Many people were disheartened by the demotion of Pluto as a planet, which left only eight official planets in our solar system. But Pluto might soon have a replacement. Growing evidence suggests that our solar system hosts another planet about 10 times the mass of Earth, which may have an orbit 20 times further from the sun than Neptune, Space.com reported in 2016.
Scientists have yet to catch a glimpse of this so-called "Planet Nine," but believe the chances are extremely high that it exists given contextual clues gleaned from the orbits of other known bodies. If it does exist, it will be just the third planet discovered in our solar system in modern times. (Uranus was discovered in 1781 and Neptune in 1846.)
The emergence of Planet Nine
The first bit of evidence pointing to Planet Nine comes from the curious orbits of six Kuiper Belt objects (KBOs) that have been discovered in our outer solar system. Even though these KBOs are all moving at different speeds, they're on elliptical paths that all point in the same direction. To put this coincidence in perspective, the odds of it happening by chance are roughly 0.007 percent.
"Basically, it shouldn't happen randomly," said Mike Brown of the California Institute of Technology, one of the researchers searching for Planet Nine, said in 2016. "So we thought something else must be shaping these orbits."
That's not all. Brown and colleague Konstantin Batygin performed computer simulations that showed that a 10-Earth-mass planet in an anti-aligned orbit could precisely explain the KBOs' odd orbits. Furthermore, the proposed Planet Nine was also consistent with the orbits of two dwarf planets in the Kuiper Belt, Sedna and 2012 VP113.
"We plotted up the positions of those objects and their orbits, and they matched the simulations exactly. When we found that, my jaw sort of hit the floor," said Brown.
And the new planet candidate may have an even broader influence. Research released in October 2016 suggests Planet Nine is responsible for the sun's tilt — a mystery that has befuddled scientists for years. The answer has to do with angular momentum — which boils down to the planet's mass times its distance from the sun, so even though Planet Nine is a long way away, it exerts a massive force on the spin of the whole system.
"Because Planet Nine is so massive and has an orbit tilted compared to the other planets, the solar system has no choice but to slowly twist out of alignment," Elizabeth Bailey, a graduate student at Caltech and lead author of a study announcing the discovery, told Phys.org.
More evidence points to Planet Nine
Since Bailey, Brown and Batygin's study in 2016, more researchers have explored the edges of the solar system. These researchers include the team responsible for a 2017 study published The Astronomical Journal that used computer simulations to create the effects that a Planet Nine would have on Trans-Neptunian objects (TNOs), a classification umbrella that covers the KBOs discussed above.
Led by Juliette Becker, a graduate student at the University of Michigan's astronomy department, the simulations created different versions of Planet Nine that could possibly explain the shape of our solar system and why these TNOs haven't either smashed into other planets, been thrown into the sun or simply spun out into space. Becker's team determined two things through these simulations.
The first is "that there are preferred versions of Planet Nine that make the TNOs stay stable for longer, so it basically increases the probability that our solar system exists the way it does," Becker told Phys.org. "Through these computer simulations, we were able to determine which realization of Planet Nine creates our solar system—the whole caveat here being, if Planet Nine is real."
The second is what Becker's team calls "resonance hopping." This is the process through which these TNOs jump between stable orbits and thus keep themselves from getting ejected out of the solar system.
There's still much to know
The distance of Planet Nine from our sun would be quite extraordinary. It would take this far-out world 15,000 years to loop around the sun, in a strange elliptical orbit. By comparison, Neptune takes only 165 years to complete its loop. This is part of the reason why Planet Nine has eluded our telescopes so far. Given all of this new evidence, however, researchers are optimistic that Planet Nine will be spied within the next five years. That is, assuming it exists.
It's becoming increasingly likely that this Planet Nine does exist, however. A NASA press release from October 2017 suggested that it's becoming harder to imagine our solar system without a Planet Nine at this point.
"There are now five different lines of observational evidence pointing to the existence of Planet Nine," said Batygin said in the press release. "If you were to remove this explanation and imagine Planet Nine does not exist, then you generate more problems than you solve. All of a sudden, you have five different puzzles, and you must come up with five different theories to explain them."
Brown is particularly motivated to discover Planet Nine because it was his co-discoveries of Sedna and the dwarf planet Eris, which is about the same size as Pluto, that led to the demotion of Pluto from planet to dwarf planet. Call it astronomer's guilt, but he'd feel vindicated if a new ninth planet could be discovered to take Pluto's place.
"All those people who are mad that Pluto is no longer a planet can be thrilled to know that there is a real planet out there still to be found," he said in 2016. "Now we can go and find this planet and make the solar system have nine planets once again."
Brown and Batygin are relying on the Subaru Telescope at Mauna Kea Observatory in Hawaii to try to find Planet Nine. The telescope is the "best tool" for picking out dim, extremely distant objects lost in huge swaths of sky, Batygin said.
Becker's team argued that their research could help narrow down the search, too. Given Planet Nine's tilting and stabilizing effects on TNOs, it means the planet must be in an orbit that can actually achieve these effects. Brown plans to incorporate Becker's study's data into his calculations once the technical details are provided.
Editor's note: This story was first published in January 2016 and has been updated with new information since then.