Astronomers say they've found hints that an unseen planetary-mass object may lurk in the outer reaches of our solar system. And no, they're not talking about Planet Nine, Planet X, Nibiru or any of the other previously hypothesized worlds out there.
Kat Volk and Renu Malhotra of the University of Arizona's Lunar and Planetary Laboratory say their analysis points to an eight-degree tilt in the average planes of orbits for the most distant objects in the Kuiper Belt, a ring of icy mini-worlds that lie beyond the orbit of Neptune.
"The most likely explanation for our results is that there is some unseen mass," Volk said in a news release. "According to our calculations, something as massive as Mars would be needed to cause the warp that we measured."
In a paper to be published by the Astronomical Journal, Volk and Malhotra say the gravitational influence of a Mars-size object at a distance of 60 astronomical units, or 60 AU, could explain the orbital warp. In comparison, Earth is 1 AU from the sun, and the Kuiper Belt extends from roughly 30 to 80 AU.
The warp was detected only for objects at distances from 50 to 80 AU, which fits the model proposed in the paper.
Last year, a separate team of astronomers proposed the existence of a world they call Planet Nine, on the basis of a different type of analysis looking at distant objects with oddball orbits. That object would have to be farther out, coming no closer than about 200 AU.
Astronomers have been looking for Planet Nine ever since its existence was proposed but have not yet detected it.
Volk and Malhotra say their mystery object (Planet Ten?) may have eluded detection if it lies in the galactic plane, which is so densely packed with stars that sky surveys could easily miss it.
"The chance that we have not found such an object of the right brightness and distance simply because of the limitations of the surveys is estimated to be about 30 percent," Volk said.
It's also possible that the warp was caused by a different phenomenon – for example, the gravitational disruption caused by a passing star.
"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," Malhotra said.
The next best opportunity to catch a glimpse of the mystery object should come with the completion of the Large Synoptic Survey Telescope in Chile. Scheduled for first light in 2020, that instrument will take unprecedented real-time surveys of the sky, night after night.
"We expect LSST to bring the number of observed KBOs from currently about 2,000 to 40,000," Malhotra said. "There are a lot more KBOs out there — we just have not seen them yet. Some of them are too far and dim even for LSST to spot, but because the telescope will cover the sky much more comprehensively than current surveys, it should be able to detect this object, if it's out there."