Astronomers from Caltech have <a href="">announced</a> that they believe they have found evidence of a <a href="">giant ninth planet</a> in the solar system. This rendering depicts a distant view of Planet Nine.
Astronomers from Caltech have announced that they believe they have found evidence of a giant ninth planet in the solar system. This rendering depicts a distant view of Planet Nine. R. Hurt (IPAC)/Caltech
Solar System photo

A mysterious planet might lurk on the edge of our solar system

Scientists have long suspected that our solar system might harbor a hidden planet. Now, after decades of searching, they may be on to something.

This week, in The Astronomical Journal, astronomer Mike Brown and planetary scientist Konstantin Batygin published what may be a map to the hypothetical planet.

To find it, the pair of scientists analyzed the wonky orbits of 6 dwarf planets in the Kuiper Belt–a giant ring of rocky debris, just past Neptune. Something knocked these 6 objects out of alignment, and Brown and Batygin think a large planet could be out there tugging on them. The scientists think this hypothetical ‘Planet X’ could weigh as much as 10 Earth masses.

overlapping orbits

Evidence of Planet Nine

The six most distant known objects in the solar system with orbits exclusively beyond Neptune (magenta) all mysteriously line up in a single direction. Moreover, when viewed in three-dimensions, they are all tilted nearly identically away from the plane of the solar system. Such an orbital alignment can only be maintained by some outside force. In their new paper, Batygin and Brown show that a planet with 10 times the mass of the earth in a distant eccentric orbit anti-aligned with the other six objects (orange) is required to maintain this configuration.

This is not the first time scientists have thought they’ve pinned down a secret planet in the outer solar system, but the evidence is stronger now than it’s been before, says Jim Green, director of NASA’s Planetary Science Division.

How Could We Miss Such A Massive Planet?

If Planet X exists, Brown and Batygin have calculated that the closest it would ever come to the Sun is a distance of 200 astronomical units. One AU equals the distance between the Sun and Earth, so at its closest, this hypothetical planet is 200 times farther from the Sun than we are. And at its furthest point, it may be as much as 1200 AU from the Sun.

For context, Pluto is 40 AU from the Sun, and it took ten years for the New Horizons spacecraft to get there. The Voyager spacecraft–the fastest spacecraft ever–have traveled about 120 AU in the nearly 40 years since they launched.

So if Planet X is out there, it’s really, really far away.

“These are very long orbitals,” says Green, “right at the edge of visibility with the top telescopes in the world, so it’s going to take a little while to work through this.”

How Skeptical Should We Be?

Although Brown and Batygin are careful, respected scientists, it’s still possible they could be mistaken.

“Everyone is skeptical because we’ve cried wolf on this a bunch of times,” says Green. “Wishing doesn’t make it so. It has to withstand the scrutiny of the science community, and in the past it’s always fallen apart.”

If Planet X is out there, it’s really, really far away.

Brown and Batygin’s educated guess as to where Planet X might be is based on the crooked orbits of 6 very distant objects. Because they’re so far away, it takes them hundreds of years or more to orbit the sun. That means we’ve only seen a fraction of their orbits, so the scientists’ calculations of their pathways may not be entirely accurate.

For comparison, says Green, “when we flew by Pluto last summer, we really didn’t know where Pluto was.” That’s because it takes Pluto 247 Earth-years to circle the Sun, and since we’ve only known about Pluto since 1930, we’ve only seen a small piece of its orbit. Fortunately, the New Horizons spacecraft was able to pinpoint Pluto’s position on the fly, “but Pluto could have actually been 1000 kilometers ahead of or behind where it really was. So calculating orbits really is a tough business.”

How Will We Find Out Whether It’s Really There Or Not?

In addition to checking the orbits that Brown and Batygin calculated, scientists will be searching for evidence of Planet X in the data they already have. It’s a big sky, and now that scientists have a clearer idea of where to look for Planet X, that could help to track down details that might’ve been overlooked before.

Green says the data from NASA’s Wide-Field Infrared Survey Explorer (WISE) could be a good place to start. Large planets tend to radiate heat, so if Planet X is out there, WISE’s infrared sensors could have detected the signature.

Scientists have looked for Planet X in WISE data before, “but they were looking at the whole entire sky and not all the wavelengths were checked,” says Green. “So I’m sure there are scientists right now looking back at the WISE data and saying, ‘Did we miss it?’”

The next step will be to look for Planet X using a powerful telescope. Brown and Batygin have already secured time on the Subaru telescope in Hawaii to do just that.

“If these guys are right, we’re going to be looking at Planet X in 2018.”

Similarly, Green is optimistic that if Planet X is out there, NASA’s James Webb Space Telescope, expected to launch into space in 2018, could find it. The large infrared telescope is Hubble’s successor. “If these guys are right, we’re going to be looking at [Planet X] in 2018.”

Finding Planet X will be a challenge since we don’t really know how big it is. Although Brown and Batygin estimate that it weighs 10 Earth masses, that mass could potentially be compacted into a small and very dense terrestrial planet, or spread out into a large and easier-to-spot gas giant.

Looking for more dwarf planets with skewed orbits could also provide some helpful hints, perhaps pinpointing exactly where Planet X is hiding, or else show that it was something else that disturbed these objects’ orbit.

What Else Could It Be?

The weird orbits of the 6 dwarf planets suggests that something scattered them.

“Sometimes orbital dynamics are like a pool game where you’ve got two balls that hit each other and it changes their trajectories,” says Green. “But in space they don’t actually have to hit each other, their gravity can change their orbits.”

It’s too soon to say exactly what was responsible for knocking those objects askew. Could it have been the exiled Planet X, formed in the inner solar system but later knocked out when things got too crowded? Alternately, perhaps it was a series of smaller perturbations that scattered the objects out of alignment, suggests Green, or even a rogue planet—a planet without a star—wandering through our solar system.

Without hard evidence, it’s hard to say right now.

“Of course, finding another planet would be pretty spectacular,” says Green.