22nd January 2016

Strong evidence of a ninth planet in our Solar System

Astronomers at the California Institute of Technology have found the strongest evidence yet that a ninth planet – or "Planet X" – is present in our Solar System, orbiting the Sun every 15,000 years.

Researchers at the California Institute of Technology (Caltech) have found evidence of a giant planet tracing a highly elongated orbit in the far distant reaches of our Solar System. The object, which has been 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 (which orbits the Sun at an average distance of 2.8 billion miles). With a semi-major axis of 300,000 light seconds, or about 0.01 light years, it would take this new planet an estimated 15,000 years to make just one full orbit around the Sun.

The researchers, Konstantin Batygin and Mike Brown, discovered the planet's existence through mathematical modelling and computer simulations but have not yet observed the object directly.

"This would be a real ninth planet," says Brown. "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."

Brown notes that Planet Nine, at 5,000 times the mass of Pluto, is sufficiently large that there should be no debate about whether it is a true planet. Unlike the class of smaller objects now known as dwarf planets, it gravitationally dominates its neighbourhood. In fact, it dominates a region larger than any of the other known planets, a fact that Brown says makes it "the most planet-y of the planets in the whole solar system."

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