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Watch out Pluto; Planet Nine is here

Watch out Pluto; Planet Nine is here

Step aside Pluto and make way for ninth planet. Caltech researchers have found evidence of a giant planet tracing a bizarre, highly elongated orbit in the outer solar system. The object, which the researchers have nicknamed Planet Nine, has a mass about 10 times that of Earth and orbits about 20 times farther from the sun on average than does Neptune.

This would be a real ninth planet, says Mike Brown, the Richard and Barbara Rosenberg Professor of Planetary Astronomy. There have only been two true planets discovered since ancient times, and this would be a third. It's a pretty substantial chunk of our solar system that's still out there to be found, which is pretty exciting.

Konstantin Batygin and Brown describe their work in the current issue of the Astronomical Journal and show how Planet Nine helps explain a number of mysterious features of the field of icy objects and debris beyond Neptune known as the Kuiper Belt.

Planet Nine is unique because it is located much farther out from the sun than the rest of the planets in our solar system, which are clustered closely to the sun. In fact, on its orbit, Planet Nine comes no closer to the sun than about 250 astronomical units. One astronomical unit, the distance from the Earth to the sun, is about 93 million miles.

Brown took the problem down the hall to Batygin, and the two started what became a year-and-a-half-long collaboration to investigate the distant objects. Fairly quickly Batygin and Brown realized that the six most distant objects from Trujillo and Sheppard's original collection all follow elliptical orbits that point in the same direction in physical space. That is particularly surprising because the outermost points of their orbits move around the solar system, and they travel at different rates.

The first possibility they investigated was that perhaps there are enough distant Kuiper Belt objects some of which have not yet been discovered to exert the gravity needed to keep that subpopulation clustered together. The researchers quickly ruled this out when it turned out that such a scenario would require the Kuiper Belt to have about 100 times the mass it has today.

That left them with the idea of a planet. Then, effectively by accident, Batygin and Brown noticed that if they ran their simulations with a massive planet in an anti-aligned orbit an orbit in which the planet's closest approach to the sun, or perihelion, is 180 degrees across from the perihelion of all the other objects and known planets the distant Kuiper Belt objects in the simulation assumed the alignment that is actually observed.

And indeed Planet Nine's existence helps explain more than just the alignment of the distant Kuiper Belt objects. It also provides an explanation for the mysterious orbits that two of them trace. The first of those objects, dubbed Sedna, was discovered by Brown in 2003. Batygin and Brown found that the presence of Planet Nine in its proposed orbit naturally produces Sedna-like objects by taking a standard Kuiper Belt object and slowly pulling it away into an orbit less connected to Neptune.

Where did Planet Nine come from and how did it end up in the outer solar system Scientists have long believed that the early solar system began with four planetary cores that went on to grab all of the gas around them, forming the four gas planets Jupiter, Saturn, Uranus, and Neptune. Over time, collisions and ejections shaped them and moved them out to their present locations. But there is no reason that there could not have been five cores, rather than four, says Brown. Planet Nine could represent that fifth core, and if it got too close to Jupiter or Saturn, it could have been ejected into its distant, eccentric orbit.

Batygin and Brown continue to refine their simulations and learn more about the planet's orbit and its influence on the distant solar system. Meanwhile, Brown and other colleagues have begun searching the skies for Planet Nine.

The paper is titled Evidence for a Distant Giant Planet in the Solar System.

 

Watch out Pluto; Planet Nine is here