The unusual orbit of the hypothetical 'Planet 9' could explain one of the mysteries of the Solar System – why the Sun appears to be tilted. A team of scientists has said the angular orbit of Planet 9 would influence the orbits of the rest the planets in the Solar System – creating the 6 degree tilt we see.

Planet 9, which is said to be about 10 times the size of Earth, has long been predicted to exist by scientists, but until recently there was no evidence for it. However, earlier this year a team at Caltech noticed several objects in the Kuiper Belt with unusual orbits.

These unusual orbits could be explained by the presence of a massive planet sitting at the edge of the Solar System, producing a gravitational effect on the objects.

From the orbits of the Kuiper Belt Objects, scientists were able to work out the size and mass of the planet, along with its orbit around the Sun. Unlike the other eight planets in the Solar System, Planet 9's orbit sits on an angle of around 30 degrees compared to the other planets.

By examining what effect Planet 9 would have on the rest of the Solar System, scientists found evidence to show its existence would explain the unusual tilt of the sun – a feature that has so far defied explanation.

planet 9
Artist's impression of Planet Nine, the planet speculated to exist in our Solar System by scientists Caltech/R. Hurt IPAC

The team, from Caltech, are set to publish their study in the Astrophysical Journal. They presented their findings at the American Astronomical Society's Division for Planetary Sciences.

All of the known planets in the Solar System orbit the Sun within a couple of degrees of each other, on a flat plane. However, this plane is at a 6 degree tilt to the Sun, making our star look like it is at an angle, when really it should all be aligned.

Mike Brown, who predicted the existence of Planet 9 with Konstantin Batygin in January, said: "It's such a deep-rooted mystery and so difficult to explain that people just don't talk about it."

By carrying out mathematical calculations of Planet 9's orbit and the effect it would have on the rest of the solar system, scientists discovered it would explain the tilt of the rest of the planets – its angular momentum produces a force on the rest of the Solar System's spin. The 6 degree tilt fits with their calculations perfectly.

"Because Planet 9 is so massive and has an orbit tilted compared to the other planets, the solar system has no choice but to slowly twist out of alignment," says Elizabeth Bailey, lead author of the study.

"It continues to amaze us; every time we look carefully we continue to find that Planet 9 explains something about the solar system that had long been a mystery," Batygin added.