Astronomers have found compelling evidence for a new giant planet orbiting the Sun in the depths of the Solar System, they revealed tonight. The US team have not yet spotted the new world, which they say is around ten times the size of Earth. But they believe it has given its existence away by the pulling effect it has on other distant objects.

An artist’s impression of Planet Nine, looking back towards the Sun and showing flashes of lightning in its clouds. Image credit: Caltech/R. Hurt (IPAC)

One of the astronomers is Professor Mike Brown, whose discoveries of many icy bodies in the outer Solar System helped bring about Pluto’s demotion from full planet status.

He and colleague Dr Konstantin Batygin, both from the California Institute of Technology, are calling their new world Planet Nine for now. They say it goes round the Sun in a highly elliptical orbit at an average distance of 55 billion miles, or around 20 times the distance of present outer planet Neptune. It is so distant that a year on the planet would last between 10,000 and 20,000 years because that is how long it would take to orbit the Sun!

Brown said in announcing their findings: “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.”

His research has led him to discover several small icy objects in a zone of the Solar System beyond Pluto called the Kuiper Belt. They include a handful of worlds comparable in size to Pluto, which is now labelled a dwarf planet.

When Brown kept track of these objects, he found that 13 are behaving in a way that suggests a powerful influence on them. Though he felt it unlikely there could be another giant planet, Brown joined forces with theorist Batygin to examine it.

Orbits of the six most distant known objects in the Solar System whose orbits all mysteriously line up in such a way as to point to the existence of Planet Nine in the orbit shown. Image credit: Caltech/R. Hurt (IPAC)

Batygin, an assistant professor of planetary science, said: “Although we were initially quite skeptical that this planet could exist, as we continued to investigate its orbit and what it would mean for the outer solar system, we become increasingly convinced that it is out there.

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

Their work together showed that the six most distant objects being studied in the Kuiper Belt were all in elliptical orbits pointing in the same direction in space, and tilted in a similar way. They realised that something out there was affecting them.

Then the astronomers found that if they ran a computer simulation involving a massive planet, the effect on the icy bodies matched that which is being seen in real life. It also explains the peculiar orbits of many other Kuiper Belt objects.

Brown and colleagues have now begun a search of the sky with some of the world’s biggest optical telescopes in a bid to find Planet Nine.

Brown said: “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.

“Now we can go and find this planet and make the Solar System have nine planets once again.”