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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:
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.
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.
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