Sciency Words: Binary Planet

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:

Binary Planet

If Pluto isn’t a planet, what is it? In 2006, the International Astronomy Union reclassified Pluto as a dwarf planet, but they considered other options as well. One idea was to classify Pluto and its largest moon Charon as binary planets.


Bonus points to anyone who can translate this.

The term “binary planets” comes by analogy with the term “binary stars,” which are stars that orbit each other. Proposed technical definitions of binary planets include:

  • A pair of planetary bodies that orbit a point located somewhere between them (it’s not clear how close to the middle that point needs to be).
  • A pair of planetary bodies co-orbiting a star that have close to the same mass (it’s not clear how similar their masses have to be).


Isaac Asimov, the grandmaster of science fiction and one of the greatest science communicators of his day, proposed his own definition for binary planets, or rather double planets, as he called them. Asimov’s definition was based on the gravitational attraction each planet had for the other.

In his books on science, Asimov applied the term double planet not only to Pluto and Charon but also to Earth and the Moon. After all, the Moon does exert a pretty strong gravitational pull on the Earth, arguably comparable to the gravitational pull the Earth exerts on the Moon.

Pluto and Charon have such an unusual relationship with each other that modern scientific literature often still calls them binary planets, even though the I.A.U. has rejected that terminology. Occasionally, the Earth/Moon system is also referred to this way.

The existence of two binary or almost binary planet systems in our own Solar System suggests that we may find other binary worlds orbiting distant stars. Binary habitable planets may even be possible. As this article from Discovery News suggests, civilizations on one or both planets might end up in “a fevered space race that would dwarf our space race of the 1960’s.”

At the very least, such a setting could offer loads of potential for a science fiction story.

Written by James Pailly.

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

  • Kirov

    Great article. I am by no means an authority on the subject, but I draw the distinction between a planet-moon relationship and binary planet depending on where the center of mass of the system is. If it’s inside one of the bodies, it’s a planet/moon; if it’s outside, then they’re a binary planet. That seems like an objective enough distinction to me. I also think that binary planets are much more likely to harbor life than singleton planets. Tidal forces seem to promote life, and a species may be more likely to become space-faring if they have a giant planet hanging over them that causes them to wonder.

    Also, I’m assuming that binary is ascii, but I’m too lazy to translate it and don’t know ascii by heart yet. That’d be really cool though if it’s actually some obscure encryption with a secret message.

    • That does sound like an easy enough rule-of-thumb when it comes to such orbital mechanical quirks of planetary formation: If the Barycenter is outside the larger body, then it’s a Binary/Double Planet. If the Barycenter is within the larger body, then it’s a planet-moon system. Though, considering the current IAU terminology, Pluto and Charon would be classified as a Binary/Double Dwarf Planet since, well, there’s that whole “clearing its orbital path of other co-orbital debris and similar bodies” criteria.

      Though it does beg the question as to what would be, in the eyes of the IAU, considered co-orbital debris if we take into consideration Moons, Binary/Double Planets, Lagrangian Worlds and other similar gravitational quirks that I can only think off the top of my head for the moment. Okay, well granted there’s the whole “Hill sphere” for Moons and Binary/Double Planets, but the Lagrangian Worlds does throw a wrench to the whole “clearing its orbital path of debris” criteria unless there’s some obscure Lagrange Point rule of such planetary bodies that I’m missing….

  • Leonardo Faria

    Pluto will be always a planet to me, what you all say doesn’t matter.