And yet, another possibility remains, which is that the only solutions people find are the ones they have access to, like the proverbial story of the person who lost their keys at night and only looks for them under the streetlight. “Scientists are often good at doing contrastive assessments, like a Sherlock Holmes–style argument: Here’s my suspects, and here’s the one most likely to have done it,” says Christopher Smeenk, a philosopher of science at the University of Western Ontario. “But do you have the right list of suspects?”
There have been many planetary misses in history, Smeenk points out, such as 17th-century claims of a moon orbiting Venus, which better data demonstrated not to exist. Two centuries later, astronomers attributed Mercury’s slightly peculiar orbit to the gravitational forces from an unseen inner planet, dubbed Vulcan. But when Albert Einstein’s theory of general relativity came along, it explained the orbit, debunking Vulcan claims.
In the case of Planet Nine, Ann-Marie Madigan, an astrophysicist at the University of Colorado Boulder, believes everyone has missed a key suspect. The gravitational forces in the outer solar system could be more complicated, and the unlikely alignment of those icy bodies could all be a temporary coincidence.
She argues that there are millions, if not billions, of planetoids—more than previously thought—orbiting in that distant, dusty disk of material around our solar system. Most astronomers have assumed that the forces of these tiny objects are so small that they can be ignored in models, and it’s difficult to model their behavior. But Madigan includes them all in her models, and has found that, as they orbit over and over again, the nebulous pull of their gravity gently and gradually clumps some objects together over time. This “self-gravity” mechanism, as she calls it, could explain the other lines of evidence brought forth by Batygin as well.
“People think gravity’s dominated by Jupiter, Neptune, Uranus, and Saturn, and they’re not really thinking of the collective effects of all the smaller bodies,” Madigan says. “The main criticism I get from Planet Nine advocates is that there’s no evidence for such a mass of small bodies. But I don’t pay too much attention to that, because they haven’t seen Planet Nine yet either.”
Incidentally, both Batygin and Madigan invoke the principle of Occam’s razor, the notion that the simplest explanation is likely the correct one. But they come to completely different conclusions, highlighting that this seemingly straightforward principle is actually rather complicated, with no clear answer yet in sight.