Sciency Words: Planet X

Sciency Words PHYS copy

Today’s post is part of a special series here on Planet Pailly called Sciency Words. Each week, we take a closer look at an interesting science or science-related term to help us all expand our scientific vocabularies together. Today’s word is:


Planet X is perhaps the most abused term in modern astronomy. The name has been coopted 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.

Dc08 Perturbing Orbits

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

Dc08 Orbits Unperturbed

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.

Dc08 Planet X

The term also remains annoyingly popular among conspiracy theorists.

P.S.: Planet X discovery announcements seem to pop up every few months. Just last week, 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.

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Today’s post is part of Pluto/Kuiper belt month for the 2015 Mission to the Solar System. Click here to learn more about this series.

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