spaceSpace and Physics

Astronomers Think They've Discovered A Neptune-Sized Ninth Planet Beyond Pluto


Jonathan O'Callaghan

Senior Staff Writer

761 Astronomers Think They've Discovered A Neptune-Sized Ninth Planet Beyond Pluto
Artist's impression of the view from "Planet Nine". Caltech/R. Hurt (IPAC)

In 2005, the discovery of the dwarf planet Eris by Caltech astronomer Mike Brown and his colleagues ultimately led to Pluto being demoted as the ninth planet of the Solar System a year later. Brown took to his status as the man who relegated Pluto with aplomb; his handle on Twitter is, rather appropriately, “plutokiller.”

But new research, published today in The Astronomical Journal by Brown and his colleague Konstantin Batygin, is sure to cause a stir. He is proposing the existence of a real ninth planet of the Solar System, dubbed “Planet Nine” and ostentatiously nicknamed “Phattie,” that would be almost the size of Neptune.


The planet has not been observed; rather, the astronomers have put together a mathematical model that infers its existence. “We have a gravitational signature of a giant planet in the outer Solar System,” Batygin told Nature. But interestingly, they say that some of the most powerful telescopes on Earth at the moment may be capable of spotting it – and it may already be hiding in existing images.

Evidence for Planet Nine comes from the observed motion of objects in the Kuiper Belt, a vast region of comets beyond the orbit of Pluto. According to the paper, it suggests there is a planet ten times the mass of Earth on a hugely elliptical orbit around the Sun, completing an orbit every 10,000 to 20,000 years and never getting closer than 200 times the Earth-Sun distance.

A hefty degree of skepticism is certainly needed, though. After all, the infamous Planet X and the mythical Nibiru have been circling in astronomy and conspiracy circles for years. Planet Nine has not even been seen yet; it’s too early to say it exists for definite. But Brown himself is confident.

“OK, OK, I am now willing to admit: I DO believe that the solar system has nine planets,” he wrote on Twitter.


Shown is the predicted elliptical orbit for Planet Nine, and other orbits for known distant objects in the Solar System. Caltech/R. Hurt (IPAC) 

In their paper, Brown and Batygin say there is only a 0.007 percent chance that the observed clustering of Kuiper Belt Objects (KBOs) is “due to chance,” suggesting another origin. “We find that the observed orbital alignment can be maintained by a distant eccentric planet with mass [greater than 10 Earths],” they wrote. The planet could also explain the elliptical orbits of dwarf planets like Sedna.

One possible explanation for the planet’s existence, according to the authors, is that it was a giant planet core that was ejected during the early Solar System, something that may be common in planetary systems.

The discovery of a ninth planet in the Solar System would be huge, and that’s an understatement. Astronomers have previously predicted the existence of hundreds of dwarf planets beyond the orbit of Pluto in the Kuiper Belt, but so far no solid theories exist for a large planet like Planet Nine.


This paper is sure to be pored over, scrutinized, and perhaps even discredited, so don’t expect to have to learn a new mnemonic for the planets any time soon. But be prepared; the man who killed Pluto might just have given a new lease of life to the hypothesized existence of a ninth world in our Solar System.

“For the first time in over 150 years, there is solid evidence that the Solar System’s planetary census is incomplete,” said Batygin in a statement.


spaceSpace and Physics
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  • ninth planet,

  • Mike Brown,

  • Phattie,

  • Planet Nine