TIME space

New ‘Planet 9’ May Be Next to Join Our Solar System

"We have convincing evidence that the census of the solar system is incomplete."

Michael Brown doesn’t care how much you love Pluto. He loves it too, but that didn’t stop him from leading the charge in 2006 to strip it of its “planet” designation and bust it down to a mere dwarf planet. But now, the Caltech astronomer is making amends in a big way: along with Caltech planetary scientist Konstantin Batygin, he has just released a paper announcing the highly likely discovery of a new planet to replace Pluto—and no one’s ever going to be able to demote this one.

“The existence of another planet has been spoken about 100 times before,” Brown says. “But this is the first time in 150 years that we can say we have convincing evidence that the census of the solar system is incomplete.”

The discovery of the world that Brown and Batygin refer to in The Astronomical Journal simply as “Planet 9” began in 2003, with the discovery of a far more modest object named Sedna. A dwarf planet even smaller than Pluto, Sedna is a Kuiper Belt object (KBO), like Pluto one of a vast band of icy, rocky objects that surround the solar system beyond the orbit of Neptune. Brown was part of the team that found Sedna too, and if anything made the new world remarkable, it was its extreme distance from the sun—one that has it completing a single orbit in 11,400 years, compared to Pluto’s 248.

Read next: 500 Planets on One Exquisite Chart

But something else was strange about Sedna as well—or at least about the company it keeps. In the years that followed, astronomers discovered five other KBOs whose closest approach to the sun (or perihelion) matches Sedna’s almost perfectly, both in distance and in the angle of the orbit relative to the horizon of the solar system.

This couldn’t be a coincidence. The matching perihelia have only a 0.7% likelihood of occurring by chance; the matching angles have only a 1% chance. The double match among all six objects factors out to a vanishingly small .007% likelihood of being random. “It was very exciting to notice this collection of objects in this super obscure arrangement,” says Brown. “The orbits are physically lined up in space.”

If it wasn’t chance that was behind such a tidy clustering, it had to be gravity—the influence of a passing object that herds the KBOs together, in the same way Saturn’s moons influence the positioning of particles in the rings. Using mathematical modeling and computer simulations, Brown and Batygin concluded that Sedna and the other members of its flock must be herded by an as-yet-unobserved planet with about ten times the mass of Earth, circling the sun at an average distance 20 times greater than that of Neptune—making one orbit every 20,000 years. A body with different mass or a different distance simply couldn’t explain the behavior of the six KBOs.

Read more: Pluto — No Longer a Planet — Has a Twin Sister

How Planet 9 found itself in so remote a place is unclear. It’s unlikely that it formed in situ, essentially coalescing out of the primordial cloud of dust and gas that formed the rest of the solar system, since there probably wouldn’t have been enough raw material to form so big a world out that far. Rather, the new planet might have accreted closer in like the other planets, and then been ejected by their gravitational forces.

“Planet 9 is part of the sun’s family,” says Brown. “The birth cluster turned it around so it was placed in a distant orbit. It has been there all along and has witnessed the development of the solar system from afar.”

Now, Brown and Batygin need earthbound witnesses to confirm that Planet 9 exists. Just as Neptune was originally inferred by wobbles spotted in Uranus’s orbit, the existence of Planet 9 can be inferred by the aligned KBOs. But without a verifiable observation, it’s still a theoretical discovery. The pair are hoping to crowdsource that job, getting as many telescopes looking as possible across the globe. They’ve made the challenge easier by mapping Planet 9’s orbit; now skywatchers have to pinpoint where it is on that very long path.

The name of the planet will be crowd-sourced too, if the researchers get their way — as opposed to being proposed by the discoverer and then approved by the International Astronomical Union (IAU), which is the usual way of doing things. Brown’s and Batygin’s personal name preference is “George,” a hat-tip to British astronomer William Herschel, who discovered Uranus and wanted to name it Georgium Sidus (the Georgian Planet) after King George III. That might be a hard sell to the IAU—to say nothing of nearly all other stargazers, who tend to like a little more lyricism in their cosmos.

Read next: A Month-by-Month Guide to the Best Places to Travel in 2016

Whatever the planet is eventually called, its very existence will do more than simply add to the population of the solar system. It will also add to its mystery. Even in our tiny corner of the universe, it seems, there can still be big surprises lurking.

Your browser is out of date. Please update your browser at http://update.microsoft.com


YOU BROKE TIME.COM!

Dear TIME Readers,

As a regular visitor to TIME.com, we are sure you enjoy all the great journalism created by our editors and reporters. Great journalism has great value, and it costs money to make it. One of the main ways we cover our costs is through advertising.

The use of software that blocks ads limits our ability to provide you with the journalism you enjoy. Consider turning your Ad Blocker off so that we can continue to provide the world class journalism you have become accustomed to.

The TIME Team