1. How Can Anyone Study the Inside of a Planet With No Ground?
Jupiter is a gas giant. It is about 11 times wider than Earth, and if it were a hollow shell, 1,000 Earths could fit inside [1]. NASA explains that Jupiter has no true surface: it is mostly swirling gases and liquids, and the extreme pressures and temperatures deep inside would crush, melt and vaporize a spacecraft trying to fly in [1].
Its makeup is close to the Sun's: mostly hydrogen and helium [1]. So the question is not "what rock is it made of?" but "how are the hydrogen, the helium and a small amount of heavier material arranged on the way down?" Nobody can take a sample from deep inside, so scientists work from the outside in. They weigh the planet's pull from space, they look at the one place a probe has actually been, and they squeeze hydrogen and helium in laboratories to see what they do.
2. What Can Jupiter's Gravity Tell Us About Its Inside?
The Juno spacecraft, which has been orbiting Jupiter since 2016, measured the planet's gravity field in very fine detail. A team that builds models of Jupiter's inside describes the gravity numbers Juno measured (called J2 to J8) as having "unprecedented" precision [2].
Why would gravity say anything about the inside? Jupiter spins quickly and is slightly squashed as a result, and the way its mass is spread from the top layers to the center changes the shape of its gravity field. A simple way to picture it is a spinning ball of dough: a lump of heavier material in the middle would change how the pull looks from far away. This is an analogy, not a statement from the sources.
Scientists build computer models of the inside and keep the ones that reproduce Juno's numbers. There is a catch. The researchers say the predictions are strongly affected by the choice of "equation of state", which is the set of rules for how hydrogen and helium behave under huge pressure, and that there are still difficulties in reconciling the measured numbers with these rules [2]. So gravity narrows the possibilities but does not hand over one final answer. The other two kinds of clue help.
3. What Happens to Hydrogen the Deeper You Go Into Jupiter?
Going down, pressure and temperature climb. According to NASA, deep in the atmosphere hydrogen gas is compressed into a liquid, which gives Jupiter the largest ocean in the solar system, an ocean made of hydrogen instead of water [1].
Deeper still, perhaps about halfway to the planet's center, scientists think the pressure becomes so great that electrons are squeezed off the hydrogen atoms. The liquid then conducts electricity like a metal, so it is called metallic hydrogen. The fast spin of this layer is thought to act like a dynamo and generate Jupiter's powerful magnetic field [1]. These depths come from models, since no instrument has measured them directly.
Beneath the metallic hydrogen sits the question in the title. Before the data from Juno, the standard idea was a small solid core of ice, rock and metal inside a mostly uniform mix of hydrogen and helium [1]. But first, there is one place inside Jupiter that a machine did visit, and it left a surprise about helium.
4. What Did the Galileo Probe Measure When It Dropped Into Jupiter?
NASA's Galileo spacecraft released an entry probe in July 1995. On December 7, 1995, the probe hit Jupiter's atmosphere and sent back data for 58 minutes. NASA calls it the first spacecraft to enter Jupiter's atmosphere [3]. NASA's page gives the last reading as a depth of about 180 km (112 miles) below the entry point, at a pressure of 22.7 atmospheres [3].
That sounds deep, but Jupiter's radius is about 69,900 km [1]. By my own arithmetic, 180 km is only about a quarter of one percent of the way to the center. The probe measured the very top of the atmosphere, not the core.
It still brought back a puzzle. NASA reports that the probe found less lightning, less water vapor and half the helium that had been expected in the upper atmosphere [3]. Researchers at the University of California, Berkeley, add that neon turned out to be only about one-tenth as abundant as expected [4]. If Jupiter formed from material like the Sun's, where did the helium and neon go?
5. Could Helium Rain Down Inside Jupiter and Carry Neon With It?
In 2010, two Berkeley researchers proposed a possible answer from computer simulations. Deep inside Jupiter, they said, helium can condense into droplets that fall inward, and neon dissolves into those droplets and sinks with them. That would leave the upper layers short of both elements, just as the probe saw [4].
In their calculations the droplets form about 10,000 to 13,000 km (6,000 to 8,000 miles) below the cloud tops. By my arithmetic that is roughly 14 to 19 percent of the way to the center. The researchers themselves point out that "rain" is an imperfect word: at those pressures and temperatures it is hard to say whether hydrogen and helium are gas or liquid, so the falling droplets are fluid helium mixed with neon, moving through fluid metallic hydrogen [4].
At the time, the team noted that such conditions could not yet be reproduced in a laboratory [4]. Since then, experiments have come closer. A study published in Nature in May 2021 used powerful lasers to compress hydrogen-helium mixtures to about 10,000 °C and pressures of several million atmospheres, and observed the two separating. The French research agency CEA, whose scientists took part, says the results help constrain current models of planet interiors [5].
Note the order of events. First came an observation (missing helium and neon), then a calculated explanation, then a lab test of one ingredient of that explanation. Nobody has watched helium rain inside Jupiter, so it remains a well-supported idea rather than a proven fact.
6. So Is Jupiter's Core a Solid Rocky Ball?
Probably not a simple one. In 2017, a team led by S. M. Wahl compared interior models to Juno's gravity numbers. They found that a "dilute core", where heavy elements are mixed into the hydrogen and helium and spread out over a significant fraction of the planet's radius, helps the models match what Juno measured. They estimated that the core region holds about 7 to 25 Earth masses of heavy elements [2].
NASA describes the same picture in plain words: the core is larger than expected, partly dissolved, and has no clear boundary with the metallic hydrogen around it, which is why researchers call it "dilute" or "fuzzy" [1].
Here is how strongly to read it. The range of 7 to 25 Earth masses is an estimate from models, and the same paper warns that the results depend strongly on the equation of state [2]. Gravity gives how mass is spread out, not directly what the material is. So "the core is dissolved" is best read as a leading explanation, not a closed case.
7. How Can You See What "Doesn't Mix" Looks Like?
Do this with an adult. Half-fill a clear jar with a lid with water, add a splash of cooking oil, close the lid tightly and shake. The oil breaks into tiny droplets. Leave the jar still for a few minutes and the droplets gather and float back into a layer on top. That is a small-scale feel for two liquids that refuse to blend. Do not drink the mixture.
One difference matters: in Jupiter, helium droplets are thought to sink through lighter hydrogen, while oil floats because it is lighter than water. So the jar shows the idea of separating, not the same event.
To go further, the NASA pages and the Wahl paper listed below show the diagrams and numbers. When you read them, ask of each number: was this measured, or was it calculated from a model? Mass and size are measured. Depths such as 10,000 km are model results. The shape of the core is somewhere in between.