For the last year and a half, the man who killed Pluto has been trying to make up for it by finding another ninth planet. This planet, once nicknamed “Planet X,” would be a theoretical source of gravity beyond the icy rocks that endlessly tumble past Neptune — the Kuiper Belt — explaining some odd orbits observed over the last few years. Today, in a open-access study published in The Astronomical Journal, Brown and his colleague Konstantin Batygin offer the best evidence yet that a dim hulk circling past the Kuiper Belt would be the ninth planet in the solar system.

“For the first time in over 150 years, there is solid evidence that the solar system’s planetary census is incomplete,” said Batygin in a press release.

The six most distant objects in the solar system (magenta) all mysteriously align. Planet Nine, if it had 10 times Earth’s gravity and an anti-aligned orbit to these distant objects (orange), would explain these paths almost perfectly. (Image: Caltech)

The search for “Planet X,” now nicknamed “Planet Nine” by Brown and Batygin, began in 2014 when Chad Trujillo and Scott Sheppard first noticed that the orbits of 13 outer-solar system objects were oddly aligned. The time at which these objects passed through the plane of the planet’s orbits, and when they made their closest approach to the Sun, called the “argument of perihelion,” synced up in a way that chance should exclude.

When Brown and Batygin looked at this data with fresh eyes, they found that the coincidences compounded. Not only did the argument of perihelion for the outer-solar system objects match up, six of those objects (colored magenta in the graphic above) follow elliptical orbits pointing the same direction. Because these orbits took the objects across the entire solar system and at different rates, something had to be lining them up.

Then came the mathematical modeling. What could produce such an odd set of orbits? Brown and Batygin settled on a theoretical planet, one with 10 times the gravity of Earth, that could gravitationally corral smaller objects. Modeling Planet Nine produced paths that six known Kuiper Belt objects traced exactly.

The existence of Planet Nine (orange) predicts that some Kuiper Belt objects should assume orbits that are perpendicular to the plane of the solar system (in blue). That’s exactly what Brown and Batygin confirmed when checking the orbits of known objects. (Image: Caltech)

Recreating the Kuiper Belt orbits was just one line of evidence that makes Brown and Batygin’s Planet Nine so compelling. According to the models, Planet Nine’s orbit would also produce orbits in other Kuiper Belt objects that were perpendicular—imagine two circles passing through each other to make an “X”—to the orbit of the solar system’s planets. Five known objects (blue in graphic above) fit the bill.

A year on Planet Nine would take at least 10,000 Earth-years to elapse. It would have an orbital distance 20 times that of Neptune—56 billion miles. All of this theoretical language is necessary because no one has actually seen Planet Nine. It lives exclusively in computer models. To find it, it would take the world’s most powerful telescopes, like those at the W. M. Keck Observatory or the Subaru Telescope, because that distance means an exceptionally dim planet. If the timing is right, astronomers might even be able to spot it in previous images. With the best evidence yet for the third planet found since ancient times, the hunt is now on.

“All those people who are mad that Pluto is no longer a planet can be thrilled to know that there is a real planet out there still to be found,” Brown concludes. “Now we can go and find this planet and make the solar system have nine planets once again.”

—

IMAGES: Powered by deviantART//Hameed; Caltech

STUDY: Evidence for a distant giant planet in the solar system