Have astronomers discovered a ninth planet?

Since the declassification of Pluto in 2006, our Solar System has been considered an eight-planet system. But a new discovery by astronomers may have revealed a new contender for the ninth member.

An artist's impression of the hypothetical Planet 9.
Published: January 21, 2016 at 12:00 pm

Researchers at Caltech say they have found evidence of a ninth planet in our Solar System.


The planet is thought to be gaseous, like Neptune or Uranus, and follow an elongated orbit in the outer edges of the Solar System

It would have a mass about 10 times that of Earth and orbit about 20 times farther from the Sun than Neptune.

The planet would take between 10,000 and 20,000 years to make a full orbit of the Sun, the researchers say.

Evidence for Planet Nine, as it is being called, is a result of computer simulations created by Konstantin Batygin and Mike Brown, although the planet has not yet been directly observed.

The work began in 2014 with Chad Trujillo and Scott Sheppard, two researchers who observe the outer edges of the Solar System to look for distant objects.

They had observed that 13 of the most distant objects in the Kuiper Belt follow a similar orbit, which could potentially be explained by the presence of a small planet.

Brown and Batygin then began exploring the concept and realised that the six most distant objects from Tujillo and Sheppard’s original selection all follow elliptical orbits that point in the same direction.

As well as this, the six objects are all tilted in the same way.

Artist's concept and animation: Robert Hurt (Caltech/IPAC) & WorldWide Telescope/Images: NASA, Keck Observatory, Cerro Tolo Observatory/Motion graphics & editing: Caltech AMT
Produced in partnership with Caltech Academic Media Technologies and the Office of Strategic Communications © 2016 California Institute of Technology

"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," says Brown.

"Basically it shouldn't happen randomly, so we thought something else must be shaping these orbits."

Brown and Batygin first explored the idea that distant, undiscovered objects in the Kuiper Belt are exerting gravity that keeps the orbits together, but realised that this would require the Kuiper Belt to have about 100 times the mass it has today.

The pair then ran simulations that assumed the existence of a huge planet in an anti-aligned orbit.

This means an orbit in which the planet’s perihelion is 180 degrees across from the perihelion of all the other objects and planets.

Perihelion is the point in an object's orbit at which it is closest to its host star.

The inclusion of this ninth planet made the distant Kuiper Belt objects in the simulation follow the alignment that Brown and Batygin were observing.

With this model, however, the idea surfaced that such orbital patterns would eventually cause the planet and the Kuiper Belt object to collide.

But the stability of the orbits could potentially be maintained through a force called ‘mean-motion resonance’, which sees the ninth planet’s anti-aligned orbit preventing the Kuiper Belt objects from colliding with it.

When orbiting planets approach each other they exchange energy, so Planet Nine could simply be nudging the orbits of the Kuiper Belt objects out of its way, keeping both orbital trails in sync and meaning the two never collide.

It is still unknown how Planet Nine came to be found in our Solar System, but one theory is that it could be a fifth ‘planetary core’.

Scientists believe that our Solar System formed originally as the four gas giants - Jupiter, Saturn, Uranus and Neptune - but that over time these planets were shaped and moved into their current orbits by collisions.

Brown and Batygin believe that Planet Nine could be a fifth core that was expelled from the inner Solar System by one of the other gas giants.

Whatever the reason for its formation, if this ninth planet does exist, Brown believes it should be possible to find it.

While the precise location of the planet within its orbit is not known, he says it could be found using images captured in previous surveys, or by using some of the planet’s largest telescopes.

The idea of a distant, ninth planet might seem a surprise, but the astronomers believe the object’s mass and orbit would actually fit in with how other planetary systems work, and make our own appear more ‘normal’.

"One of the most startling discoveries about other planetary systems has been that the most common type of planet out there has a mass between that of Earth and that of Neptune," says Batygin.

"Until now, we've thought that the Solar System was lacking in this most common type of planet.


Maybe we're more normal after all."


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