The Growing Case for an Elusive Ninth Planet

Astronomers have found another quirky object that suggests a giant body lurks at the fringes of the solar system.

Artist's impression of a planet in space
Artist's impression of a still-unseen ninth planet (Roberto Molar Candanosa / Scott Sheppard / Carnegie Institution for Science)

Astronomy has really wreaked some havoc on science textbooks over the years, particularly when it comes to cataloging the solar system. For most of the 20th century, there were nine planets, taught to schoolchildren with the help of quirky mnemonics like My Very Excellent Mother Just Served Us Nine Pizzas. Then, in 2006, the Pizzas were dropped; a set of astronomers determined that Pluto was better classified as a dwarf planet than as a full-fledged one. And now, after more than a decade of relative peace, astronomers wish to add a new ninth planet, upending humanity’s understanding of our solar system, not to mention the current school curriculum.

For the past several years, some astronomers have been thinking about a new planet in our cosmic neighborhood. The thing is, they’ve never seen it. They have only observed evidence that it may be out there: a cluster of small celestial bodies that move in unusual orbits compared with the rest of the solar system. This configuration, astronomers say, suggests these objects were jostled by a powerful unseen force: a huge planet, about 10 times the mass of Earth, orbiting in the fringes of the solar system, well beyond Pluto.

Telescope searches haven’t yet found this hypothetical planet, if it exists. But they’ve detected about a dozen objects with distant, unusual orbits that bolster its case. On Tuesday, astronomers announced that they’ve added a new object to the list.

The object in question is 2015 TG387, and it sits right in the middle of the mysterious, strange-orbited cluster astronomers have observed in the solar system. When the celestial body was first discovered in 2015, it was about two and a half times farther away from the sun than Pluto is right now. It took astronomers three more years and many more follow-up observations with powerful instruments like the Hubble Space Telescope to track the object and calculate its dramatic orbit:

Roberto Molar Candanosa / Scott Sheppard / Carnegie Institution for Science

2015 TG387 takes a whopping 40,000 years to circle the sun. It never actually comes close enough to the solar system’s giant planets—Jupiter, Saturn, Uranus, and Neptune—to feel their gravitational pull. This makes 2015 TG387, as well as other far-flung objects, such as 2012 VP113 and Sedna (also pictured), great candidates for studying the outer solar system.

“It never interacts with anything that we know of in the solar system,” says Scott Sheppard, an astronomer at the Carnegie Institution for Science and a co-discoverer of 2015 TG387. “Somehow, it had to get on this elongated orbit in the past, and that’s the big question: What did it interact with to get [there]?”

To figure that out, Sheppard and his colleagues ran computer simulations of a space environment that included a hypothetical ninth planet. They used calculations proposed by Michael Brown and Konstantin Batygin, a pair of California Institute of Technology astronomers who are also searching for this mystery body, which they call Planet Nine. It was a success: The simulations showed that a distant planet had shepherded 2015 TG387 into its funky orbit.

“This fits in perfectly with what we would predict for Planet Nine, so I’m happy to see it discovered,” says Brown, the Caltech astronomer. (Brown, coincidentally, is among those responsible for Pluto’s reclassification; he discovered many large objects beyond Neptune that made scientists rethink their definition of planets.)

Batygin was also excited. “I’m running code as we speak that evaluates how the inferred orbit and mass of [the hypothetical planet] are affected by this new object,” he said, when I contacted him the day before the discovery was announced.

Aside from its elongated orbit, little is known about 2015 TG387. The object is too far for astronomers to determine its composition or color. “It’s a point of light, it’s clear, it’s there, but it’s very faint,” Sheppard said.

Based on the little light they can see, Sheppard and his team estimate the object is about 300 kilometers (186 miles) across, which would make it a smallish dwarf planet.

Roberto Molar Candanosa / Scott Sheppard / Carnegie Institution for Science

The researchers think there could be thousands of small bodies like 2015 TG387 at the edge of the solar system. But finding them isn’t easy. For about 99 percent of its orbit, 2015 TG387 is too faint for our most powerful telescopes to detect. Astronomers only caught it when it made its closest approach to the sun.

And even then: “At the closest point in its orbit, this object is still more than two times farther from the sun than Neptune, which is pretty remarkable,” says Juliette Becker, an astronomer at the University of Michigan and a fellow hypothetical-planet hunter who was not involved in the study. “Objects at this distance are extremely hard to discover.”

Astronomers need to find more of these objects to sharpen their search for a new neighbor in our solar system. Their strange orbits reveal clues about the gravitational forces acting on them, which in turn provide information about the location of the hypothetical planet.

Or maybe not. The hypothetical-planet theory has its critics. They chalk it up to observational bias, the human tendency to see things that we expect or want to see. They suggest that one of the arguments for this mystery planet, the strange clustering of objects beyond Neptune, may not be that strange at all. Astronomers have only surveyed a fraction of the sky in their search; perhaps the pattern they’ve spotted in one slice of the solar system only looks like a pattern because they can’t see the rest, and these objects might actually be evenly distributed.

Or maybe these objects are indeed strange, but not because they’re being tugged at by some massive planet. Ann-Marie Madigan, an astrophysicist at the University of Colorado at Boulder, says the discovery of objects with strange orbits bolsters a different theory, which she calls the “self-gravity mechanism”: The edges of the solar system are littered with small objects and, sometimes, when they orbit near one another, their collective gravity is powerful enough to jostle their bigger neighbors, pushing them off the plane of the solar system.

“I’m excited as the discovery of TG387 points to a large population of small bodies in the outer solar system,” Madigan said. “The more bodies we detect, the more likely it is that their collective gravity is important, so it’s good news for my theory, too.”

The debate over a new planet in the solar system has many layers, including a rather trivial but contentious one: what to call it. Brown and Batygin’s favored name, Planet Nine, has irked some astronomers who never quite got over the injustice of Pluto’s demotion. Sheppard is aware of this tension and chooses to avoid it altogether by calling it Planet X. But even that title carries some baggage. In the early-20th century, the American astronomer Percival Lowell predicted that a planet seven times the mass of Earth orbited beyond Neptune. But Lowell never found the planet—it didn’t exist.

The nomenclature ultimately matters little to the search. To borrow from Shakespeare, that which we call a hypothetical planet by any other name would be as elusive.