Plot thickens in the hunt for a ninth planet

The outer reaches of the solar system could harbor a ninth planet, or evidence for a modification of the laws of gravity
Contact: Vige Barrie
How many planets are there in the solar system? What laws govern their motion? These questions of antiquity led to the birth of astronomy and the dawn of science. Even today in the twenty-first century, the solar system may harbor secrets that could lead to insights about the universe on much larger scales.

In an article published last week, physicists report that the same observations that motivate the hunt for a ninth planet might actually be the first solar system signatures of an alternate formulation of the laws of gravity. 

The article, titled “Modified Newtonian Dynamics as an Alternative to the Planet Nine Hypothesis”, studies the effect that 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 that Newton’s famous law of gravitation is valid up to a point – when the gravitational acceleration predicted by Newton’s law becomes small enough, MOND allows a different regime of gravitational behavior to take over. 

The MOND acceleration scale is tiny by earthly standards (one hundred billionth of one “g”, the amount of gravitational acceleration near the surface of the Earth). But in the far reaches of the solar system, the acceleration predicted by Newtonian gravity is small enough that MOND effects could be significant.

This surprised Harsh Mathur, a professor of physics at Case Western Reserve University who co-authored the study with Katherine Brown, associate professor of physics at Hamilton College. “MOND is really good at explaining galactic-scale observations. But I hadn’t expected that it would have noticeable effects on the outer solar system,” he said.

The observational success of MOND on galactic scales is why some scientists consider it an alternative to ‘dark matter’, a term physicists use to describe a hypothesized form of matter that would have gravitational effects but not emit any light.

One of the first observational motivations for dark matter occurred in the 1970s when Vera Rubin showed that most objects in galaxies were rotating in a manner that suggested their motion was dominated by a large amount of unseen mass. MOND requires no dark matter because it changes the laws of gravity to mimic the effect of additional mass.

In the decades since Rubin’s discoveries, physicists have come to believe that dark matter makes up the majority of the matter in the universe.  But decades of experimental searches to directly detect it have come up empty-handed.

Brown and Mathur had studied MOND’s effect on galactic dynamics before, but 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.

Orbital peculiarities have led to historic discoveries before: Neptune was discovered through its gravitational tug on the orbits of nearby objects, the minute precession of Mercury provided early evidence in support of Einstein’s theory of general relativity, and astronomers have recently used orbital dynamics to infer the presence of supermassive black holes at the centers of most galaxies.

Brown realized MOND’s predictions might be at odds with the observations that had motivated the search for a ninth planet. “We wanted to see if the data that support the Planet Nine hypothesis would effectively rule out MOND,” she said.

Instead, they found that MOND predicts precisely clustering that astronomers have observed. Over millions of years, they argue, the orbits of some objects in the outer solar system would be dragged into alignment with the galaxy’s own gravitational field. When they plotted the orbits of the objects from the Planet Nine dataset against the galaxy’s own gravitational field, “the alignment was striking,” Mathur said.

But the authors also note that the current dataset is too small to draw reliable conclusions and that any number of possibilities might prove to be correct; other astronomers have argued that the orbital peculiarities are merely a spurious result of observational bias, for example.

“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 says.

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