Katherine Brown (Jones-Smith)

Modified Newtonian Dynamics as an Alternative to the Planet Nine Hypothesis,” co-authored by Associate Professor of Physics Kate Brown, was recently published in the October issue of The Astronomical Journal.

Written with Harsh Mathur of Case Western Reserve University, the article presents the results of the co-authors’ study of the effect the Milky Way galaxy would have on objects in the outer solar system if the laws of gravity were governed by a theory known as Modified Newtonian Dynamics (MOND).

MOND proposes that Newton’s famous law of gravitation is valid up to the point at which the gravitational acceleration predicted by Newton’s law becomes small enough and MOND will allow a different regime of gravitational behavior to take over. 

Brown and Mathur, who had previously studied MOND’s effect on galactic dynamics, found a renewed interest in it after astronomers announced in 2016 that a handful of objects in the outer solar system showed orbital anomalies that could be explained by a ninth planet.

“We wanted to see if the data that support the Planet Nine hypothesis would effectively rule out MOND,” Brown said. Instead, they found that MOND predicts precisely clustering that astronomers have observed, noting that the current dataset is too small to draw reliable conclusions and that any number of possibilities might prove to be correct.

Brown said that “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.”

The Astronomical Journal is a monthly, peer-reviewed open access publication of the American Astronomical Association.

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