A possible ninth planet at the edge of the solar system could be the reason behind one of the biggest mysteries in astronomy.

For years astronomers have been unsure why the solar system lies on a strange tilt. This wobble also gives the appearance that the Sun is slightly tilted.

Now, astronomers believe that because Planet Nine is so massive (it is thought to have a mass 10 times that of Earth) and has a tilted orbit compared to the other planets, "the solar system has no choice but to slowly twist out of alignment," according to Elizabeth Bailey, a graduate student at the California Institute of Technology (Caltech) and lead author of a study announcing the discovery.

A diagram showing Planet Nine's distance from the Sun. Credit: Reuters/NASA

Researchers first discovered "the most planet-y of the planets in the whole solar system" in January, but thought it could have escaped telescope's notice because of its extreme distance from the Sun (it orbits the Sun at an average distance of 56 billion miles).

"It continues to amaze us; every time we look carefully we continue to find that Planet Nine explains something about the solar system that had long been a mystery," explains Konstantin Batygin, an assistant professor of planetary science at Caltech and one of those who first predicted the presence of Planet Nine.

For many years astronomers had been confused about why the solar system lies at a slight tilt because it formed as a spinning cloud which slowly collapsed first into a disc and then into objects orbiting a central star (the Sun).

The other planets in the solar system orbit the Sun in an essentially flat plane, and this plane itself rotates at nearly a six-degree angle, giving the appearance that the Sun is angled.

Planet Nine has a large impact on the solar system due to its mass and distance from the Sun, together these exert a force which affects the overall spin of the solar system.

A giant planet with a strange orbit, about 30 degrees off the other planets’ plane, could account for that wobble, the scientists suggested.

However, astronomers are still unsure why Planet Nine has such an unusual orbit.

One suggestion is that it was ejected from the neighbourhood of Jupiter, while another is that it could have been influenced by the gravitational pull of other stars in the solar system's extreme past.

They also still have to find the illusive planet, a search which they believe could take three years or more.