“New Planet Found” is about as exciting a headline nowadays as “Dog Bites Man,” which is to say, not very. Thanks largely to the space-based Kepler Mission, astronomers have identified about 2,000 new worlds, orbiting stars that lie tens or even hundreds of light-years from Earth, in the last two decades. Collectively, these are scientifically important, but with so many in hand no single addition to the list is likely to be much of a big deal. But a new planet announcement today from the California Institute of Technology is a very different proposition, because the world it describes does not circle a distant star. It is part of our own solar system—a place you would think we had explored pretty well by now.

Evidently not: in an analysis accepted for publication at The Astronomical Journal, California Institute of Technology planetary scientists Konstantin Batygin and Mike Brown present what they say is strong circumstantial evidence for a very large undiscovered planet, perhaps 10 times as massive as Earth, orbiting in the solar system’s outer darkness beyond Pluto. The scientists infer its presence from anomalies in the orbits of a handful of smaller bodies they can see. “I haven’t been this excited about something in quite a while,” says Greg Laughlin, an expert on planet formation and dynamics at the University of California, Santa Cruz, who was not involved in the research.

The object, which the researchers have provisionally named “Planet Nine,” comes no closer than 30.5 billion or so kilometers from the sun, or five times farther than Pluto’s average distance. Despite its enormous size, it would be so dim, the authors say, that it is unsurprising that nobody has spotted it yet.

If it exists, that is. “Sadly,” Brown says, “we don’t have an actual detection yet.” But the evidence is strong enough that other experts are taking very serious notice. “I think it’s pretty convincing,” says Chad Trujillo of the Gemini Observatory in Hawaii. David Nesvorny, a solar system theorist at the Southwest Research Institute (SwRI), in Boulder, Colo., is impressed as well. “These guys are really good,” he says. “They make a good case.”

Strange orbits
Batygin and Brown are not the first to argue for an extra planet in our solar system. In 2014 Trujillo and Scott Sheppard, of the Carnegie Institution for Science, argued in Nature that their own discovery of a much smaller object, called 2012 VP113 , along with the existence of a handful of previously identified bodies in the outer solar system, hinted that there might be something planet-size out there. The evidence lay with their orbits, specifically with an obscure parameter called the “argument of perihelion”—the relationship between the time a body makes its closest approach to the sun and the time it passes through the plane of the solar system. The objects Trujillo and Shepherd identified all had uncannily similar arguments of perihelion, which could mean they were being shepherded by the gravity of an unseen world. “We noticed something curious,” Trujillo says, “and said ‘someone should go explore this further.’” (Scientific American is part of Nature Publishing Group.)

Several groups did, and agreed that the case for a hidden planet was plausible but still quite speculative. (See “The Search for Planet X” in the current Scientific American). The new analysis strengthens that case dramatically, however. The similarity of the arguments of perihelion turns out to be “just the tip of the iceberg,” Batygin says.

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The first thing he and Brown did, he says, was to analyze Trujillo and Sheppard’s data with entirely fresh eyes. “What we noticed,” Batygin says, “was that the long axes of these objects’ orbits fall into the same quadrant of the sky.” In other words, they point in the same direction. That outcome was not guaranteed; two bodies can have similar arguments of perihelion even if their orbits are not otherwise physically similar. But when Brown and Batygin plotted the orbits of those outer solar system objects, they noticed that their highly elliptical orbit shapes were closely aligned. “Shouldn’t something like that be hard to miss?” Brown asks. “Yes, you would think so. This a case where we had our noses buried in the data, never stepping back and looking at the solar system from above. I couldn’t believe I’d never noticed this before,” he says. “It’s ridiculous.”

The directionality of the orbits was an even stronger hint that something was physically herding these distant objects. “At first,” Brown says, “we said ‘there can’t be a planet out there—that’s crazy.’” So they examined the most likely alternative—that the Kuiper Belt of icy objects beyond Pluto had formed all of its bodies into a clump naturally, much as galaxies pulled themselves into shape gravitationally out of the cosmic cloud of gas that emerged from the big bang.

The problem with that scenario, the authors realized, was that the Kuiper Belt lacks the mass to make it happen. When the scientists turned to the “crazy” notion of a planet, however, their simulations generated just the right kind of aligned orbits. They also revealed something else: The gravity of a giant planet should lead to an entirely independent set of objects whose orbits are not aligned with one another but are sharply tilted compared with the orbits of the planets—up to 90 degrees away from the plane of the solar system or even more. “That seemed really puzzling,” Batygin says. “But then Mike said, ‘I think I’ve seen something like these in the data.’” Sure enough, observers have spotted a half dozen or so objects just like this and nobody had come up with a good explanation of how they might have gotten there. Now Batygin and Brown’s simulation was providing one. “The fact that they’re now marshaling two new, independent lines of evidence for a hypothetical planet,” Laughlin says, “makes their case even stronger.”

The gravity of a hypothetical Planet Nine could explain the peculiar orbits of two different sets of objects that lie out beyond Pluto.

The diagram was created using WorldWide Telescope.

Caltech/R. Hurt (IPAC)

Super Earth
The planet that best fits the data would be on the order of 10 times as massive as Earth—putting it in the so-called “Super Earth” category, which includes many planets around other stars but none, until now, in our own solar system—and smaller than Neptune, the fourth-largest known planet orbiting the sun, which has about 17 Earth masses. Its most probable orbit is a highly elongated one that brings it to within 35 billion kilometers of the sun at the closest (“that’s where it does all the damage,” Brown says) and between three and six times as far away at its most distant.

Even at that enormous distance, Planet Nine could in principle be spotted with existing telescopes—most easily with the Japanese Subaru Telescope in Hawaii, which not only has a huge mirror for trapping faint light but also a wide field of view that would allow searchers to efficiently scan big swaths of sky. “Unfortunately, we don’t own the Subaru,” Brown says, “which means we’re unlikely to be the ones who find it. So we’re telling everyone else where to look.”

Until they actually see it, astronomers cannot say definitively that Planet Nine is real. “I tend to be very suspicious of claims of an extra planet in the solar system,” says Hal Levison of SwRI. “I have seen many, many such claims in my career, and all of them have been wrong.” The orbital alignment is genuine, he acknowledges. “Something is creating it. But what that something is needs to be explored a bit more.”

Overall, however, planetary scientists are clearly thrilled by the prospect that we might be on the verge of such a major discovery. “When I was growing up,” Sheppard says, “we thought the big planets had all been found. It would be very exciting and very surprising to learn that we were wrong.”

The mood of the astronomical community is perfectly captured, Laughlin says, by something British astronomer John Herschel said to the British Association for the Advancement of Science in a talk on September 10, 1846. Irregularities had been spotted in the orbit of Uranus, suggesting that the gravity of an unknown, massive planet was tugging on it. Referring to the mystery object, Hershel said:

“We see it as Columbus saw America from the shores of Spain. Its movements have been felt along the far-reaching line of our analysis with a certainly hardly inferior to ocular demonstration.” Just two weeks later Neptune was discovered, right where the theorists’ calculations said it should be.