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