planet edge of solar system
Artist's concept of a planet at the outer edges of our solar system. This particular planet was catalogued in 2005 as 2003UB313. NASA/JPL-Caltech

The search for Pluto’s replacement has begun. The astronomer leading the charge is Caltech’s Mike Brown, also known as the man who killed Pluto. He has already discovered 16 dwarf planets in the outer solar system, but when it comes to real planets—bodies with enough mass to make themselves round, clear their neighborhood of debris, and orbit the Sun—there’s only one new contender. And they call it Planet X.

“We have found evidence that there’s a giant planet in the outer solar system,” Brown says. “By ‘giant’ we mean the size of Neptune, and when we say ‘outer solar system’ we mean 10 to 20 times farther away than Pluto.”

planet edge of solar system

Lonely Planet

Artist’s concept of a planet at the outer edges of our solar system. This particular planet was catalogued in 2005 as 2003UB313.

Granted, Brown is not the first to make such a claim. Theories of a Planet X have been around for a century, including two published in the arXiv last month. So far, none have turned out to be true planets. But Brown has evidence to make a pretty strong case that his is a real one. After connecting the dots between half a dozen dwarf planets far beyond the Kuiper Belt, he thinks he has a treasure map with a big red Planet X on it–a map that he published in The Astronomical Journal today. He has made the map open access, hoping that others will join in the search.

Where It All Began

In 2003, Brown and his team discovered a dwarf planet called Sedna; it was the farthest object ever seen in our solar system. Its extremely elliptical orbit is so large it takes 10,000 years to make one trip around the sun. Its perihelion, or closest approach to the Sun, is 76 astronomical units from the sun (1 AU = the distance between Earth and the Sun). The farthest it gets from the sun is 937 AU. For comparison, Pluto lives at 36 AU.

More than a decade later, in 2014, scientists discovered another Sedna-like body, called 2012VP113, that lived far out past the Kuiper Belt and also had an eccentric orbit. Its perihelion is 80 AU. Eventually four more objects—bringing the total to six—were found with similar lopsided orbits, and they all happened to sweep out in the same direction.

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.

But it wasn’t the sweeping that intrigued Brown. He noticed a pattern: all six objects were also lined up in space. “You can think of them like different hands on the clock,” Brown says. “They’re all moving at different rates in the same direction, and every once in a while you look up and they are all in the same spot.” There is a less than one percent chance, according to Brown, that this is just a coincidence.

Calling For Backup

In order to figure out what could be keeping six dwarf planets in this position, Brown enlisted the help of Konstantin Batygin, a theoretical astrophysicist at Caltech who specializes in solar system dynamics.

Batygin input data from his mathematical theories, along with Brown’s observations, into Caltech’s CITerra supercomputer. For a year, the team cranked out computer simulations and filled the office chalkboards with complicated Hamiltonian Dynamics to test every possible explanation for the alignment. What they figured out was this: something massive must be keeping these six bodies in line with one another. That something could be Planet X.

lab photo brown batygin

In the Lab

Brown and Batygin get nerdy

“Think of the planet as a dog that shepherds the six objects, like it would sheep,” Batygin says. Planet X’s influence on the six bodies is powerful and effective at keeping them in line, not unlike the way our sun keeps our eight planets aligned. Planet X, on the other hand, only interacts with these six bodies every 50,000 years or so, on average. Because its influence is so strong, they calculate the planet would need to be about 10 times the mass of Earth to have that effect.

The Formation Of Planet X

This is how Brown and Batygin think Planet X formed: in the early stages of the solar system, some 4 billion years ago, the large planets (including Planet X) were still rocky cores. If Planet X’s core had been able to stay in the inner solar system and carry out the rest of its formation, it could have accumulated enough gas or ice to become another giant like Jupiter or Neptune.

But because the large cores of the other planets were packed so tightly in the inner solar system, there wasn’t enough room for them all to develop. So one got kicked out. “There would have been a gas nebula around the solar system at the time that would have slowed it down as it plowed through the gas, putting it into this eccentric orbit,” Brown explains.

If such a large and far-out planet does exist, it would likely be a rocky core covered in methane and nitrogen ices. Its orbit would be anti-aligned, meaning it swings out the opposite way of everything else in our solar system, and it would probably take around 20,000 years to orbit the sun. “There’s no rule in the universe that says we get to keep all of our planets,” says Batygin. “And as it turns out, maybe one of them is still lurking out there.”

The Proof

The data is still being compiled, but the theory checks out, at least mathematically. “This predicts exactly where Planet X should be,” Brown says. The only thing left is to find it.

To do that, the team secured time on the Subaru 8.2-meter telescope atop Mauna Kea in Hawaii—one of the most powerful telescopes on Earth. It has already started scanning the skies for any movement from anything beyond the Kuiper Belt. In the meantime, the team expects to find more objects like Sedna and 2012VP113, which will help serve as further evidence to narrow down Planet X’s current location.

They’re on a mission to reclaim an original member of our solar system—to find our true ninth planet.

Even though they know the general area where the planet should reside, Brown and Batygin estimate it could take them between 5 and 15 years to find it. And that’s okay. They’re on a mission to reclaim an original member of our solar system—to find our true ninth planet. It might sound bold, but most claims that have the potential to change our understanding of the Universe usually are. And if they locate it, Brown is no longer a planet killer, but a planet savior.


It’s a quiet Wednesday at the California Institute of Technology. These famous arched hallways have been graced the by the likes of Einstein, Feynman, and Hawking, just to name a few. Brown and Batygin work in offices next door to one another. Batygin’s chalkboard is a mix of complicated physics and a big note-to-self that says: “Give Mike data!”

Brown has tokens on his desk, shiny meteorites, a jade sculpture of Sedna, and other inspirations—such as photos of his wife and daughter.

The team published two papers in The Astronomical Journal this week with their findings and theories about this potential planet. Their goal is to release as much information as possible so that other astronomers and scientists can join the search.

Brown and Batygin sat down with Popular Science to talk about the possible existence of our long-lost 9th planet and the beginning of the worldwide hunt.

Batygin's blackboard

Batygin’s chalkboard

Give Mike data! 🙂

How long did you think it would take you to solve this problem?

Mike Brown: I brought it to Konstantin thinking he would need a few weeks at most.

Konstantin Batygin: I thought it would take me no longer than an afternoon to figure out.

MB: A year or so later, here we are.

How do you think the scientific community will react to your news?

MB: If you ever say, ‘We think we have found evidence for Planet X,’ the first reaction of any astronomer is, ‘Oh, you’re one of those crazy people.’ And that’s a perfectly reasonable reaction. There’s been so much craziness over these things over the past century that there’s no reason to believe that somebody is going to come along who’s not crazy and say the same thing. Except, I think we’re right this time rather than crazy.

KB: Hopefully our work will be met with follow-up searches.

What was the goal in publishing the paper now rather than just searching for it yourself?

MB: The paper is a roadmap for anyone to look. I wanted to make it as clear as possible where to look, and where I have looked already.

What makes you want to involve other people in the search?

MB: I want to know the answer. I need to know the answer more than I need to be the discoverer. Don’t get me wrong; I would love to be the discoverer, given the choice, but I just want to know it’s really there.

What will people need if they want to begin searching for Planet X?

KB: Make sure you’re stocked up on coffee and Red Bull.

Where does the drive come from to find planets, and this one in particular?

MB: For me, the answer is exploration; it’s just an exciting thing to do. If there’s something out there, I want to go and find it. You can check off the scientific reasons why it’s a useful endeavor and write grant proposals, but mostly I just want to know what’s going on there. And this—the possibility that somewhere out there, at the edge of our solar system, there is this 10-Earth-mass planet that nobody knew about until now—it’s like, ‘Wow!’

KB: Opportunities to discover something drastically new about our cosmic environment don’t come up everyday, so I feel fortunate that I have had a chance to play this game. I’m also genuinely excited to find out if our theoretical calculations have yielded a prediction that is on-point or not.

Mike, you’re known for being big on names for planets, have you thought about what you would like this to be called if it’s discovered?

MB: My daughter, Lilah, has suggested that we call it Pluto. That way Pluto can be a planet again.