Astronomers at Caltech believe they’ve found an ice giant out beyond Neptune. Illustration by Aleks Sennwald

Very briefly some years ago, Mike Brown discovered the tenth planet in the solar system. This was in 2005; Brown, an astronomer at Caltech, had spotted an object that officially became known as Eris (he preferred the nickname Xena). Eris was about as big as Pluto, which was still a planet back then, and it orbited the sun at a distance nearly three times greater. But the existence of Eris raised troubling questions, such as: What’s a planet, exactly? And if Eris is a planet, why not also various other small spheres that orbit the sun? In the end, the International Astronomical Union categorized Eris as a dwarf planet—a polite phrase for “not a planet at all”—and, with Brown’s encouragement, Pluto was demoted, too. Instead of ten planets, the solar system now had eight. Brown still gets letters and late-night obscene calls from people who miss having a ninth planet, but he has no regrets. (In 2010, he wrote a book called “How I Killed Pluto and Why It Had It Coming.”) Last week he told me, “When all this happened, ten years ago, people would say, ‘Are there any other planets out there?’ And I would say, ‘Nope, that’s it. There are just eight planets, and we’ll never have any more.’ ”

Brown now thinks he was wrong. Today, in The Astronomical Journal, Brown and his colleague Konstantin Batygin have published a paper with the title “Evidence for a Distant Giant Planet in the Solar System,” in which they make a persuasive case that there actually is a ninth planet out there. They have not observed it directly, only inferred its presence from the behavior of a handful of faraway objects, which have been caught in its gravitational sway. After more than a year of watching, calculating, and conducting computer simulations, Brown and Batygin write, “We motivate the existence of a distant, eccentric perturber.”

As best they can determine, the perturber is perhaps ten times more massive than Earth, or roughly half as massive as Neptune, and it is very distant indeed. It follows an eccentric orbit, meaning one that is more elliptical than circular, and comes no closer to the sun than about two hundred and fifty astronomical units. (An astronomical unit is the distance from the sun to Earth, or ninety-three million miles. Jupiter is roughly five astronomical units from the sun, and Pluto averages nearly forty.) At its farthest, the new planet is between six hundred and twelve hundred astronomical units away; if the sun were on Fifth Avenue and Earth were one block west, Jupiter would be on the West Side Highway, Pluto would be in Montclair, New Jersey, and the new planet would be somewhere near Cleveland. It takes between twelve and twenty thousand years to go once around the sun. It is an ice giant, a lonely wanderer and the gravitational bully of the outer solar system. Brown and Batygin call it Planet Nine, and Jehoshaphat, and George. “We actually call it Fatty when we’re just talking to each other,” Brown said.

Brown acknowledged that the history of astronomy is riddled with false hopes. Urbain Le Verrier, the French mathematician who correctly predicted the existence of Neptune, in 1846, also predicted the existence of a planet orbiting between the sun and Mercury. He called it Vulcan, and it turned out not to exist. Every few years, someone announces the discovery of Planet X, some large object that Galileo and four centuries of his descendants missed, only to retract it. “If somebody proposed this—if I picked up a newspaper and read a headline—my first reaction would be, Oh my God, these guys are crazy,” Brown said of his and Batygin’s finding. “But if somebody then looked at the evidence, they’d have a hard time disagreeing that the evidence is there.”

Greg Laughlin, an astronomer at the University of California, Santa Cruz, and one of the few scientists who knew in advance of the paper, said, “It’s a very solid dynamical analysis. It’s top-notch. If anybody else was making this claim, you’d have to discount it to, at best, a one-per-cent chance of being there. But the combination of Mike Brown, who has a really solid observational sense of what’s out there, and Konstantin’s theoretical brilliance—if it’s out there, they’ve found it.” Alessandro Morbidelli, an astronomer and planetary scientist at the Observatoire de la Côte d’Azur, in Nice, France, and a referee for The Astronomical Journal, said, “This paper for the first time gives a smoking gun for the existence of an additional planet.”

The possibility of Planet Nine illustrates just how much our knowledge of the solar system has expanded in recent decades. In 1992, astronomers observed the first evidence of the Kuiper Belt, a population of icy objects—more than a thousand, at latest count—orbiting the sun at a distance of between thirty and fifty astronomical units. That same year, astrophysicists began planning a mission to Pluto (technically a Kuiper member itself). By the time the spacecraft, eventually called New Horizons, arrived at its destination, last July, Pluto was less an ending than a beginning. In the interim, Brown and his colleagues Chad Trujillo and David Rabinowitz had spotted Sedna, a small, icy object with an eccentric orbit well outside the Kuiper Belt; it comes no closer to the sun than seventy-six astronomical units and, at its farthest, strays more than nine hundred astronomical units away. “The very first clue that something else was out there was our discovery of Sedna,” Brown said. “That was the first object that didn’t quite fit any of the existing categories.”

Sedna is typically referred to as an extreme Kuiper Belt object, although Brown has also placed it in the Oort Cloud, another reservoir of icy scraps, which is thought to occupy the utmost edges of the solar system. Several other objects like it have since emerged, with license-plate names like 2012 VP 113 , which was discovered two years ago by Trujillo and his colleague Scott Shepard. (They nicknamed it “Biden.”) Such objects spend so much of their time so far away that only a few are visible during our lifetime. Of those that have been seen, however, what is remarkable is how closely their orbits align. Draw the solar system as you might view it from the top down—that is, perpendicular to the ecliptic, the plane on which the planets orbit. Neptune and all the planets within its orbit fit in a small circle; Sedna and the other extreme Kuiper Belt objects fan out to one side, their near poles overlapping. View the solar system from the side and it’s clear that Sedna and its kin share their own ecliptic, tilted at an odd angle from, and crossing, the main one. In their 2014 paper, Trujillo and Shepard noted that the similar orbits of these objects suggest "that an unknown massive perturbing body may be shepherding these objects into these similar orbital configurations.”

A little-noted fact about planets, and one inherent to their definition, is that they do stuff: they have sufficient mass and gravity to affect other objects in the solar system. As Brown and Batygin began to look more closely at the alignment of Sedna, 2012 VP 113 , and the rest, they, too, began to think that a larger organizing force might be at work. “We started scratching our heads,” Brown said. “And after a long analysis, a year and a half of back-and-forth, we realized that the answer is—and we can’t come up with any other answer—that there’s a giant planet that is sculpting the orbits of these objects, forcing these objects into this one particular location. This one giant planet that’s very far away, in the very distant part of the solar system.” The probability that the alignment occurred randomly, they calculated, was 0.007 per cent. “That’s not exactly a good gamble,” Batygin said.

At first, the pair envisaged a planet whose orbit encircled Sedna and the aligned objects, as a shepherd might its flock. (Astronomers refer to this model as “secular perturbation theory.”) But computer simulations made it clear that that arrangement didn’t explain the observable data. “We nearly gave up,” Batygin said. Instead, they posited a more interventionist prime mover: a giant planet on an eccentric orbit that crosses the objects’ orbits but is aligned against them, sending the planet far off in the opposite direction. This arrangement, too, seemed too peculiar to be real. But it allowed for a prediction: if there really was a planet with the size and orbit they had calculated, there should be a small class of Kuiper Belt objects in its path that have been tilted on their sides. A quick search through the data sets of the Minor Planet Center, at Harvard University, revealed precisely these objects, located precisely where they should be.