Why is it called an exoplanet?

Dear Astroquizzical: why is it called an EXOplanet? What’s the opposite of an exoplanet?

We haven’t had a naming question in a while!

An exoplanet is also called an “extrasolar planet” - both terms simply mean a planet which is in orbit around a star which is not ours.

The ‘exo’ part comes from the same root as an “exoskeleton”, “exothermic” or “exotic”. The first tells us that an animal’s skeleton is an ‘outer’ skeleton such as those of spiders and insects, not an interior skeleton like mammals have. If you’re a chemistry person, you’ll recognize “exothermic” as a sign that a chemical reaction produces more heat than it consumes. Put another way, it’s dumping heat “outside” of the reaction. “Exotic” simply means that it comes “from outside” where you’re from.

The opposite of “exo-“ is “endo-“, which means “internal” instead of “external”. While we don’t tend to use the word “endoskeleton” to mean an internal skeleton, we do use “endothermic” to mean something that must suck energy out of its environment.

An exoplanet is simply an “external” planet, in that it isn’t in our solar system. So the opposite of that would be something internal to our solar system, which also is not a planet! Any of the many moons, comets, and asteroids in our solar system (or Pluto) would qualify.

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