Something very odd seems to be going on out beyond Pluto. Astronomers have known for more than two decades that the tiny former planet is not alone at the edge of the solar system: it is part of a vast cloud of icy objects known collectively as the Kuiper belt. But unlike most of their fellow travelers, and unlike the planets and most asteroids, which orbit between Mars and Jupiter, a small handful of Kuiper belt objects, or KBOs, have orbits that are decidedly weird. For one thing, they take unusually elongated paths around the sun, unlike the roughly circular orbits of most planetary bodies.

These badly behaving objects, which number between four and a dozen, depending on who is counting, share another orbital peculiarity as well. Like most KBOs, they orbit at an angle to the pancake-shaped plane where the planets live, rising above the pancake for part of the time, then plunging through to dip below for the rest. But unlike their frozen brethren, these objects all pass through the planetary plane at the same time that they make their closest swing toward the sun.

Or, to use a term even many astronomers find arcane, they have remarkably similar arguments of perihelion (AOP). “Normally,” says Scott Sheppard, a planetary scientist at the Carnegie Institution for Science, “you would expect the arguments of perihelion to have been randomized over the life of the solar system.” Maybe it is just a coincidence that these few bodies ended up with the same AOP: such a thing should happen, purely by chance, less than a few percent of the time. Those odds are something like getting 10 heads in a row when you flip a coin: pretty unusual but nowhere near impossible.

But those 10 heads could also mean your coin is loaded, and the same goes for these KBOs. Something may have forced the objects into this strange configuration—and that something could be a huge, unknown planet, significantly more massive than Earth, lurking out at the edge of the solar system: a super Earth (super Earths are planets up to roughly 10 times more massive than Earth). If such a hidden object—sometimes whimsically called “Planet X”—exists, it would orbit at least 10 times farther from the sun than Neptune—too distant and too faint to have been spotted by any telescope to date. Yet its sizable mass would have gravitational effects on the rest of the solar system—effects that might explain the odd orbits astronomers have seen.

“We don't have definitive proof yet that there's a planetary-mass body out there,” says Nathan Kaib, a planet-formation theorist now at the University of Oklahoma. “But something funny is going on that we don't understand.” And a growing number of astronomers are putting stock in the once ridiculed notion of the presence of a super Earth in our midst, Planet X.

As Kaib suggests, the evidence for a hidden planet is far from ironclad. Many astronomers still doubt the idea, and even those who invoke it as a possibility say they are not yet fully convinced. The history of astronomy is full of invisible mystery planets, their existence inferred from the peculiar orbits of other objects. Some have turned out to be major discoveries. Others were false alarms. It may be that we do not know our solar system nearly as well as we thought we did. If there is a Planet X out there, it will necessitate a wholesale rewriting of some key chapters of the solar system's history.

On the Trail of Hidden Worlds

The first search for a hidden planet circling the sun came in the early 1800s, when scientists became increasingly convinced that Uranus, discovered accidentally in 1781 by the musician-turned-astronomer William Herschel, was not orbiting quite as Newton's law of gravity said it should. Several scientists posited that the gravity of a large, undiscovered planet was to blame—and in 1846 German astronomer Johan Galle spied the gas giant Neptune, basically where his French colleague Urbain Le Verrier had calculated it should be. (There is good evidence that Galileo had actually seen Neptune as early as 1612 with his small, crude telescope but had assumed it was a star.)

In the early 1900s Boston aristocrat Percival Lowell began a search for another hidden planet based at his own personal observatory in Flagstaff, Ariz. This time the evidence came in the form of anomalies in the orbits of both Uranus and Neptune, pointing to the existence of yet another unseen giant planet. Early in 1930 a young assistant at Lowell Observatory named Clyde Tombaugh found a planet more or less where the calculations said it should be—a replay of the discovery of Neptune. “The Sphere, Possibly Larger than Jupiter and 4,000,000,000 Miles Away, Meets Predictions,” the New York Times announced on March 14, 1930.

It did not, though. Within a few decades it became clear that Pluto is far from Jupiter's size and is actually smaller than Earth's moon. Its meager gravity could not possibly explain anomalies in the orbits of Neptune and Uranus—which turned out to be just as well because those anomalies faded away on further inspection. In that sense, Pluto was a false alarm.

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Credit: Jan Willem Tulp; Sources: International Astronomical Union Minor Planet Center's List of Transneptunian objects (Kuiper belt object data); Scott S. Sheppard Carnegie Institution for Science (extreme outer solar system object data)

In the big picture, however, its discovery was extraordinarily important. By the 1980s planetary scientists had begun to suspect that Pluto was not a puny planet orbiting all alone in the solar system's frozen outskirts but simply the brightest member of a vast, richly populated region known as the Kuiper belt. In 1992 the first KBO (besides Pluto, that is) was spotted with a telescope in Hawaii, and since then, observers have tallied another 1,500 or so. The 2005 discovery of Eris, which rivals Pluto in size and significantly outweighs it, threatened to open a floodgate that could have added several more planets to the existing roster of nine. That specter prompted the International Astronomical Union to demote Pluto from planet to dwarf planet in 2006.

Reshuffling the Solar System

The discovery of the Kuiper belt, in turn, lends credence to the latest search for a Planet X because it helps explain how such an object might have ended up so far from the sun that we still have not seen it. Computer simulations suggest that the icy bodies of the Kuiper belt should have formed somewhere in the neighborhood Neptune now occupies. Something must have flung them much farther out (or scattered them, to use the technical term) to their present positions. This observation led astronomers to theorize that a disruption took place during a chaotic period soon after the nascent planets congealed from the “protoplanetary disk” of gas and dust that swirled around the newborn sun. During this unsettled time, Jupiter, Saturn, Uranus and Neptune most likely shifted by hundreds of millions of kilometers from their initial orbits, their gravity sending the KBOs flying outward. Some simulations even point to the existence of a possible fifth gas giant that was ejected from the solar system entirely as the others adjusted their positions.

It is easily plausible that if a super Earth existed, it, too, could have been flung outward during this period of general mayhem. And because super Earths have proved to be common among the roughly 2,000 exoplanets discovered around other stars over the past couple of decades, it is also reasonable to suppose that there could once have been one circling our own sun. With that in mind, says Ben Bromley of the University of Utah, who collaborated with Scott Kenyon of the Harvard-Smithsonian Center for Astrophysics, “we ran some mock-ups of what would happen to a super Earth scattered from the region where Jupiter and Saturn are today.” In most cases, they found that the super Earth would be flung into a highly elliptical orbit, which would gradually become more and more stretched out until the planet was ejected from the solar system entirely. But if the scattering happened early enough—within about 10 million years after the formation of the planets, before the protoplanetary gas dissipated, Bromley says, “the super Earth could interact with the gas [gravitationally] and settle out in the boondocks in a more or less circular orbit.”

That scenario is one way to make the kind of massive Planet X that Lowell set out to find in the early 1900s and the kind Galle and Le Verrier did find when they collaborated to discover Neptune half a century earlier. Another way to do it, Kenyon and Bromley found, was for the super Earth to form in place at perhaps 200 astronomical units (AU) from the sun, which is to say, 200 times the sun-Earth distance of 93 million miles. (Neptune, in contrast, orbits at about 30 AU from the sun.) Such in situ formation would be possible only if there were sufficient planet-forming material—pebble-size bits of rock and ice—orbiting out that far.

There is no direct evidence that this was ever the case in our own solar system, but there is quite good evidence that it happens with stars that are very much like the sun. “If you look at nearby solar-type stars,” however, Kenyon says, “some of them have these debris disks with material extending to 200-ish AU away from the star itself. So it wouldn't be unprecedented.” And although there is no proof that super Earths have formed at such a distance around these nearby stars, he says, “at least you have the basic ingredients.” All these simulations were purely speculative when Kenyon and Bromley began working on them a decade or so ago. Nobody had seen even a hint that a super Earth was actually out there.

Enter Sedna

That situation began to change with Sedna. In 2003 Mike Brown of the California Institute of Technology, along with two colleagues, observed what was perhaps the strangest solar system object ever discovered up until that point. It was an icy body, now estimated to be about 1,000 kilometers across, and similar in many ways to Pluto, Eris and other KBOs. Its orbit, however, had not been seen before. Sedna never comes closer than 76 AU from the sun, or more than twice as far away as Neptune, on its highly elongated 11,400-year orbit, and it retreats to more than 930 AU—31 times as far as Neptune—at its most distant.

“Sedna was really surprising,” says co-discoverer Chad Trujillo, now at Gemini Observatory in Hawaii, “because it was completely unexplained.” Its stretched-out orbit resembled those of long-period comets, but unlike Sedna, these have one orbital end firmly anchored by the gravity of the giant planets. Sedna appeared unanchored to anything. “Nobody had thought an object like this could exist,” Trujillo says, “and nobody had an explanation of how it got there.”

Over the next decade or so observers found 10 other, smaller objects whose orbits are also elongated and that never come within shouting distance of Neptune. By itself, this was not especially noteworthy: none of them was nearly as extreme as Sedna either in the shape of its orbit or in how far beyond Neptune its perihelion came—that is, its closest approach to the sun. But all of them, along with Sedna itself, shared a similar argument of perihelion, the unusual orbital parameter that describes how far above or below the plane of the solar system an object is when it reaches perihelion. And that seemed … odd.

Things got significantly odder in 2014, when Trujillo and Sheppard announced in Nature that they had discovered a second Sedna-like object, about half as big as Sedna itself, after searching for something like it for a decade. “If you're a biologist,” Trujillo says, “and you find some weird creature, you're pretty sure there's got to be more like it out there.” Likewise in astronomy, he says—unless that first creature was a total fluke. “Maybe this one object happened to be thrown in this orbit for reasons we don't understand,” he says, “but you don't really know until you find another one.” Now they had.

Known provisionally as 2012 VP113, its eccentric, 4,300-year orbit has a perihelion of 80 AU and an aphelion—its farthest retreat from the sun—of 446 AU. Like Sedna, 2012 VP113 is fully detached from Neptune gravitationally. And crucially, its argument of perihelion is very similar to Sedna's, as well as to that of a handful of other, less Sedna-like KBOs. It was that last factor that led to a provocative line buried well down in the Nature paper. “This suggests,” Trujillo and Sheppard wrote, “that a massive outer Solar system perturber may exist.” The perturber, they hypothesized, could be a super Earth orbiting up to 250 AU from the sun, whose gravity might have influenced the smaller objects and synchronized their arguments of perihelion. “I don't think anyone was really thinking seriously about a massive, undetected planet before this,” says Meg Schwamb of Yale University. “But the Trujillo and Sheppard paper really brought it into play.”

Then, in September 2014, a paper in Monthly Notices of the Royal Astronomical Society Letters by two relatively obscure Spanish astronomers, brothers Raúl and Carlos de la Fuente Marcos of the Complutense University of Madrid, upped the ante. Based on the orbits of Sedna, 2012 VP113 and smaller bodies, they argued that there might not be just one super Earth. Their analysis “strongly suggest[ed]” that at least two planets might exist beyond Pluto. “Our unpublished calculations,” Raúl says, “suggest that the hypothetical planets should be at least two, but probably fewer than 15, Earth masses.”

Like Sheppard and Trujillo, the de la Fuente Marcos brothers do not claim to be making a solid prediction. Both teams say only that the existence of a super Earth is plausible. If it exists, however, astronomers' confidence that they fully understand our own solar system will be upended.

Doubts Remain

Although a hidden Planet X is a tempting explanation for the oddities of Sedna and its ilk, it is not the only option. Another possibility, says planet-formation theorist Hal Levison of the Southwest Research Institute is that Sedna, 2012 VP113 and the rest were thrown into their distinctive orbits while the sun was still part of its original birth cluster of thousands of stars that congealed from a single gas cloud. Before the cluster drifted apart, those stars would have been nearby enough to distort the orbits of objects in the outer solar system, sending them inward on long, stretched-out trajectories. Or, Sheppard says, the orbital elongations could have come from galactic tides—that is, a stronger pull from one direction than from another as the sun passes close to higher-density regions on its orbit around the center of the Milky Way. “We've run some simulations like that,” Sheppard says, “and nothing has shown up. So it doesn't appear that that's likely, but there are a lot of other possibilities kind of like that out there.”

Any of these effects could have put the objects on their oblong orbits, but only the super-Earth hypothesis could give them such closely matching arguments of perihelion. That or pure chance. The 12 objects Sheppard and Trujillo cite in their paper may sound like a lot, but given that there are millions of Kuiper belt objects, Sheppard says, “that's a statistically marginal number.”

The case for Planet X from the odd orbits of Sedna and its fellows gets even more marginal if you agree with Schwamb and her collaborator Ramon Brasser of the Tokyo Institute of Technology. “Work we've done recently,” Schwamb says, “shows there are really only four objects like Sedna.” The rest of the 12 do not come all that close to Neptune, she says, but they come close enough that they could be feeling its gravity. Neptune itself could therefore be the Planet X that explains their closely matching arguments of perihelion. And if 12 objects are considered statistically marginal, four is even more so—although “marginal” in science means something slightly different than it does in the everyday world. “The alignment of the four remaining objects,” Brasser says, “would happen by chance only 1 percent of the time.” Long odds, however, are not a slam dunk. “Just because you can say a planet is possible,” Schwamb says—which she agrees it is—“doesn't rule it in.”

Planetary scientists have learned that lesson more than once. In the 1980s University of California, Berkeley, physicist Richard Muller thought he could explain many past mass extinctions of species on Earth by invoking a dim star or a brown dwarf—an object less massive than a star but more massive than a planet—orbiting the sun at a distance of roughly 100,000 AU, or about 1.5 light-years. Once every 26 million years, more or less, goes the theory, the object he called Nemesis kicks a knot of comets out of the Oort cloud, a still hypothetical shell of icy bodies that surrounds the solar system far beyond Sedna or anything else astronomers have ever seen. The Oort comets fall in toward the sun. Some of them slam into Earth, and there goes a large fraction of the species on our planet.

But just like today's arguments for Planet X, the theory was only barely plausible, and searches for Nemesis have consistently turned up empty. More recently, John Matese and Daniel Whitmire, while at the University of Louisiana at Lafayette, postulated a Jupiter-size planet in the far outer solar system to explain what seemed to be an excess of long-period comets coming in from one direction in the sky. “It was,” Schwamb says, “a paper in the literature,” which is scientific shorthand for “I never bought it in the first place.” Sure enough, a sensitive survey by nasa's Wide-field Infrared Survey Explorer (WISE) telescope saw nothing. “We would have been able to see an object as small as Jupiter out to about maybe 30,000 to 40,000 AU from the sun,” says Kevin Luhman of Pennsylvania State University, who conducted the search, “and we would have been able to see an object as small as Saturn out to maybe 10,000 or 15,000 AU from the sun.” They found nothing. A super-Earth-size Planet X would be much closer but also so much dimmer that it would not have shown up in this survey.

So Is It There or Not?

With no more than 12 unusual objects to guide them, planetary scientists cannot say at this point whether our solar system is host to a super Earth or not. They can say only that the hypothesis is consistent with the observations. Identifying more objects with similar orbital characteristics is crucial, which is why astronomers are so excited about a new object announced in November 2015. Known as V774104, it has a perihelion even farther from the sun than Sedna's. It is too early to know whether its orbit confirms or rejects the possibility of an undiscovered giant planet, says Sheppard, who led the discovery team. It is also too early to say much about the 40 or so other distant objects Sheppard's team found at the same time in what he calls “the deepest, widest survey of the outer solar system ever conducted.” But the more the researchers find, the better their chances of saying definitively whether something massive lurks out there.

To improve their chances even more, planetary scientists are eager to get their hands on the Large Synoptic Survey Telescope (LSST), scheduled to come online in northern Chile by 2019. It will not be any bigger than the largest telescopes currently in use, but its field of view will be much wider, allowing it to search much broader swaths of sky. At present, Trujillo says, astronomers have surveyed about 10 square degrees of sky—the full moon, for comparison, covers a quarter of a square degree—in search of faint, distant objects. The LSST, he says, “will be able to survey tens of thousands.”

If a super Earth is out there and if it is large and bright enough, the LSST could find it. Or perhaps someone else already has. In December 2015 observers claimed they had taken direct images of what could be a different super Earth using the Atacama Large Millimeter/submillimeter Array in Chile. Most of their colleagues were highly skeptical, but more observations could change that. Or perhaps some other telescope has inadvertently imaged our local super Earth. “Maybe it's sitting around on somebody's hard drive, and they just never noticed it because they weren't looking for it, or they weren't looking in the right way,” Trujillo says. “People tend to see only what they're looking for.”

Editor's note: Even as this story was first being published in our February issue, Caltech astronomers Mike Brown and Konstantin Batygin announced even stronger evidence that Planet X is orbiting in the darkness out beyond Pluto—and they narrowed down the part of the sky where astronomers should look. Targeted searches for this unseen world are now under way.