBOs. In the 2020s, that’s expected to shoot up to 40,000. All thanks to the Large Synoptic Survey Telescope (LSST). Once completed, the LSST will take real-time surveys of the sky every night from its perch atop Cerro Pachón in Chile. If an icy rock the size of Mars is lurking out there, LSST should find it.

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