Could There Be a Ninth Planet?

A new study re-examined work from 2016 and proposed additional evidence of a ninth planet. While this investigation has been met with much skepticism, Scott Engle, PhD, assistant professor of astrophysics and planetary sciences at Villanova University, contends Planet Nine's existence is a possibility.

"Proposing that the Solar System contains a yet undiscovered planet is always going to be a remarkable claim, so it makes complete sense that there's been a lot of excitement from the public and also a divided response from the field," says Dr. Engle. "This recent study certainly makes a more convincing claim than in 2016. The authors seem to understand the weight behind such a claim, and they appear to have aimed this study to address some of the claim's criticisms. Naturally, though, its existence is just an interesting possibility."

If Planet Nine is confirmed, what would this change in the field of astronomy?

Alongside the excitement of a new planet's discovery, a swell of studies would emerge to continue to investigate the planet. "First, we'll want to pin down the planet's orbit, which will continually improve as we track the planet over time. Once we get a good idea of its orbit, there's also the possibility of going back through the image archives—maybe we'll get really lucky, and Planet Nine photobombed some image that Hubble was taking of an unrelated object."

Planet Nine would be unique, particularly because of its position so far from the Sun. "Theoreticians will have a field day determining if the planet formed that far from the Sun or if it formed closer to the Sun and 'migrated out' to its current distance... and then [picking apart the] implications those results have for the current structure of the Solar System and our theories on how it formed. Astronomers will also want to know the composition of Planet Nine—whether it 'belongs' with the Solar System or is a captured rogue planet."

Can we expect any extraterrestrial discoveries?

Before we start to imagine life on Planet Nine, Dr. Engle noted it is probably not habitable: "Likely not in any human sense of the word. There is a wide range of properties allowed by the latest results, but as the authors state, a gas giant seems probable. Also, the planet orbits at such a distance that it receives 1/57,000th the amount of sunlight that we receive here on Earth, or less."

But a discovery of a ninth planet could spark novel findings and "eureka" moments. "The detection would also serve as a great reminder that even 'right in our own backyard' there are still incredible discoveries to be made," said Dr. Engle.