Reddit reviews JUPITER 2E PB (Smithsonian Library of the Solar System)

As several people have pointed out, the question is not "what is Jupiter's structure" or "When did scientists realize that Jupiter had an atmosphere", it's "when did scientists realize it had no solid ground?". It's about the history of knowledge of the interior composition. Since, recent spacecraft results suggest it may gradually transition into something like a small solid core, let's interpret the question as, "When did we first realize that Jupiter wasn't a rocky planet"?

To solve this problem, you need to calculate the mass of Jupiter and its size, so you can find its density. The mass of Jupiter can be calculated using Newtonian orbital mechanics, provided you know the orbital period and orbital diameter of its moons. Period is easy, but a major historical problem in solar system studies was finding the relative scale of everything: it's easy to measure angles and relative sizes with a telescope, but we need the actual length dimensions. You need a baseline distance to start with, such as the distance from the Earth to the Sun: this was worked out in the late 1600s: from that, the size of Jupiter, the size of its moons' orbits, and thus its mass and density could be found.

So even before 1700, astronomers realized that Jupiter was 300 times the mass of Earth, but with a density much less than that of rock. Since the density known today is about 1.4 times that of water, I suppose that without knowledge of high-pressure physics they couldn't rule out a liquid water or ice planet, but scientists have known for over 300 years that Jupiter is definitely not a rocky world. H. G. Wells was a fantastic writer, but he maybe wasn't so meticulous about doing his library research.

The story is laid out by Reta Beebe in Jupiter: The Giant Planet (1997).