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Plot Thickens in Hunt for Ninth Planet

Outer reaches of solar system could harbor another planet—or evidence modifying laws of gravity

A pair of theoretical physicists are reporting that the same observations inspiring the hunt for a ninth planet might instead be evidence within the solar system of a modified law of gravity originally developed to understand the rotation of galaxies.

Harsh Mathur, Professor, Department of Physics

Researchers Harsh Mathur,  a professor of physics at Case Western Reserve University, and Katherine Brown, an associate professor of physics at Hamilton College, made the assertion after studying 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 (or MOND).

MOND proposes Isaac Newton’s famous law of gravity is valid up to a point. That is, when the gravitational acceleration predicted by Newton’s law becomes small enough, MOND allows for a different gravitational behavior to take over. 

The observational success of MOND 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 not emit any light.

“MOND is really good at explaining galactic-scale observations,” Mathur said, “but I hadn’t expected that it would have noticeable effects on the outer solar system.”

Their work was recently published in The Astronomical Journal.

A ‘striking’ alignment

Mathur and Brown had studied MOND’s effect on galactic dynamics before. But they became interested in MOND’s more local effects 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.

See full story in The Daily – Plot Thickens in Hunt for Ninth Planet