In January last year, astronomers from Caltech suggested that a gigantic so-far-undiscovered planet might be lurking on the fringes of the Solar System. Now researchers from the University of Arizona (UA) have found that if it exists, this so-called Planet Nine might not be alone out there. Weirdly wobbling objects in the Kuiper Belt seem to indicate the influence of yet another planetary body at least as large as Mars.

The Caltech team first hypothesized the existence of Planet Nine based on the odd orbital plane of six Kuiper Belt Objects (KBOs). These were all tilted on an angle about 30 degrees off the average, suggesting that a huge planet about 10 times larger than Earth was tugging on them from way out in the shadows. Since then, evidence for the existence of this planet has mounted, including its influence on other trans-Neptunian objects and even the Sun.

Using a similar method, the UA researchers observed the tilt angles of some 600 KBOs, and found that the most distant objects have an average orbital plane that’s off-kilter by about eight degrees. That suggests the presence of another currently-undiscovered planetary body about the size of Mars, orbiting 60 times further from the Sun than Earth.

“The most likely explanation for our results is that there is some unseen mass,” says Kat Volk, lead author of the study. “According to our calculations, something as massive as Mars would be needed to cause the warp that we measured.”

The suggestion of a currently-unknown planetary body was made by studying the odd orbital planes of...
The suggestion of a currently-unknown planetary body was made by studying the odd orbital planes of objects in the Kuiper Belt(Credit: Heather Roper/LPL)

The team gives the findings a less than two percent chance of being a statistical anomaly, and the effect is also not likely to be caused by the predicted Planet Nine. That distant giant is believed to orbit between 500 and 700 Astronomical Units (AU), but to have the observed impact it would need to be closer than 100 AU. A rogue star swinging past could have created the wobble as well, but the researchers don’t consider that likely because the time scales don’t quite line up.

Without having observed the mysterious object directly, the team is careful to call it a “planetary mass object” for now. The official definition of a planet, as voted by the International Astronomical Union in 2006, is “a celestial body that (a) is in orbit around the Sun, (b) has sufficient mass for its self-gravity to overcome rigid body forces so that it assumes a hydrostatic equilibrium (nearly round) shape, and (c) has cleared the neighbourhood around its orbit.”

It was that last part that got poor Pluto kicked out of the club, so until the hypothesized object has been found, it’s best not to attach the label of “planet” just yet. The researchers believe that the mystery may be solved when the Large Synoptic Survey Telescope (LSST) comes online in 2020.

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