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The term "gas giant" was invented in the 1950s, but I think the concept has been known longer than that.

When did we first learn that Jupiter was a gas giant, not a terrestrial-type planet?

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We need to clear one thing up here, and that is the definition of a gas giant. As an example, take Wikipedia's definition:

A gas giant is a giant planet composed mainly of hydrogen and helium.

All we have to do to classify Jupiter as a gas giant is to prove that it's made of . . . gas.

The answer leads to Giovanni Cassini, who observed Jupiter's differential rotation in the 1690s, thus showing that it could not be solid.

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    $\begingroup$ Thanks for the feedback. I've clarified the question a bit, hopefully getting rid of the ambiguity you described. (Though I'm not so sure Cassini was doing much research in the 1960s.) $\endgroup$ – Joe Oct 18 '15 at 16:17
  • $\begingroup$ @Joe Ack! Typo. I also edited out the now irrelevant stuff. $\endgroup$ – HDE 226868 Oct 18 '15 at 16:18

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