I'm asking for historical scientific knowledge of our understanding of the outer planets, which seems hard to come by.

Pioneer 10 was the first probe to an outer planet (Jupiter). Before then, did we believe that the outer planets were rocky with huge atmospheres? This would seem to be the default position.

Or did we have good reason to believe they were truly gaseous planets? If so, why?

EDIT: as an edition, can anyone find the best photos we had of Jupiter before Pioneer 10? Pre-probe photos of the other gas planets would be good too.

  • $\begingroup$ Don't we still believe the Gas Giants have a rocky core? It's just the mass and size ratio of solids vs. gases that are in dispute. Jupiter's core is an Earth-like Iron and Silicon core than is 14 to 18 times the mass of Earth. That's a bit much to sweep under the carpet and pretend it does not exist. $\endgroup$ Nov 29, 2021 at 15:18

3 Answers 3


My 1965-vintage copy of Sourcebook On The Space Sciences has this to say:

The model developed by DeMarcus in 1958 is regarded at present as the most acceptable for the interior of Jupiter. It postulates that the planet consists of 78% by weight of hydrogen molecules and the remainder helium ... calculations indicate that at a distance of about 0.8 times the planet's radius from the center, the pressure becomes so high ... that molecular hydrogen turns into a metallic form ... most of the internal volume (and mass) of Jupiter would thus appear to consist of this solid hydrogen ... it should be made clear that the model just described is used largely as a basis for calculation and is not meant to represent the exact condition of Jupiter's interior.


Nothing is known about the nature of the surface of Jupiter, but it is probably a strange and complex system. Although much of the interior of the planet may be solid hydrogen (and helium), the pressures in the outer parts are too low and the temperatures probably too high to permit hydrogen to solidify. Hence, there may be some liquid hydrogen (and possibly methane) near the surface of Jupiter. Between this and the atmosphere, it is expected that there will be a transition layer containing solid or liquid ammonia (or both), methane, and possibly ice. The foregoing is admittedly speculation, since the temperature distributions in the atmosphere and interior of the planet are quite unknown.

This seems to agree to some extent with the modern picture of Jupiter's structure as given by Wikipedia:

Jupiter is thought to consist of a dense core with a mixture of elements, a surrounding layer of liquid metallic hydrogen with some helium, and an outer layer predominantly of molecular hydrogen ... The core is often described as rocky, but its detailed composition is unknown, as are the properties of materials at the temperatures and pressures of those depths ... The presence of a core during at least part of Jupiter's history is suggested by models of planetary formation that require the formation of a rocky or icy core massive enough to collect its bulk of hydrogen and helium from the protosolar nebula ... A core may now be entirely absent, as gravitational measurements are not yet precise enough to rule that possibility out entirely ... The core region is surrounded by dense metallic hydrogen, which extends outward to about 78% of the radius of the planet.

That is to say, both then and now, the belief is that Jupiter and the other giant planets have dense, non-gaseous interiors.

Since the 1960s, the understanding of the behavior of hydrogen under extremely high pressure has changed, and it's believed that the bulk of Jupiter's and Saturn's volumes consist of metallic liquid hydrogen.

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    $\begingroup$ I'm not sure Jupiter's core is exactly "Rocky", but I like your quote. By studying Jupiter's (and Saturn's), moon's orbits, mass could be accurately calculated and along with that, density. The low density would imply a planet that was mostly gas. I'm not sure when that was recognized, but I suspect, well before Pioneer made it out there. $\endgroup$
    – userLTK
    May 27, 2015 at 8:00
  • $\begingroup$ I'd upvote this if it weren't for your last sentence, as it seems to disagree with the quote and everything else available on wikipedia, NASA's websites etc $\endgroup$
    – Rory Alsop
    May 27, 2015 at 11:20
  • $\begingroup$ If by rocky, you mean consisting of chunks of solid mineral material, then I think the last sentence is technically accurate $\endgroup$
    – neelsg
    May 27, 2015 at 11:58
  • $\begingroup$ Revised the conclusion a bit. $\endgroup$ May 27, 2015 at 14:40
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    $\begingroup$ Here's a 1950 picture from Mt. Wilson & Palomar: anamorfose.be/night-photography/solar-system-and-comets/… -- there's a picture of comparable quality in my mid-60s book. $\endgroup$ May 28, 2015 at 2:42

As I remember, Saturn was often described as being less dense than water and thus able to float in a giant ocean, long before the probes. The orbiting natural satellites were used to measure the total masses and thus densities of the gas planets.

James Blish coined the term "gas giant" to describe the outer planets about 1952 in a science fiction story, so by then it was accepted that they had low density and very deep dense atmospheres above any "surfaces" they might have.


The orbital period of a satellite tells you the mass of the object it's orbiting. Thus we could weigh the gas giants and know that they were so light they had to be almost all gas.


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