In recent years, strange observations in our solar system’s outer reaches have led scientists on a hunt for a ninth planet. However, two researchers believe these mysterious sightings might not be pointing towards a hidden planet after all. Instead, their new research suggests they could be evidence of a changed law of gravity itself.
Harsh Mathur, a professor of physics at Case Western Reserve University, and Katherine Brown, an associate professor of physics at Hamilton College, have put forth the idea that these observations might be evidence of a theory known as Modified Newtonian Dynamics (MOND).
To break this down, Sir Isaac Newton’s famed law of gravity states that two bodies will attract each other with a force directly proportional to the product of their masses and inversely proportional to the square of the distance between them. However, MOND suggests that there’s an exception to this law: when the gravitational pull becomes exceptionally weak, it might behave differently.
But why is this significant? MOND is often considered as an alternative explanation to “dark matter.” Scientists have theorized dark matter as a form of matter that, although it doesn’t emit any light, has gravitational effects that shape the universe. “MOND is really good at explaining galactic-scale observations,” Mathur says in a media release, “but I hadn’t expected that it would have noticeable effects on the outer solar system.”
The duo’s interest in MOND’s potential local effects was piqued in 2016 when astronomers found a few objects in the far corners of our solar system behaving oddly—movements which some believed could be explained by the existence of a ninth planet. Historically, odd orbital behaviors have unveiled significant discoveries. For example, the planet Neptune was discovered due to its gravitational influence on nearby objects.
While many were looking for a ninth planet, Brown began to think that MOND’s predictions might offer another explanation. “We wanted to see if the data that support the Planet Nine hypothesis would effectively rule out MOND,” she noted. Surprisingly, their study found that MOND could indeed account for the strange alignments of these distant objects. When they compared these object orbits with the gravitational field of our Milky Way galaxy, Mathur described the alignment as “striking.”
However, before we toss out the idea of a ninth planet or fully embrace MOND, it’s crucial to note that the dataset used is relatively small. Many other explanations could also account for the mysterious orbital behaviors, including the possibility that scientists are merely seeing what they expect to see—a phenomenon known as observational bias.
Brown emphasizes the broader significance of their findings: “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 study is published in The Astronomical Journal.