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:
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.
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.
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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.
Planet Pailly 
I’m your moon
You’re my moon
We go round and round
(I’ll provide an address for delivery of the bonus points 🙂 )
As a teenager, I read ‘Earthblood’, a sci-fi novel by Keith Laumer where a far future protagonist is looking for Earth. When they find it, they call it a “double planet”, which threw me for a loop back then because I didn’t yet understand how large Earth’s moon is in comparison to the usual moon-planet relationship. (Although as I understand it, in the case of Earth, the center of gravity remains inside the Earth.)
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That’s my understanding as well. I suspect the whole double or binary planet concept will turn out to be far more relevant for other planets than our own.
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