Astronomers have long been puzzled by the orbital mechanics of objects that act in strange and largely unexplained ways in the outer reaches of the solar system, beyond the orbit of Neptune.

Observations have shown that these trans-Neptunian objects (TNOs) are moving as if they were being pulled by the gravitational force of a sizeable unseen mass.

One prevailing — and controversial — theory is that there's a ninth and enormous planet lurking in the area, dubbed "Planet Nine," which could account for the strange behavior.

Now, a pair of planetary scientists are suggesting it may be a far smaller, Earth-like planet — no bigger than three times the Earth's mass — that could explain the unusual movement of TNOs.

"We predict the existence of an Earth-like planet and several trans-Neptunian objects on peculiar orbits in the outer solar system, which can serve as observationally testable signatures of the putative planet's perturbations," Patryk Sofia Lykawka of Kindai University in Japan and Takashi Ito of the National Astronomical Observatory of Japan, write in a new paper published in The Astronomical Journal.

This theoretical planet could also be a lot closer to us than Planet Nine, which scientists have suggested could be orbiting anywhere between 400 and 800 astronomical units from the Sun.

The planet could be just 250 to 500 astronomical units away from the Sun, according to the researchers, bringing it far closer to the Kuiper Belt, a collection of icy objects lurking between the orbit of Neptune and around 50 astronomical units from the Sun.

As telescopes become more sensitive over time, allowing us to peer ever farther into the furthest reaches of the solar system, more and more TNOs are being identified — complicating the picture further.

Some of these objects appear to cluster and move in groups, behavior that's consistent with the existence of a much more massive object in the vicinity.

Using the latest wealth of data we've been collecting about these objects, the two scientists revisited prevailing theories as to what may be lurking past the orbit of Neptune.

"We determined that an Earth-like planet located on a distant and inclined orbit can explain three fundamental properties of the distant Kuiper Belt," they wrote. "A prominent population of TNOs with orbits beyond Neptune's gravitational influence, a significant population of high-inclination objects, and the existence of some extreme objects with peculiar orbits."

This planet could explain why some of these objects orbit the Sun at extreme angles, such as dwarf planet Sedna, which has an extremely elongated, 11,408-Earth-year orbit around the Sun.

In fact, the hypothetical Earthlike planet would itself be orbiting the Sun at an inclination of roughly 30 degrees, which would also make it an outlier.

But we'll have to be patient and wait until we have "more detailed knowledge of the orbital structure in the distant Kuiper Belt," the researchers argue, which could "reveal or rule out the existence of any hypothetical planet in the outer solar system."

More on TNOs: Solar System May Have Captured an Extra Planet, Scientists Say


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