Sciency Words: Chthonian Planet

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 expand our scientific vocabularies together. Today’s term is:


In ancient Greece, there were two ways to pray: with your arms raised up to the gods of Olympus or with your arms lowered in deference to the gods of the underworld, also known as the chthonic or chthonian deities.

Regarding spelling and pronunciation, the “chth” thing makes more sense if you’re familiar with how the Greek alphabet works. Much like the “p” in psychology or the “h” in rhinoceros, the “ch” in chthonian becomes silent in English.

English often retains these silent letters as a way to remind us of a word’s origin and history. Also, we have to do something to keep our spelling bees interesting.

Chthonian became an astronomy term in 2003 thanks to this paper: “Evaporation rate of hot Jupiters and formation of Chthonian planets.” The paper describes a scenario in which a hot Jupiter—a gas giant orbiting waaaaay to close to its parent star—has its entire atmosphere stripped away by solar radiation. Only the planet’s rocky and/or metallic core remains. It would probably look something like this:

This is actually a pretty clever play on the original meaning of chthonian, which could refer to the underworld and all things death-related OR could just mean the earth and everything beneath its surface.

In one sense, chthonian planets are dead. Very, very dead. Also, because a chthonian planet is still located dangerously near to its parent star, conditions there would be truly hellish. But in another sense, these chthonian planets would look like any other Earth-like exoplanets, meaning they are rocky, terrestrial worlds, as opposed to Jupiter or Saturn-like gas giants.

For the time being, the idea of chthonian planets is still more or less theoretical. We have not yet proven definitively that such worlds exist. However, several candidate chthonian planets have been identified. I’ll introduce you to two of them next week.

4 thoughts on “Sciency Words: Chthonian Planet

    1. That’s my understanding of it as well, though I’m sure there’s a lot of nuance to planetary and solar system development that I don’t understand. I guess the key difference between a Chthonian planet and a “normal” terrestrial planet like Earth is that the Chthonians used to be gas giants.

      Liked by 1 person

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