Our solar system has eight planets, but some scientists say there may be a ninth planet. This isn't just the opinion of Pluto enthusiasts - there's evidence that there's a vast undiscovered world lurking at the dark edge of the solar system. Now, a new study finds that weird phenomena in the outer solar system can be explained by a modified theory of gravity, another idea for the existence of dark matter.
In the 19th century, astronomers measuring Uranus' orbit noticed some inconsistencies between observations and predictions and concluded that Uranus was affected by the gravitational pull of a large, unknown body. Sure enough, Neptune was soon discovered.
In 2016, astronomers made a similar prediction: Based on the strange orbital patterns of six icy objects in the Kuiper Belt, an unknown planet with a mass of about 10 Earths may be secretly tugging on them. Further evidence from other celestial bodies and even the tilt of the Sun seems to strengthen this speculation.
However, other scientists have proposed other explanations, including gravitational "bumper car" interactions between planets, distant rock and ice disks, and even a small black hole. Now, a team of researchers has come up with an even bolder idea - revising the theory of gravity itself.
This isn't as outrageous as it sounds. While Newton's law of universal gravitation does a good job of explaining the large-scale structure and motion of the universe, it does fall short in some cases. For more than a century, dark matter has been the first choice to fill the holes left by Newton's laws in our models of the universe. This mysterious substance is thought to permeate the universe, does not reflect or emit any light, and only interacts with regular matter through its strong gravity.
Although dark matter is widely accepted, it has never been directly detected, despite many experiments looking for it. Some scientists believe we may need to adjust Newton's law of gravity. For example, at low accelerations, the effect of gravity may be stronger than Newton described, eliminating the need for dark matter. This model is called Modified Newtonian Dynamics (MOND), and evidence supporting it has recently been found in star clusters and more than 150 galaxies.
In a new study, theoretical physicists applied MOND to the data set used to find Planet Nine. According to their model, if MOND is at work, the orbits of some objects in the outer solar system should align with the Milky Way's gravitational field for millions of years. To the team's surprise, they found that the orbital clusters of these objects matched observations of the solar system perfectly.
Harsh Mathur, a co-author of the study, said: "The agreement is shocking. MOND does explain the galactic-scale observations very well, but I did not expect it to have obvious effects in the outer solar system."
While the study is intriguing, the team acknowledges that the data set is still small and there are many other possible explanations. In the meantime, the search for dark matter and Planet Nine is likely to continue.
"Whatever the outcome, this work highlights the potential of the outer solar system as a laboratory for testing gravity and studying fundamental questions in physics," said study co-author Katherine Brown.
The research was published in Acta Astromica.