Our solar system officially houses eight planets, but some scientists say there could be a ninth. And that’s not just Pluto aficionados – evidence suggests a huge undiscovered world lurks on the dark fringes out there. Now, a new study has found the outer solar system oddities could be explained by modified theories of gravity, an alternative idea to dark matter.

In the 19th century, astronomers measuring the orbit of Uranus noticed some inconsistencies between observations and predictions, and concluded that it was being influenced by the gravity of a large unseen body. Sure enough, the planet Neptune was soon discovered as a result.

In 2016 astronomers made a similar prediction: based on the bizarre orbital patterns of six icy objects in the Kuiper belt, an unknown planet with the mass of about 10 Earths could be tugging on them from the shadows. Further evidence from other objects and even the Sun’s tilt seemed to strengthen the case.

Other scientists, however, have put forward alternative explanations, including gravitational “bumper car” interactions between planets, a distant disc of rock and ice, and even a mini black hole. And now, a team has proposed an even wilder idea – modifying the theory of gravity itself.

It’s not as outrageous a claim as it might sound. While Newton’s law of universal gravitation holds up pretty well to explain the large-scale structure and motion of the universe, it does fall short in some situations. For the better part of a century, dark matter has been the favored plug to fill the holes Newton’s law leaves in our cosmological models. This mysterious substance is thought to pervade the universe, not reflecting or emitting any light and only interacting with regular matter through its strong gravity.

As widely accepted as it is, dark matter has never been directly detected despite many experiments searching for it. Some scientists suggest that instead, we may need to adjust Newton’s law of gravity. For instance, gravity’s effects may be stronger at low accelerations than Newton described, which would cancel out the need for dark matter. This model is known as Modified Newtonian dynamics (MOND), and evidence supporting it has recently been discovered in star clusters and over 150 galaxies.

In a new study, theoretical physicists have applied MOND to the dataset used in the hunt for Planet Nine. According to their model, if MOND is at work then the orbits of some objects in the outer solar system should, over millions of years, move into alignment with the Milky Way galaxy’s gravitational field. And to the team’s surprise, they found that the clustering of orbits for these objects exactly matched the observations in our solar system.

“The alignment was striking,” said Harsh Mathur, co-author of the study. “MOND is really good at explaining galactic-scale observations, but I hadn’t expected that it would have noticeable effects on the outer solar system.”

As intriguing as the study is, the team acknowledges that the dataset is rather small, and that there are still plenty of other possible explanations. In the meantime, the hunts for both dark matter and Planet Nine will likely continue.

“Regardless of the outcome, this work highlights the potential for the outer solar system to serve as a laboratory for testing gravity and studying fundamental problems of physics,” said Katherine Brown, co-author of the study.

The research was published in The Astronomical Journal.

Source: Case Western Reserve University