The Planet X may not be a planet

The mysterious Planet X, said to be somewhere beyond the orbit of Pluto, has so far eluded all our efforts to find it. We have found it. It is very dark, very cold, and very far away, which is not conducive to detection. And we have only a vague idea of where it might be, since it could be a grain of sand against a dark sky. There are various hypotheses regarding its existence: we haven't seen it yet, it doesn't exist at all, or it could just be a swarm of rocks. Or, according to new research by physicists at Hamilton College and Case Western Reserve University, what we have interpreted as evidence that a planet exists may instead be a sign that something is missing from our current General Theory of Relativity.

Artist's impression of Planet X. Credit: Caltech / R. Hurt (IPAC)

A possible flaw in general relativity?

According to their recent study, the subtle anomalies in the orbits of various objects that some attribute to a hidden planet are also consistent with another type of gravitational model, called Modified Newtonian Dynamics (or MOND). Newton's laws are important, no doubt, but they are not perfect. And the flaws in Newton's law of universal gravitation were solved by Einstein's theory of general relativity.

MOND is an alternative solution that proposes that other gravitational effects take over when Newtonian gravitational acceleration falls below a certain threshold. Because MOND can describe the gravitational behavior of galaxies that does not seem to be related to normal matter - such as rotation curves and unexpected lensing effects - it is an interesting alternative to dark matter. But its implications have not been studied on a smaller scale.

The existence of Planet Nine has been inferred from some strange orbital groupings observed in the outer Solar System. There, large numbers of small icy rocks are found on large circumsolar orbits. The way some of these rocks have been observed to cluster together in their orbits may suggest that they have been gathered together by the gravitational influence of a large planet.

Physicists initially thought that these clustering observations might be inconsistent with MOND, so they undertook an investigation, modeling the orbits of objects in the Kuiper belt of the outer Solar System to observe how they behaved. Surprisingly, the result was a group of rocks that behaved almost exactly like the observed cluster. Their calculations suggest that, according to the MOND theory, some of the objects in the outer Solar System should be pulled into the gravitational field of the Milky Way over time.

It is not enough to say with confidence that MOND is responsible for the strange behavior of these distant rocks simulating the presence of a planet, and there is not enough data to confirm it. But it does put MOND in the mix of things that could be out there instead of Planet Nine. Dynamic simulations would allow a more detailed study of the MOND hypothesis and help to confirm or rule it out. Whatever the outcome, this work highlights the potential of the outer solar system to serve as a laboratory for testing gravity and studying fundamental problems in physics.