Unseen planet dents the old order

This artist's concept illustration shows a distant view from Planet Nine back towards the sun. The planet is thought to be gaseous, similar to Uranus and Neptune. Hypothetical lightning lights up the Show more

When a pair of scientists noticed objects in the Kuiper Belt behaving 'in a very strange way that shouldn't happen', they put 2 and 2 together and concluded the anomaly was being caused by a distant ninth planet.

It could be the loneliest and coldest place in the Solar System, a world of rock and ice permanently shrouded in darkness on an epic 20,000-year journey around our Sun.

If it exists at all. The news that a ninth planet may exist has both excited and divided the scientific community.

Evidence presented on Thursday by astronomers at the California Institute of Technology points to “Planet 9” as the only logical explanation for the behaviour of another group of distant objects billions of miles from Earth.

Konstantin Batygin and ­­Michael E. Brown admit that they have not actually seen the new planet, but have published the evidence in 12 pages of closely argued calculations in the February issue of The Astronomical Journal. Dr Brown now hopes to enlist the help of ordinary astronomers to capture the elusive celestial body.

“There are many telescopes on the Earth that actually have a chance of being able to find it and I’m really hoping that as we announce this, people start a worldwide search to go find this ninth planet,” he said.

Their evidence rests on the behaviour of objects in the Kuiper Belt, a band of rock and ice that includes Pluto, whose own credentials as a planet have been repeatedly questioned.

Several of the larger objects in the Belt “swing out in one direction in a very strange way that shouldn’t happen”, Dr Brown said. The pair’s findings report: “Such a clustering has only a probability of 0.007 per cent to be due to chance, thus requiring a dynamic origin.

“We find that the observed orbital alignment can be maintained by a distant eccentric planet.”

The planet has not been seen so far because it orbits differently to the other eight in the Solar System, which line up in a single direction.

According to the scientists’ calculations, “Planet 9” would be at least the size of Earth and possible much bigger, with a mass 4,500 times that of Pluto.

Its orbit would take it between 20 billion and 100 billion miles from the Sun, which would appear as little more than a brighter than normal star. Each year on the planet would last between 10,000 and 20,000 years. The Caltech team admit they do not yet know the orbit of the planet.

Scientists have long speculated about the existence of a “Planet X” but until now have failed to find any clear evidence of its existence.

Dr Brown said he hoped the new planet would be viewed within five years. One theory is that it is the core of a gas giant like Saturn, formed in the early Solar System and ejected during a close pass by Jupiter.

Scepticism about their claims came from Nasa’s chief scientist Ellen Stofan, who said proof would need to be provided by telescopic evidence. However, she admitted that planets similar to Planet 9 had been observed outside the Solar System.

“The fact that we don’t have a planet in that size class between Earth and Neptune makes us think, ‘well, maybe we are missing one’, and maybe they’ve predicted it,” she told the BBC.

Dr Brown’s credentials include the discovery of the dwarf planet Eris in 2005. At 2,236 kilometres wide, Eris has a mass 27 per cent greater than Pluto, making it the ninth largest object in the Solar System.

As a result, the International Astronomical Union the following year created a new category of dwarf planet, including Pluto and effectively ending its status as a planet proper.

Dr Brown, whose Twitter title is "planet killer", says his daughter is still upset at her father's role in the demotion of Pluto. He told The Washington Post: "She's still kind of mad about Pluto being demoted, even though she was barely born at that time – she suggested a few years ago that she'd forgive me if I found a new planet. So I guess I've been working on this for her."