Could Planet 9 Actually Just Be Controversial Physics?
For years, the existence of a missing Planet 9 has been one of the predominate theories attempting to explain some of the eccentric orbits found in the outer Solar System. But so far, no such planet has been found.
A new study suggests that the problem may not be a missing planet, but instead, a missing piece of physics that’s addressed by a hypothesis known as Modified Newtonian Dynamics (MOND).
This theory joins a growing list of hypotheses that attempt to rectify some of the gravitational weirdness that is still observed in our Solar System.
Solving gravitational anomalies in our solar system often involve planets—both real and hypothetical. Famously, in 1845, French astronomer Urbain Le Verrier pinpointed the planet Neptune based solely on mathematical calculations to solve for disturbances in Uranus’ orbit. Le Verrier tried a similar trick to explain Mercury’s inconsistencies and created the hypothetical planet Vulcan—but that gravitational mystery was instead solved by Einstein’s Theory of General Relativity in 1915.
Fast forward to a century later, and astronomers are once again in the search of a missing planet, known as Planet 9. This time, astronomers can’t explain why certain celestial bodies in the Kuiper belt—such as the dwarf planet Sedna—have such strange orbits. And once again, two possibilities emerge: either we’re missing a planet, or we’re missing some physics.
Many stories have been written about the missing-planet option, and now a new study from Ohio’s Case Western Reserve University (CWRU) and New York’s Hamilton College are exploring the alternative. Instead of relying on the Planet 9 theory, scientists Harsh Mathur and Katherine Brown suggest that a theory known as Modified Newtonian Dynamics (MOND) could explain the orbital weirdness witnessed in the Kuiper belt. The results of the study were published in The Astrophysical Journal in late September.
“MOND is really good at explaining galactic-scale observations,” CWRU physics professor and study co-author Mathur said in a press statement. “But I hadn’t expected that it would have noticeable effects on the outer Solar System.”
When it comes to science theories, MOND is a bit of an underdog. The theory was first proposed in 1983 as an alternative hypothesis to the notoriously slippery concept known as dark matter. As its name states, MOND tweaks Newtonian physics (specifically, the second law of motion) by challenging the rules around extremely low-accelerating objects, which include stars in the outer galaxy. Long story short, MOND suggests that these stars in the outer Milky Way experience a greater gravitational pull than Newton’s law suggests.
This new study suggests that an additional gravitational pull experienced by these far-flung stars could edge the needle just enough to explain the gravitational behavior seen in the Kuiper belt. Mathur says that the original aim of the study was to “rule out” MOND, but when they aligned Planet 9 data sets with the galaxy’s MOND gravitational field, the results were “striking.”
MOND and a far flung Planet 9 are far from the only hypotheses vying to explain the strange orbits of Sedna and its companions. Some astrophysicists, for example, think a miniature black hole could explain things.
For now, though, until a telescope can finally locate an outer space interloper, the theories will likely never end.
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