Was it due to appearance only or spectroscopic methods were used? Venus also has a thick cloud cover, how was it determined to be a rocky planet?
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2$\begingroup$ I think that there will be two interesting answers; when it was first suspected that it was made out of gas will come from an estimate of their average densities from their sizes and masses, but the first observational confirmation of the large gaseous fractions of these planets (their cores are not gas!) may have come much later, perhaps via spectroscopy, perhaps infrared or radio. $\endgroup$– uhohFeb 27 at 9:07
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2$\begingroup$ Pretty sure Newton did this in his Principia. $\endgroup$– Greg MillerFeb 27 at 13:48
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3$\begingroup$ The internal structure of Jupiter is a bit of a puzzle. As I said here gravitational data from Juno told us that our old model is wrong. $\endgroup$– PM 2RingFeb 27 at 20:48
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1$\begingroup$ "gas" and "fluid" are often used interchangeably in this context. I suspect the Soviet Union would claim the honor of proving that Venus had a rocky surface by the simple act of landing/crashing on it and send back photos. Say what you like about the Soviet Union but "bloody minded" is one description of their series of probes to Venus. See the Venera probes. $\endgroup$– StephenG - Help UkraineMar 1 at 11:41
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3 Answers
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I'm unsure of the "history of science" aspect of this, but an actual deduction that these are gas giants would require Kepler's laws and Newton's law of gravity combined with a modest telescope.
It can be deduced from Kepler's laws that the giant planets are much further from the Sun than Earth. If one measures the angular diameter of their visible discs ($\theta_p$) and the angular diameter ($\theta_m$) and periods ($P_m$) of the orbits of their moons then the following analysis was possible.
The radii of the planets was $R = d\theta_p$. The semi-major axes of the moon orbits $a_m = d \theta_m$.
Newton's gravitational law (published 1687) then allows the mass of the planet to be estimated: $$ M =\frac{4\pi a_m^3}{GP_m^2} = \frac{4\pi d^3\theta_m^{3}}{GP_m^2}\ .$$
The mass and radius are then combined to give density, $3M/4\pi R^3$ and we see that, even without knowing $d$ we can accurately estimate the density of the giant planets by the late 17th century. Newton did something like this in the third book of Principia (see Cohen 1998) and got the relative densities of the Earth/Jupiter/Saturn/Sun with reasonable accuracy. Absolute values do require a knowledge of $G$ which could come from accurate measurement of the scale of the solar system from the transits of Venus in 1761 and 1769.
So, assuming we find that these massive objects have an average density of $\sim 1000$ kg/m$^3$ (or about a fifth of the density of the Earth, according to Newton) combined with the observations of gaseous motions at the surface and the much higher mean density of the Earth, I think one could arrive at the conclusion that a large fraction of these bodies must be in the form of a gas/fluid. This would not rule out a solid centre and indeed, Saturn does have some sort of solid core and Jupiter may have one.
A similar calculation can be done for the Earth and for Venus (the mass of Venus is estimated from its effects on the orbits of the Earth and Mercury). I am unsure when accurate mass measurements were available but certainly by the mid 19th century when the precession of Mercury was being investigated. This would establish that the density of Venus and the Earth were similar and much higher than that of the gas giants.
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By 1690, Giovanni Cassini was able to estimate the rotation period of the planet and noticed that the atmosphere of Jupiter undergoes differential rotation which confirmed that Jupiter was made of gas instead of having a solid surface. See here
For more details, check the articles.
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11$\begingroup$ Why does differential rotation mean the planet is gas? Could it not be the case that a solid planet had an atmosphere with differential rotation? Did they recognize this fact back in the late 1600's or did they "just" discover the differential rotation that was later used as a piece of evidence in the argument for it being a gas planet? $\endgroup$ Feb 27 at 19:35
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4$\begingroup$ @DustinWyatt So, not all of Jupiter is gas; there is a core that isn't gas (we aren't sure what it is). But, the only thing we can see is gas. What more, this gas isn't bound to a nearby solid, like on Venus. $\endgroup$ Feb 27 at 20:13
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2$\begingroup$ @Yakk-AdamNevraumont See: astronomy.stackexchange.com/questions/8251/… $\endgroup$ Feb 28 at 4:48
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2$\begingroup$ @DustinWyatt See: astronomy.stackexchange.com/questions/39752/… $\endgroup$ Feb 28 at 4:51
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5$\begingroup$ By this evidence you might claim that the Earth or Venus were made of gas. I don't think this addresses the Q - it demonstrates that what we can see is not solid. $\endgroup$– ProfRobFeb 28 at 18:11
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According the answers to this question:
Astronomers didn't discover that the giant planets didn't have solid surfaces until the first half of the 20th century.
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$\begingroup$ Another answer here demonstrates that a fluid surface was established in the 17th century. $\endgroup$– ProfRobMar 1 at 5:38