The plot thickens in the search for Planet Nine

Two theoretical physicists report that the same observations that inspired the search for a ninth planet may instead be evidence within the solar system of a modified law of gravity originally developed to understand the rotation of galaxies.

Researchers Harsh Mathur, a professor of physics at Case Western Reserve University, and Kathryn Brown, an assistant professor of physics at Hamilton College, made this assertion after studying the effect the Milky Way might have on objects in the outer solar system – if the laws of gravity hold true. It is governed by a theory known as Modified Newtonian Dynamics (or MOND).

MOND suggests that Isaac Newton’s famous law of gravitation is valid to some extent. That is, when the gravitational acceleration predicted by Newton’s law becomes small enough, MOND allows a different gravitational behavior to take over.

MOND’s observational success on galactic scales is why some scientists consider it an alternative to “dark matter,” the term physicists use to describe a hypothesized form of matter that would have gravitational effects but emit no light.

“MOND is really good at explaining galaxy-scale observations, but I didn’t expect it would have such noticeable effects on the outer solar system,” Mathur said.

Their work has recently been published in Astronomical magazine.

“Amazing” alignment.

Mathur and Brown have studied MOND’s effect on galactic dynamics before. But they became interested in MOND’s local effects after astronomers announced in 2016 that a group of objects in the outer solar system showed orbital anomalies that could be explained by the presence of a ninth planet.

Orbital properties have led to landmark discoveries before: Neptune was discovered by the force of its gravity on the orbits of a nearby body, the precise precession of Mercury provided early evidence supporting Einstein’s theory of general relativity, and astronomers have recently used orbital dynamics to infer the existence of a supermassive black hole at the center of our galaxy.

Brown realized that MOND’s predictions might conflict with the observations that had spurred the search for a ninth planet. “We wanted to see if the data supporting the Planet Nine hypothesis would effectively rule out the existence of MOND,” she said.

Instead, Mathur and Brown found that MOND accurately predicts the clusters observed by astronomers. They say that over millions of years, the orbits of some objects in the outer solar system will be pulled into line with the galaxy’s gravitational field.

When they plotted the orbits of objects from the Planet Nine dataset against the galaxy’s gravitational field, “the alignment was amazing,” Mathur said.

The authors caution that the current data set is small and that any number of other possibilities could be true; For example, other astronomers have argued that orbital properties are the result of observational bias.

“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,” Brown said.