Caltech/R. Hurt (IPAC)

Is there anybody out there? Two astronomers say they have found evidence that a planet around 10 times the mass of Earth is lurking in the outer reaches of the solar system, on an orbit that comes no closer than 200 times the distance between the sun and Earth. Dubbed Planet Nine, it hasn’t been seen directly. Instead, Konstantin Batygin and Mike Brown of the California Institute of Technology in Pasadena have inferred its existence from the strange orbits of other, smaller bodies.

If the planet is confirmed, it could steal the crown lost by the now-demoted Pluto – ironic, as Brown, who calls himself “plutokiller” on Twitter, was instrumental in that world’s downfall. In 2005 he discovered a Pluto-sized object, now known as Eris, in a distant region of the solar system called the Kuiper belt, leading to the reclassification of both objects as dwarf planets.


Brown and others have continued to explore the Kuiper belt and have discovered many small bodies. One called 2012 VP113, which was discovered in 2014, raised the possibility of a large, distant planet, after astronomers realised its orbit was strangely aligned with a group of other objects. Now Brown and Batygin have studied these orbits in detail and found that six follow elliptical orbits that point in the same direction and are similarly tilted away from the plane of the solar system.

Caltech/R. Hurt (IPAC)

According to the pair’s simulations, that something is a planet that orbits on the opposite side of the sun to the six smaller bodies. Gravitational resonance between this Planet Nine and the rest keep everything in order. The planet’s high, elongated orbit keeps it at least 200 times further away from the sun than Earth, and it would take between 10,000 and 20,000 Earth years just to complete a single orbit.

Get hunting

The pair have detailed their findings in a paper published in the Astronomical Journal today, but as yet they have no direct evidence for the existence of the new planet, which means it’s time for astronomers to fire up their telescopes and get hunting.

“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,” said Brown. “Now we can go and find this planet and make the solar system have nine planets once again.”

Journal reference: The Astronomical Journal, DOI: 10.3847/0004-6256/151/2/22

Click here to find out more about the background to our hunt for Planet X