Sciency Words: Planet X

Today’s post is part of a special series that first appeared on Planet Pailly. Every week, we take a look at a new and interesting scientific term to help us all expand our scientific vocabularies together. Today’s term is:

Planet X

Planet X is perhaps the most abused term in modern astronomy. The name has been co-opted by astrologers, conspiracy theorists, and on occasion science fiction writers.

The name originated with Percival Lowell, better known as that guy who thought he saw canals on Mars. Based on apparent inconsistencies in the orbits of Uranus and Neptune, Lowell predicted that a ninth planet must exist: something massive enough that its gravity would perturb Uranus and Neptune’s orbits.

With the discovery of Pluto in the 1930’s, Lowell’s Planet X hypothesis seemed to be confirmed.


Later, it became apparent that Pluto was tiny. In fact, it looked like Pluto was barely large enough to be a planet at all.


Then in the late 1980’s and early 1990’s, Voyager 2 revealed that we had miscalculated the mass of Neptune. Uranus and Neptune were exactly where they should have been all along. It was our math that was faulty.

The original Planet X hypothesis is now thoroughly defunct, just like that whole Martian canals thing. However, the term is still used as a placeholder name for any hypothetical as-yet-undiscovered planet hiding in the outer Solar System.

The term also remains annoyingly popular among conspiracy theorists.


P.S.: Planet X discovery announcements seem to pop up every few months. Just a few weeks ago, astronomers announced the possible discovery of a Planet X and a Planet Y. Maybe this time it’s for real, but based on past experiences I’m guessing it’s not. Everyone stay skeptical and don’t get caught up in the hype.

Written by James Pailly.

To read all the articles in the ‘Sciency Words‘ series, visit the Planet Pailly blog.

  • Kirov

    Interesting. I had heard of the orbit perturbation theory before, but I didn’t realize it had been busted.

    • James Pailly

      In relation to Uranus and Neptune, it’s busted. Orbit perturbation in the Kuiper belt and the scattered disk are still possibilities.

      • Kirov

        Yes, I just read your latest post, and I remember all the hubbub about a ninth planet a few days ago. I know someone who’s a bit of a conspiracy theorist, so they got excited at the prospect of a potential Nibiru. If another planet really is out there, do you think Nibiru could be a potential name, just for the fun of it?

        • It’s worth pointing out that this article was actually written several weeks ago. SciFi Ideas has an agreement with James to republish his articles a few weeks after he posts to his own blog, so this was written before the current hype. Turns out it was quire prescient.
          Do you think Niburu would be a fun nod to the conspiracy theorists, or would it just fuel their irrational fire?

          • Kirov

            Both, I suppose. I would certainly think it was a fun nod, but you’re right. Many conspiracy theorists would take it as reason for more irrationality. Any suggestions for other names, other than Neo-Pluto? I think that’s more of a placeholder.

            • James Pailly

              Assuming they stick to Roman gods, I believe the names Minerva, Apollo, Persephone, Diana, and Bacchus are still available. Vulcan was also the name of a Roman god, so that’s an option too.

            • I just did a quick search on “Persephone” (because it’s a lovely name), and there’s an asteroid (399 Persephone), but that’s all.
              Wikipedia actually lists it as a potential name for a ninth planet, and it seems that several sci-fi writers have used it as such. In fact it was briefly considered as the name for Eris.

        • why not yuggoth? the planet in the Cthulhu mythos?

  • Shouldn’t it be called Planet IX as it is the ninth, not tenth?

  • I hate the term conspiracy theory, it’s not tested, therefore it is not a theory, it should be “conspiracy conjecture” or “conspiracy BS”

  • also, at least it’s not as popular as the who shot JFK thing, or the “9/11 was an inside job” thing, both of which have been thoroughly proved incorrect on multiple occasions, and each time, the conspiracy bullshitters fail to acknowledge the evidence, it’s like the man who thinks that the tv has little men in it, even when it’s explained to him at length by an engineer, he simply say’s “I don’t suppose there aren’t just a few little men in the tv though?”

  • Francis KENNEDY

    it’s there, it’s called nibiru by locals