A planetary body with the mass of Mars could be lurking in the outer edge of the Solar System, according to new research. The object would be far beyond Pluto, but closer than the hypothetical Planet 9, thought to also be located in the outer reaches of the Solar System.


Evidence for this new planetary mass object comes from observations of the Kuiper Belt, a ring of icy bodies encircling our Solar System.

Most Kuiper Belt objects (KBOs) orbit the Sun with tilts that average out to the ‘invariable plane’ of the solar system.

The most outer KBOs do not.

Their average plane is tilted away from the Solar System’s invariable plane by about eight degrees.

"The most likely explanation for our results is that there is some unseen mass," says Kat Volk, a postdoctoral fellow at the University of Arizona’s Lunar and Planetary Laboratory and lead author of the study.

"According to our calculations, something as massive as Mars would be needed to cause the warp that we measured."

To reach their conclusion, the team looked at the tilt angles of the orbital planes of over 600 objects in the Kuiper Belt.

"Imagine you have lots and lots of fast-spinning tops, and you give each one a slight nudge," says study co-author Renu Malhotra.

"If you then take a snapshot of them, you will find that their spin axes will be at different orientations, but on average, they will be pointing to the local gravitational field of Earth."

According to the study authors, the average orbital plane of objects in the outer Solar System should be flat past 50 Astronomical Units (AU). But looking farther out from 50 - 80 AU, they found the average plane warps away from the invariable plane.

The effect is likely to be caused by an object with the mass of Mars orbiting about 60 AU from the Sun.

The hypothetical Planet 9 is predicted to be more massive and at a distance of 500 - 700 AU.

The team have ruled out the scenario that the differing orbital plane was caused by the gravitational pull of a passing star.

"A passing star would draw all the 'spinning tops' in one direction," Malhotra explains. “Once the star is gone, all the KBOs will go back to precessing around their previous plane.


That would have required an extremely close passage at about 100 AU, and the warp would be erased within 10 million years, so we don't consider this a likely scenario."


Ezzy Pearson is the News Editor of BBC Sky at Night Magazine. Her first book about the history of robotic planetary landers is out now from The History Press.