A simple question about Jupiter’s Great Red Spot led me to a more basic conceptual difficulty: why do we call Jupiter a planet at all?
When we think of a planet, especially from our Earth-based intuition, we imagine a world with a surface — somewhere one could in principle travel to, descend toward, and eventually land on.
This picture works reasonably well for Mercury, Venus, Earth, and Mars. But it fails completely for Jupiter and Saturn.
A planet on which you cannot land
Jupiter has no solid ground.
If one descended through its atmosphere, there would be no definite moment at which the atmosphere ended and the surface began. The hydrogen and helium would simply become denser and denser as pressure and temperature increased.
One would pass gradually from a relatively thin atmosphere into dense gas, then into a supercritical fluid, and eventually into extremely dense metallic hydrogen.
There is no ordinary boundary corresponding to “air above, ground below.”
Even Jupiter's quoted “surface” is only a conventional reference level, usually associated with an atmospheric pressure of about one bar. An imaginary spacecraft could simply pass through it. There would be nothing there on which to land.
This immediately makes Jupiter seem less like Earth and much more like the Sun.
Jupiter as a kind of quasi-star
Jupiter consists mainly of hydrogen and helium, just like the Sun.
It is a huge, approximately spherical, self-gravitating fluid body, and it possesses no solid surface. From this point of view, it seems much more closely related physically to a star than to a terrestrial planet.
The crucial difference is mass.
The Sun is massive enough for gravitational compression to raise its central temperature and density until sustained nuclear fusion becomes possible. Hydrogen nuclei fuse ultimately into helium and release enormous amounts of energy.
Jupiter never became massive enough for this to happen.
I therefore find it tempting conceptually to think of Jupiter as a kind of “quasi-star”: an object made principally of stellar material that never acquired enough mass to ignite ordinary hydrogen fusion.
The expression “quasi-star,” however, already has a specific technical meaning in astrophysics, so it would not be a good formal name for Jupiter. The broader established category is that of substellar objects.
And nature actually provides a remarkable sequence:
Jupiter-like planet → brown dwarf → low-mass star.
Brown dwarfs occupy the intermediate region. They are more massive than ordinary giant planets, and some can temporarily burn deuterium, but they are not massive enough to sustain ordinary hydrogen fusion.
So there really is a physical continuity between Jupiter and stars.
In terms of composition and bulk thermodynamics, Jupiter is arguably much more Sun-like than Earth-like.
Stars have no ground either
The Sun also has no solid surface.
What we call the solar “surface” is the photosphere. But the photosphere is not a material boundary like the surface of Earth.
It is essentially the region from which visible photons can finally escape efficiently.
An imaginary indestructible spacecraft could descend through the photosphere into progressively denser and hotter plasma. There is no ground underneath.
So Jupiter and the Sun share something fundamental: both are enormous self-gravitating bodies composed mainly of hydrogen and helium, and neither possesses an ordinary solid surface.
The decisive distinction is that the Sun sustains nuclear fusion whereas Jupiter does not.
Metallic hydrogen is not solid metal
Another concept that initially confused me is “metallic hydrogen.”
The word metallic naturally suggests something like iron or copper — a solid metal.
But metallic does not mean solid.
Under the enormous pressures inside Jupiter, hydrogen undergoes a profound change in its electronic properties. Its electrons become sufficiently delocalized that the material becomes electrically conducting.
In that sense hydrogen becomes metallic.
But it is expected to exist mainly as an extremely dense conducting fluid.
So metallicity refers primarily to electronic behavior, not to whether the substance is solid or liquid.
There is therefore no enormous floor of solid metallic hydrogen hidden underneath Jupiter's atmosphere.
Does Jupiter have a core?
Yes — but probably not the kind of core one might first imagine.
The traditional simple picture was something like this: an atmosphere of hydrogen and helium, then metallic hydrogen, and finally a compact rocky or icy core.
Measurements by NASA's Juno spacecraft have suggested something considerably more interesting.
Jupiter appears to possess what is often called a “dilute” or “fuzzy” core.
Instead of having a sharply bounded solid ball at its center, heavy elements may be distributed throughout a large central region, gradually mixing into the surrounding hydrogen-helium material.
There may therefore be no simple boundary saying: hydrogen ends here, solid core begins here.
The composition changes progressively toward the center.
This idea of a fuzzy core is fascinating because it undermines our ordinary picture of a planet as a definite solid object surrounded by an atmosphere.
Do stars also have fuzzy cores?
In another sense, yes.
The Sun has what we call a fusion core. But again, this is not a solid object with a sharp wall.
It is simply the central region where temperature and density are sufficiently high for nuclear fusion to occur efficiently.
The fusion rate falls continuously as one moves outward.
The Sun then passes through the radiative zone, the convective zone, and eventually the photosphere.
These are regions defined primarily by changing physical conditions and mechanisms of energy transport, rather than by rigid material boundaries.
So there is an interesting comparison.
Jupiter's fuzzy core is fuzzy mainly in composition: the heavy elements appear to be spread through an extended central region.
The Sun's core is diffuse in another sense: the region of efficient nuclear fusion fades gradually with radius.
Astrophysical “cores” therefore need not resemble a solid ball at all.
Then Earth gives us the opposite surprise
At this point another question appears.
Earth really does have a solid inner core.
Its broad internal structure is:
solid mantle → liquid outer core → solid inner core.
But the temperature increases as one travels toward the center.
Near Earth's center, temperatures are thought to be of order several thousand degrees, roughly comparable to the temperature of the visible surface of the Sun.
So how can the inner core possibly be solid?
The answer is pressure.
Whether a substance is solid or liquid is not determined by temperature alone. It depends on both temperature and pressure.
For iron and iron-rich alloys, the melting temperature increases strongly with pressure.
In Earth's outer core, the temperature is above the local melting point, so the material is liquid.
But deeper inside, the pressure becomes so enormous that the melting temperature rises faster than Earth's actual temperature.
Eventually the melting temperature becomes higher than the local temperature.
The iron therefore crystallizes.
So, paradoxically, while descending toward hotter and hotter regions of Earth, we pass from liquid material back into solid material.
This is one of the most counterintuitive facts about the Earth's interior.
And there is an additional beautiful consequence.
As Earth slowly cools, its solid inner core gradually grows. Material from the liquid outer core crystallizes onto it.
In a very real sense, the center of Earth is slowly freezing.
High pressure does not simply mean solid matter
This comparison between Earth and Jupiter teaches an important lesson.
It would be tempting to think that because Jupiter has enormously greater pressure than Earth, its interior must eventually become solid.
But this is not true.
Pressure alone does not determine the phase of matter. Temperature and the detailed properties of the substance are equally important.
Iron under the conditions found near Earth's center enters a solid crystalline phase.
Hydrogen under much of Jupiter's interior conditions becomes instead an extraordinarily dense electrically conducting fluid.
Different substances have different phase diagrams.
So “enormous pressure” does not automatically mean “solid.”
From Earth to Jupiter to the Sun
Perhaps a more physically illuminating sequence than our ordinary astronomical labels is:
Earth → Jupiter → brown dwarf → Sun.
Earth is principally a rock-and-metal object with a genuine surface.
Jupiter is principally a self-gravitating hydrogen-helium fluid without a solid surface.
Brown dwarfs continue the progression toward increasingly massive and hotter hydrogen-helium objects.
And eventually, beyond the necessary mass threshold, we reach genuine stars in which sustained hydrogen fusion becomes possible.
Seen this way, Jupiter belongs unquestionably to the planetary family astronomically, but physically it sits remarkably close to the stellar world.
It has no ground, no conventional material surface, probably no sharply bounded solid core, and most of its mass exists as hydrogen and helium under conditions completely outside ordinary human experience.
Earth, meanwhile, gives us the opposite surprise: despite temperatures of thousands of degrees, its deepest central region remains solid because the pressure is so enormous.
The broader lesson is perhaps the most interesting one.
Our everyday categories — solid, liquid, gas, surface, atmosphere, core — come from a tiny region of the possible states of matter that we experience at ordinary terrestrial pressures and temperatures.
Inside giant planets and stars, gravity pushes matter into regimes where those familiar intuitions begin to fail.
And once we look at the problem this way, the boundary between a “world” and a “star” becomes much less obvious than ordinary language makes it seem.
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