A mirage may be Planet Nine. What once looked like evidence for a huge planet hidden at the solar system’s edge could be an illusion, a new study suggests. “We can’t rule it out,” says Kevin Napier, a physicist at the University of Michigan in Ann Arbor. “But there’s not necessarily a reason to rule it in.”
Previous research has proposed that in the solar system, a variety of far-out objects cluster in the sky as if they were being shepherded by an invisible giant planet, at least 10 times Earth’s mass. The unseen world was called Planet Nine or Planet X by astronomers.
Now, there is no support for such clustering in a recent study of 14 of those remote bodies, knocking down the primary justification to believe in Planet Nine. In 2014, when astronomers Chad Trujillo of Northern Arizona University and Scott Sheppard of the Carnegie Institution for Science announced a series of distant solar system bodies called trans-Neptunian objects with oddly bunched-up orbits, the notion of a distant planet lurking well beyond Neptune received a boost in interest.
But, says Gary Bernstein, an astronomer at the University of Pennsylvania, all those earlier studies focused on only a handful of objects that might not have reflected anything out there. In some areas of the sky, the objects may have appeared to turn up only because that’s where astronomers happened to look.
In 2016, six trans-Neptunian objects were used by Caltech planetary scientists Mike Brown and Konstantin Batygin to refine Planet Nine’s possible properties, pinning it to an orbit between 500 and 600 times as far from the sun as the Earth’s. “In addition to what you saw, it is important to know what you were unable to see,” he says.