On the hunt for Planet Nine

This illustration shows an imagined view from the surface of one of the three planets orbiting an ultracool dwarf star just 40 light-years from Earth.

European Southern Observatory/M. Kornmesser

This illustration shows an imagined view from the surface of one of the three planets orbiting an ultracool dwarf star just 40 light years from Earth.

Ever since Pluto was demoted in 2006, our solar system has gotten by with just eight planets. Now astronomers believe they’re hot on the trail of a new ninth planet — and unlike puny Pluto, this one may be so massive, there will be no mistaking its status.

The existence of this planet, which is known provisionally as Planet Nine, was raised in January in a paper by Michael Brown and Konstantin Batygin, astronomers at Caltech. Following the lead of other researchers, Brown and Batygin noticed that the orbits of the biggest objects in the Kuiper Belt — an expansive disc of rocky debris out beyond Neptune — were aligned. The unlikely alignment suggested that the gravitational force of some larger object beyond the Kuiper Belt was pulling on these objects.


“We realized something nobody had noted before, which is that all these most distant objects in the outer solar system, their orbits pointed off in the same direction,” says Brown. “Once we realized these objects are actually lined up, we realized something had to be keeping them lined up.”

This kind of reasoning has been fruitful for astronomers in the past. In the 1846, astronomers went hunting for — and immediately found — Neptune after noticing unexplained perturbations in the orbit of Uranus. Later, astronomers thought they noticed similar quirks in Uranus’s orbit, which led them to look for a so-called Planet X. That planet ended up being a phantom (and the supposed quirks in Uranus’s orbit turned out not to be there), but the search for Planet X did lead to the discovery of Pluto.

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In this case, Brown and Batygin crunched the numbers and concluded that whatever was pulling on objects in the Kuiper Belt would have to have some extraordinary properties. They estimate it would have to be 10 times as massive as Earth, have a highly elliptical orbit in which it’s as far as 1,500 astronomical units, AU, from the sun and never closer than 400 AU (one AU is the distance from the sun to Earth; Neptune averages a distance of about 30 AU), and take 15,000 years to complete that orbit (compared to 165 years for Neptune). Together, these facts would make it a complete outlier compared to the other eight planets in our solar system.

“It really changes the way we look at the solar system,” says Scott Kenyon of the Harvard-Smithsonian Center for Astrophysics, who recently coauthored a pair of papers about how Planet Nine could have come to exist at such a distance from the sun. “We have these four rocky planets, then four gas giants, and beyond Neptune we thought it was a lot of smallish objects with Pluto being the biggest. We have these nice groupings, now we have this oddball, and we have to understand who ordered this.”

Over the four months since Batygin and Brown announced the likely existence of Planet Nine, a number of astronomers have tried to explain its potentially unusual properties. In a paper forthcoming in the Astrophysical Journal Letters, Gongjie Li, a junior fellow in the Harvard astronomy department, considers the remote possibility that Planet Nine originally belonged to a different solar system and was captured by ours.


“That requires the planet to be far away from a large host star. That has a probability around 1 or 2 percent,” she says.

Two other astrophysicists — Kenyon and Ben Bromley of the University of Utah — have recently submitted a pair of articles to the Astrophysical Journal that consider two other scenarios. In the first, Planet Nine formed in place, as Pluto did, as the slow agglomeration of ice pellets over a period of 1 billion years or more. In the second, Planet Nine formed alongside the four gas giants (Jupiter, Saturn, Uranus, Neptune) in the early days of the solar system and was ejected outward by their gravitational force.

“During the formation, it was probably the runt of the litter, and it got tossed out into the outer part of the solar system,” says Kenyon.

The only way to tell which of these scenarios is correct is to actually find Planet Nine. If it’s actually spotted, its orbital pattern could provide clues about its origin. Astronomers may also be able to take spectroscope readings, which will allow them to determine what it’s made of. If the answer is ice, then the planet probably formed where it is. If it has the composition of the core of a gas giant, they know it likely originated closer to the sun and got kicked out.

Efforts to spot Planet Nine will begin in earnest in November or December. That’s when astronomers will have the best view of the tremendous swath of space likely to contain Planet Nine should it really exist. Brown and Batygin are currently working to reserve time on the Subaru Telescope in Hawaii, which is owned by the National Astronomical Observatory of Japan and is the best instrument for the task. Brown thinks it could take three to five years to actually find the planet, and he’s “willing to make a pretty big bet” that it’s there. The way detection would work is that, one night, the astronomers will see a bright, unidentified point in space. If the next night they come back and find that point has moved relative to the rest of space — indicating it’s part of our solar system — they’ll know they have it.

“Official recognition would happen at Day 1,” says Brown. “If there’s this 10-times-Earth-mass planet out there, everything about the solar system from that day forward will include that as a planet.”

Kevin Hartnett is a writer in South Carolina. He can be reached at

Correction: Because of a reporting error, Gongjie Li’s name was misspelled in an earlier version of this story.

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