Why NASA Thinks A Hidden Ninth Planet Might Be Possible

You've probably heard both astronomers and members of the public arguing about how many planets there are in our solar system. Ever since Pluto was demoted from its planet status in 2006 and downgraded to a "dwarf planet," people have been arguing that it should be reinstated. However, there's another debate about how many planets are in the solar system which is not so well known, about whether there's a mysterious as-yet-unobserved planet out there called Planet Nine.

The idea started with Caltech professors Mike Brown and Konstantin Batygin, who looked far out into the solar system beyond the orbit of Pluto. This region, called the Kuiper Belt, is a disk of ice and debris which extends out from the orbit of Neptune and is host to a number of dwarf planets.

The researchers found that the orbit of some of the bodies in this distant region, called trans-Neptunian objects (TNOs), seemed to cluster in strange ways. They hypothesized that this clustering could be due to the presence of a giant planet too far away for us to observe it directly. They called their hypothesized object Planet Nine.

Since then, the idea of Planet Nine, also known as Planet X, has captured the imagination of many. There have been all sorts of theories about the object, including the idea that it could be even bigger and closer than first thought (via Caltech) or that it could be a miniature black hole (via arXiv).

These ideas are fun and engaging, though most astronomers treat the idea of Planet Nine with healthy skepticism. After all, we've never directly detected such a planet, and we have only indirect evidence to suggest that it exists.

However, even NASA is open to the possibility that Planet Nine might exist. The agency has a page on "Hypothetical Planet X" stating that it could be 10 times the mass of Earth and could have an orbit so far out that a year would last between 10,000 and 20,000 Earth years. But the experts it quotes are careful to emphasize that the planet is only a theory at this point.

"The possibility of a new planet is certainly an exciting one for me as a planetary scientist and for all of us," said Jim Green, director of NASA's Planetary Science Division (via NASA). "This is not, however, the detection or discovery of a new planet. It's too early to say with certainty there's a so-called Planet X. What we're seeing is an early prediction based on modeling from limited observations. It's the start of a process that could lead to an exciting result."

The originators of the theory, Brown and Batygin, are continuing their work to look for more clues about whether there really is another planet out there. "I would love to find it," said Brown. "But I'd also be perfectly happy if someone else found it. That is why we're publishing this paper. We hope that other people are going to get inspired and start searching."