Finding Planet Nine

In 2005, Caltech researcher Mike Brown, spotted a tenth planet in the solar system that became known as Eris. The discovery of Eris, which was similar to the size of Pluto, created debates on what could and could not be dubbed an actual planet. As a consequence, the International Astronomical Union classified Eris as a dwarf planet and, with Brown’s help, even Pluto was demoted. Having accepted that he is the man responsible for the abandonment of Pluto, Brown wrote a book titled “How I Killed Pluto and Why It Had It Coming” five years later.

It’s been over 10 years since then, and although there have been vague claims here and there, only recently has there been solid evidence of a real ninth planet. On January 20, 2016, the Pluto-killer himself and his colleague, Konstantin Batygin, announced the discovery of Planet Nine in The Astronomical Journal. The planet is said to have a vast elliptical orbit that would take the planet 10,000 to 20,000 years to orbit the sun. Furthermore, it is thought to be gaseous (similar to Uranus and Neptune) and ten times the mass of Earth. Brown notes that Planet Nine is sufficiently large (5,000 times the mass of Pluto) that its membership as a true planet will not be debated.

Brown and Batygin have not actually seen Planet Nine directly, but used computer simulations and mathematical modeling to infer its existence. The claim is that the orbit of six, smaller objects—known as Kuiper Belt objects (KBOs)—lined up in a way that suggested the gravitational influence of a massive, unknown planet. “It’s almost like having six hands on a clock all moving at different rates, and when you happen to look up, they’re all in exactly the same place. The odds of having that happen are something like 1 in 100. But on top of that, the orbits of the six objects are also all tilted in the same way—pointing about 30 degrees downward in the same direction relative to the plane of the eight known planets. The probability of that happening is about 0.007 percent (Caltech).” says Brown. “Basically it shouldn’t happen randomly. So we thought something else must be shaping these orbits (Caltech).”

At first, Brown and Batygin investigated whether this phenomenon could have been caused by distant KBOs. However, they quickly ruled out this possibility as it would require the Kuiper Belt to have 100 times its current mass. Then, while trying to find out whether a planet could have caused this scenario, the researchers accidentally ran simulations with a planet in an anti-aligned orbit. Astonishingly, the alignment of KBOs was observed. Although they were skeptical at first, additions to the model managed to persuade the two. Eventually, they even found that Planet Nine’s presence explains more than just the observed clustering, but also the mysterious orbits of Sedna-like objects. “When the simulation aligned the distant Kuiper Belt objects and created objects like Sedna, we thought this is kind of awesome—you kill two birds with one stone. But with the existence of the planet also explaining these perpendicular orbits, not only do you kill two birds, you also take down a bird that you didn’t realize was sitting in a nearby tree (Caltech).” says Batygin.

• Discovery
• science