The astronomer who triggered Pluto's demotion from planetary status hopes to replace it with a new Planet Nine, and thinks his case is getting stronger. We still have no visual confirmation of the hypothesized planet, but evidence for its influence is growing.
Dr. Michael Brown of Caltech wears his status as an astronomical downgrader with pride: His twitter handle is @plutokiller. Pluto's demotion, voted on by the International Astronomical Union (IAU) in response to Brown's discoveries, was so controversial that 10 years later Brown still gets violent threats.
In January, however, Brown redeemed himself in some eyes with the proposal for a new Planet Nine. The hypothesized planet may be too far out for our current telescopes to see, but is proposed to have an elongated orbit that would at times bring it close enough to the Sun to disturb any comets and dwarf planets it encountered. Brown argued that patterns in the orbits of some of the outer Solar System inhabitants indicate they have all been herded into place by something with a mass similar to that of Neptune.
The latest addition to Brown's case is the announcement at a SETI colloquium of a Kuiper Belt Object (KBO), something orbiting between 30 and 50 AUs (astronomical units), one AU being Earth's distance from the Sun. The object, Brown tweeted, “Is exactly where Planet Nine says it should be.”
Hey Planet Nine fans, a new eccentric KBO was discovered. And it is exactly where Planet Nine says it should be. pic.twitter.com/oZn0RDq8JF
— Mike Brown (@plutokiller) March 24, 2016
Before the paper and new KBO, Brown's claims were attracting plenty of interest from astronomers.
Brown and Batygin previously claimed that six KBOs with eccentric orbits all make their closest approach to the Sun on the same side, in the same orbital plane and at similar distances, despite quite different orbital lengths. This, they argued, was either a staggeringly unlikely coincidence, or the sign of a larger object doing some gravitational shepherding.
Animation of Brown and Batygin's evidence for a ninth planet made before the most recent discovery. Caltech
The pair's latest work argues this object must make its closest approach to the Sun at between 200 and 350 AUs and have an orbit inclined at 30 degrees to the plane in which the other eight planets travel. These led to suggestions on where astronomers should look to find it.
The idea of locating a planet through its gravitational influence is not new. It alerted astronomers to the existence of Neptune, and resembles the method by which we discovered our first planets beyond the Solar System.
Nevertheless, its reliability has been less than perfect. In the early 20th century Percival Lowell used a similar process to predict the existence of a ninth planet. Observations to test his theory led to the discovery of Pluto, but no planet remotely large enough to match Lowell's description has ever been found. Moreover, plenty of subsequent planets have been postulated based on discrepancies or patterns in the orbits of outer Solar System objects, usually turning out to have no substance.