Something could be lurking at the edge of the solar system. Something big—two to four times larger than Earth—and dark. It might be a planet, slowly orbiting, and pitched away from the rest of the solar system. The only evidence comes from a half dozen dwarf planets circling the sun in strange, skewed, far-away orbits.
The evidence might be sparse, but it is also compelling. According to new analysis in the Astronomical Journal, few things besides gravitational nudging from a large, ninth planet would explain the several dozen rogue Kuiper Belt objects with highly elliptical orbits. The hypothesis, published by a pair of Caltech researchers, says the planet is probably a gas giant that was punted out of the ecliptic plane, in which all the other planets orbit, eons ago.
But phantom planets are nothing new. "There are dozens, if not hundreds of examples where researchers have said there must be another planet to explain some orbital anomaly," says Mike Brown, Caltech astronomer and co-author of the new planet paper. Occasionally, the phantoms are real. In the mid-1800s, European astronomers noticed something funky in Uranus' orbit, and posited that there might be another planet tugging on it. In a Newtonian coup, one French mathematician perfectly predicted Neptune's position. The day after receiving the prediction in a letter, his German astronomer colleague found the planet right where it was supposed to be.
But Neptune didn't resolve everybody's questions about Uranus. In fact, the newest planet's orbit also had a slight lag—raising the possibility of another planet beyond Uranus. The tug was so compelling that amateur astronomer and professional millionaire Percival Lowell built an observatory atop a mountain in a distant, western desert now called Arizona. Lowell died looking for his so-called Planet X.
The old man had been gone nearly 15 years before someone found it. William Tombaugh spotted a pinprick-sized discrepancy between parallel pictures of stars in the Gemini galaxy, taken five days apart. His discovery jolted the field of astronomy, and for a short time vindicated Lowell's theories about Planet X. Eventually science caught up with the hype, and this tiny world—named Pluto—was deemed too small to be responsible for Neptune's, let alone Uranus' wobble. "The only reason Lowell thought Planet X was out there was because of bad data that showed Neptune and Uranus were perturbed," says Brown. Over time, better observations showed that the two outermost gas giants were orbiting exactly as they ought to.
Pluto was the original Planet Nine, which means humans have been looking for this mysterious world for over 100 years. Here's another plot twist: "New" Planet Nine advocate Mike Brown was one of the main agitators behind the dwarf planet's ignominious demotion. Conspiracy? Perhaps a ploy to open up the highly coveted position of ninth planet for a larger, gassier, more suitable representative? "Yeah, you know Planet 10 just sounds so much worse, so it had to happen," he jokes. In other words, no. Though any humor in this ironic coincidence is probably lost on Alan Stern, and other Pluto-philes.
Some pretty sweet astronomy burns. And Stern has a point. This new planet is still just a theory. But if it does exist, it's got a kickass origin story. Hypothetically, the planet is up to four times the size of Earth, and at closest approach, 76 times as far from the Sun—a single orbit would take between 10,000 and 20,000 years. "There wouldn't have been enough material that far out to form something that big," says Scott Sheppard, astronomer at the Carnegie Institution for Science in Washington, DC. Instead, it probably formed in the same neighborhood as Jupiter, Saturn, Uranus, and Neptune.
Back then, things weren't so nicely organized, orbits not so circular. The new planet could have been on a near collision course with one of those larger giants, but instead of smashing into it, it merely passed close enough for a gravity assist. "It was a runt, and interacted with something like Jupiter, and Jupiter pushed it out to the outer solar system," says Sheppard.
If it's there, this new planet's orbit is really funky. This graphic above shows just how far off kilter the new planet is from the ecliptic plane.
Theoretically, that is. Right now, the only reason astronomers think any of this stuff is true is because in 2014 Sheppard and a co-author discovered six objects outside the Kuiper Belt with weird orbits. Most of these icy worlds beyond Pluto orbit within a certain range, and follow roughly circular tracks. "The graphic shows the distant, eccentric orbits, all pointing off in the same direction," says Brown. His theory is the new planet has been gravitationally nudging these tiny worlds. "It gently shepherds them over time," says Brown.
Figuring that out took a year's worth of supercomputer simulations, in which he and co-author Konstantin Batygin ran the orbits through models to see what could possibly explain those wide orbits. They knew about the new planet hypothesis—Sheppard originally proposed it in his 2014 work—but initially sought to disprove it. "We found what works," says Brown.
Still, six is a pretty low number. "It’s not a slam dunk," says Sheppard. "We’re dealing with low number statistics, a handful of objects." He has found a few more objects in the same general neighborhood, but they were too new to include in Brown's study. "It take s a year or two to get their orbits down and say exactly what they are doing, so we’re still following up." In the end, the only evidence that really matters will be whether someone actually captures this phantom planet in their telescope lens. "The sky is a big place, but based on this now we know where to look," says Brown. This time, though, the prediction probably won't come by letter.