Evidence suggesting the existence of a ninth planet in the Solar System could challenge the theory of gravity. This is the conclusion of two scientists who have studied the effect that the Milky Way would have on objects at the outer edges of our planetary system.
Modified Newtonian Dynamics
Galaxies rotate at the same speed without drifting apart. Typically, most scientists believe that this structural grip of galaxies suggests the existence of dark matter. The idea is that huge halos of dark matter envelop and gravitationally bind galaxies, preventing their contents from flying outward.
The theory known as Modified Newtonian Dynamics (MOND) eliminates the need for dark matter, instead suggesting that Isaac Newton’s famous law of gravity is correct, but only to a certain extent. MOND suggests that in the presence of high rotation speeds, a different type of gravitational behavior comes into play. And this type of behavior applies to rotating galaxies.
“MOND is really good at explaining observations on a galactic scale, but I didn’t expect it to have significant effects on the outer solar system” said scientist Harsh Mathur, who conducted the study (ref.) with physics professor Katherine Brown.
Ninth Planet and the Kuiper Belt
The connection between the theory of gravity and a hypothetical ninth planet may seem strange. It actually arises from the main evidence of this world being the peculiar behavior of objects in a distant structure called the Kuiper Belt. This zone is a disk of material in the outer part of our cosmic neighborhood, hosting various icy bodies like comets and asteroids.
In 2016, it was discovered that some of these icy objects exhibited orbital anomalies and clustering unlike the other objects in the region. This strange behavior, according to experts, could be the result of a yet-to-be-discovered planet. Strange orbits like these had already revealed the presence of planets, with Neptune being discovered as a result of its gravitational attraction on other objects in the Solar System.
Mathur and Brown wanted to know if the strange orbits of the Kuiper Belt could be the result of something else. Perhaps those orbits could be explained if MOND was the correct theory of gravity. “We wanted to see if the data supporting the hypothesis of the ninth planet actually ruled out MOND” Brown said.
MOND Explains the Ninth Planet
Ultimately, they found that the peculiar clustering could indeed be explained by the MOND theory. Mathur and Brown propose that over millions of years, the orbits of some inhabitants of the outer solar system may have been gravitationally dragged. Instead of aligning with the rest of the solar system, they align with the gravitational field of the Milky Way.
Mathur reported that the two discovered the alignment to be striking. However, the scientists urge caution in evaluating their results. They admit that the dataset on which this research is based is small and suggest that any other possibility could be correct.
“Regardless of the outcome, this work highlights the potential of the outer solar system to serve as a laboratory for testing gravity and studying fundamental issues in physics” Brown concluded.