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When was it realized that the gas giant planets Jupiter and Saturn, and the ice giant planets Uranus and Neptune, do not have solid surfaces?

When was that idea first proposed and when was it accepted by astronomers?

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    $\begingroup$ They do have solid surfaces... very, very, deep within the planets. $\endgroup$
    – Vikki
    Jan 19 at 0:04
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    $\begingroup$ @Vikki They're eventually solid, sure, but doesn't "surface" imply a sharp boundary between solid and not-solid, which they don't have? $\endgroup$ Jan 19 at 5:57
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    $\begingroup$ @JosephSible-ReinstateMonica: What they lack is a sharp boundary between the gaseous atmosphere and the main body of the planet (as the temperatures and pressures involved are high enough to make the planet a supercritical fluid). Assuming that there's a solid core at all, it has to have a sharp boundary between solid and not-solid (the phase transition between solid and not-solid has no critical temperature or pressure, unlike the transitions between different fluid phases), since it's impossible to grade smoothly between solid and not-solid. $\endgroup$
    – Vikki
    Jan 19 at 18:40
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    $\begingroup$ @Vikki Considering how glass behaves, I don't think a sharp boundary between solid and non-solid is strictly necessary. $\endgroup$
    – Brilliand
    Jan 19 at 21:08
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    $\begingroup$ @Brilliand Glass is not a liquid, if that's what you're implying. $\endgroup$ Jan 19 at 23:28

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Prehistory goes back to the second half of 19th century after Fraunhofer introduced astronomical spectroscopy, and then Kirchhoff identified some spectral lines in the Sun's spectrum in 1860s. Rutherfurd observed the spectrum of Jupiter in 1863, but its import remained murky until 1920s. For example, in a 1906 Scientific American article Erber noted great similarity in the observed chemical composition of Jupiter and Saturn, and mentioned that Joussen found presence of hydrogen there, but his pictures indicate that he still thought an observer could stand on Saturn's surface, complete with hills and clouds above.

Only in 1920s Jeffreys conjectured gaseous (hydrogen-rich) composition. The confirmation came in 1930s largely due to Wildt, see Taylor et al., The Composition of the Atmosphere of Jupiter:

"Modern studies of the composition of Jupiter's atmosphere date back to the mid-nineteenth century, when the near- infrared spectrum of the planet was viewed by Rutherfurd (1863) using diffraction gratings of his own manufacture. He discovered features that remained unidentified until 1932, when Wildt showed that the unknown spectral lines were due to ammonia and methane. In later years, building on the original insight of Jeffreys (1923, 1924), Wildt and others went on to note that the low density of Jupiter and the presence of these hydrogen-rich compounds in the atmosphere were consistent with a bulk composition similar to that of the Sun, that is, primarily hydrogen."

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    $\begingroup$ For those, like me, who initially misread 'Rutherfurd' as 'Rutherford', no, that is Lewis M. Rutherfurd (jstor.org/stable/3102525) the astronomer, not Ernest Rutherford the atomic physicist... $\endgroup$
    – Jon Custer
    Jan 18 at 14:12
  • $\begingroup$ I don't see the hills and clouds in the link $\endgroup$ Jan 19 at 18:29
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    $\begingroup$ @theonlygusti Fig. 3 on p. 25637 of the volume in GoogleBooks (second link). GoogleBooks sometimes shows different pages to different users. $\endgroup$
    – Conifold
    Jan 19 at 18:40

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