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Image: Distant planet
An artist’s conception shows a “super-Earth” planet far from the sun. (Credit: R. Hurt / IPAC / Caltech)

For decades, astronomers have gone back and forth over whether a “Planet X” exists on the edge of our solar system – and now two researchers have laid out new evidence supporting the claim, including a rough idea of where it could be found.

One of the most notable things about the claim has to do with one of the people who’s making it: Mike Brown, the Caltech astronomer who says he “killed” Pluto when it was the ninth planet.

“This would be a real ninth planet,” Brown said in a news release. “There have only been two true planets discovered since ancient times, and this would be the third.”

Brown’s “two true planets” refer to Uranus and Neptune, not Pluto. To emphasize the point, Brown and his collaborator at Caltech, Konstantin Batygin, have nicknamed the object “Planet Nine.” (Other nicknames are said to include George, Planet of the Apes, Jehoshaphat and Phattie.)

There’s one big gap in the argument: No such object has yet been detected. Instead, Brown and Batygin make the claim on the basis of a detailed analysis of objects that have been scattered into strange orbits in the Kuiper Belt. That’s the broad ring of icy material that lies beyond the orbit of Neptune.

The astronomers lay out their evidence in a paper published today by the Astrophysical Journal.

“Although we were initially quite skeptical that this planet could exist, as we continued to investigate its orbit and what it would mean for the outer solar system, we became increasingly convinced that it is out there,” Batygin said.

The proposed planet would have a mass about 10 times that of Earth, and should trace an eccentric orbit coming no closer to the sun than, say, 19 billion miles (200 astronomical units, where 1 AU equals the distance from Earth to the sun). In comparison, Neptune and Pluto come no closer than 30 AU. It would take 10,000 to 20,000 years for the proposed planet to make a full orbit of the sun.

Batygin and Brown are by no means the first to predict the existence of a large world on the solar system’s edge. Such claims have repeatedly been raised over the course of more than 30 years.

“This is about the fifth or 10th prediction like this. … Not one has panned out,” said Alan Stern, a planetary scientist at the Southwest Research Institute who is the principal investigator for NASA’s New Horizons mission to Pluto and beyond.

The new evidence focuses on the orbits of six distant objects that had previously been detected in the Kuiper Belt. The orbits are hard to explain, based on the gravitational interactions of the solar system’s large planets.

Batygin and Brown noticed that the long axes of the orbits all pointed in roughly the same direction, and that the orbits all had the roughly same tilt with respect to the solar system’s main planetary plane. When they ran the numbers, the astronomers found the probability of such an arrangement occurring by chance was about 0.007 percent.

The astronomers were able to tweak their orbital simulations to produce the arrangement if they added a large, distant planet with an “anti-aligned” orbit. That means the point of the planet’s closest approach to the sun would have to be placed diametrically opposite to the point of closest approach for the other six objects.

Kuiper Belt objects
The orbits of six distant objects in the solar system point roughly in the same direction. Moreover, the orbits have a similar tilt with respect to the solar system’s main plane. Astronomers say the gravitational influence of a world they call “Planet Nine” provides the best explanation for the effect. This diagram was created using WorldWide Telescope. (Credit: R. Hurt / IPAC / Caltech)

“Planet Nine” could explain the seemingly puzzling orbits traced by two of the distant worlds, known as Sedna and 2012 VP113 (which has been nicknamed “Planet Biden” in homage to a different kind of VP). What’s more, the planet should kick still other objects into orbits in perpendicular planes. That matches observations made in just the last three years.

“When the simulation aligned the distant Kuiper Belt objects and created objects like Sedna, we thought this is kind of awesome – you kill two birds with one stone,” Batygin said. “But with the existence of the planet also explaining these perpendicular orbits, not only do you kill two birds, you also take down a bird that you didn’t realize was sitting in a nearby tree.”

Brown and Batygin suggest that the unseen world coalesced along with the solar system’s four known giant planets – Jupiter, Saturn, Uranus and Neptune – and that it was kicked out into a stable but more distant orbit through gravitational interactions.

The simulation provides a rough idea of the planet’s orbital track, but it can’t point to where the planet currently lies on that track. If it’s in the farthest reaches of its orbit, it would be so dim that only the world’s largest telescopes could possibly see it. But if it’s around the point of closest approach to the sun, there’s a much better chance of spotting it. It might even show up in archived imagery.

“I would love to find it,” Brown said. “But I’d also be perfectly happy if someone else found it. That’s why we’re publishing this paper. We hope that other people are going to get inspired and start searching.”

Although the paper is certain to revive the search for Planet X, success is not certain: Previous surveys have failed to detect any Saturn-sized planets going out as far as 10,000 AU. Brown and Batygin say the planet they propose would be too small and dim to show up in those surveys.

Considering Brown’s history with Pluto, would Planet Nine fit his conception of a planet? Brown says it would, because the simulations indicate it would dominate the gravitational dynamics for a region of the solar system. The fact that it could affect the orbits of other objects over such a wide area would make it “the most planet-y of the planets in the whole solar system,” Brown argued.

If the world is detected, it would put the International Astronomical Union’s nearly 10-year-old definition of planethood to a new test. That definition put Pluto and other objects in a category known as dwarf planets, because they were deemed not to have “cleared the neighborhood around its orbit.”

If Planet Nine is found, would the IAU declare that it’s cleared out a celestial neighborhood ranging from the Kuiper Belt to the comet-filled Oort Cloud that lies beyond? Stern, who’s a sharp critic of the IAU definition, says such an object could lead to a fresh round of rethinking.

“If they do find it, it’ll be more like Number 19, not Number 9,” he told GeekWire in an email. “And if it is found, it’ll confirm lots of work predicting the Oort Cloud is littered with planets, and the solar system made dozens to hundreds of them.”

No matter how the search for Planet Nine turns out, Pluto’s devotees and detractors can now agree on one thing, at least: The solar system is more complicated than astronomers thought, back when Pluto was the ninth planet.

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