Jupiter — the first, largest, and heaviest planet
318 Earth masses of hydrogen and helium wrapped around a diffuse core the size of half the planet. The oldest world in the solar system, and the one whose gravity shaped where all the others sit. It radiates more heat than the Sun delivers to it. Four of its moons are worlds in their own right.
The name and the first planet
The Romans named it well. To ancient sky-watchers Jupiter was the slowest-moving bright wanderer — twelve years to trace the whole zodiac — and the second-brightest thing in the night sky after Venus. It could only be the king. In Greek it was Zeus; in Sanskrit Bṛhaspati; in Chinese Suìxīng, "the year-star", for the twelve-year lap.
The modern reading is that Jupiter is not just the largest planet but the oldest. It formed first, faster than any of the others, while the solar nebula was still gas-rich. Its early inward and outward migration through the young disk swept up material, cleared gaps, and set the boundary conditions for every planet that formed afterward. It is the reason the inner solar system has small rocky worlds instead of hot gas giants, the reason the asteroid belt is a belt, and the reason a lot of the outer solar system looks the way it does.
Jupiter is not, in modern classification, a "failed star" — it is nowhere close. The smallest brown dwarfs are about 13 Jupiter masses; the smallest hydrogen-fusing stars are about 80. Jupiter is one-thousandth the mass of the Sun. But it is a world made of the same broad ingredients — 89 percent hydrogen and 10 percent helium by volume — assembled around a core of heavier elements, and that composition alone tells you a great deal about how it works.
A world that is mostly not solid
Jupiter has no surface to stand on. What we call "Jupiter" is a continuous chemical gradient from a thin gaseous outer envelope down through supercritical fluid to a mantle of liquid metallic hydrogen, all wrapped around a diffuse core of heavier elements.
Descending from the top: at the visible cloud tops (defined as the 1-bar pressure level), the temperature is around −108 °C and the atmosphere is 89 percent hydrogen, 10 percent helium, and trace methane, ammonia, and water. Go deeper, and hydrogen never quite condenses to a liquid in the ordinary sense — the pressure crosses hydrogen's supercritical threshold and the gaseous and liquid phases stop being distinguishable. Deeper still, at pressures of 50 to 400 gigapascals and temperatures of 5,000 to 8,400 kelvins, hydrogen is compressed into an electrically conducting liquid metallic state. Currents in this metallic mantle generate Jupiter's colossal magnetic field.
At the centre, the picture became more interesting after Juno arrived in 2016. Older models predicted a compact, rocky-icy core, like a Neptune sitting inside a Jupiter. Juno's gravity measurements ruled that out. Instead, Jupiter has a "dilute" or "fuzzy" core: heavy elements — silicates, metals, ices — mixed continuously into the surrounding metallic hydrogen and reaching outward across 30 to 50 percent of the planet's radius. It contains something like seven to twenty-five Earth-masses of heavy elements. The centre itself sits at roughly 20,000 kelvin under 4,000 gigapascals of pressure.
And Jupiter is still shrinking. It radiates more heat than it receives from the Sun, powered by the slow Kelvin-Helmholtz contraction of its own interior — roughly one millimetre per year. Its own heat, not sunlight, drives most of its weather.
The Great Red Spot
There is a storm on Jupiter that has been continuously observed since at least 1830, and possibly since the 1660s. That is longer than any recorded storm anywhere else — including Earth.
The Great Red Spot is an anticyclonic vortex sitting at about 22° south, rotating counter-clockwise about once every four and a half Earth days. Its peripheral winds run up to 496 km/h; its interior core is stagnant. Its colour comes from photochemistry in the upper cloud layer — likely ammonia and sulphur compounds reacting under solar ultraviolet — but no one has fully identified the chromophore.
It has also been shrinking for a century. In the late 1800s it was measured at roughly 40,000 km across, about three Earth diameters wide. By April 2017 it was down to 16,350 km — 1.3 Earth diameters. At the current rate of contraction it is projected to become roughly circular around 2040. Astronomers are divided on whether that is a slow endpoint or a temporary phase.
Juno's Microwave Radiometer, which sees down through the clouds, established that the storm's thermal and chemical signature extends at least 240 km below the visible cloud tops. Independent constraints from Juno's gravity data put its full depth at somewhere between 200 and 500 km. That is remarkable next to terrestrial storms, which are confined to a 15 km troposphere — yet it is a shallow "pancake" on Jupiter's 70,000 km radius. The vortex is not rooted in the deep interior. The deep jet streams that power it are.
Above the storm, ground-based infrared spectroscopy has detected a localised temperature spike of 1,600 K in the upper atmosphere, 800 km above the visible clouds. The current explanation: acoustic and gravity waves generated by the turbulence at the base of the storm propagate upward, break in the thermosphere, and dump their mechanical energy as heat — as one paper put it, "crashing ocean waves on a beach".
Bands, jets, and aurorae
Jupiter has no solid surface for its atmosphere to grip, and the whole planet rotates in under ten hours — the fastest of any planet in the solar system. The result is a striped weather system unlike anything else in the sun's family.
Astronomers use three separate rotational reference systems because Jupiter's atmosphere rotates differentially: the equatorial band runs a few minutes faster than the mid-latitudes, and the deep interior — as read from the magnetic field — is defined as "System III" at 9 hours, 55 minutes, 30 seconds. Bands you see from Earth are alternating zones (rising, cloudy, lighter) and belts (subsiding, clearer, darker). Between them run jet streams that can hit 100 metres per second.
Juno's gravity measurements, published in 2018, settled a long-running argument about how deep those jets go. They are not skin-thin weather. They penetrate cylindrically into the planet's interior to a depth of about 3,000 km — 5% of Jupiter's radius — before hitting a hard limit at the base. Below that depth, hydrogen has become ionised enough that its electrical conductivity brakes the differential rotation via ohmic dissipation, and the deep interior rotates as a solid body.
At the poles, Jupiter's magnetic field channels charged particles from its magnetosphere down onto the atmosphere and generates the most powerful auroras in the solar system — brighter than Earth's by a factor of hundreds, and never dark. They provide the answer to a problem that puzzled planetary scientists for half a century: Jupiter's upper atmosphere is hundreds of kelvins hotter than solar heating alone can explain, averaging 600 to 1,000 K. In 2021, a Keck telescope survey mapped a smooth temperature gradient from over 1,000 K inside the auroral ovals down to 600 K at the equator — direct evidence that auroral energy, redistributed by winds, heats the whole planet. Not the Sun.
The magnetosphere and the moons
Jupiter's magnetosphere is the largest coherent structure in the solar system after the Sun's heliosphere itself. Its magnetic field is twenty thousand times stronger than Earth's.
Inside it, trapped charged particles are accelerated to extreme energies by Jupiter's rapid rotation and by the sulphur dioxide continuously vented from Io's volcanoes. Electrons reach energies above 50 MeV; protons reach gigaelectronvolt scales — more than fifty times higher than the proton trapping limit around Earth. The radiation environment near Jupiter is severe enough that any spacecraft going there has to be shielded and its orbit has to be planned around avoiding the worst of the belts.
Four of Jupiter's moons are large enough to be worlds. Galileo discovered them in January 1610 through a small telescope in Padua, and the names Galilean and Medicean still stick. Io is the innermost and the most volcanically active body in the solar system, its sulphur eruptions constantly resurfacing it and feeding the magnetosphere. Europa is a smooth ice-crusted world with a deep salt-water ocean underneath, probably in contact with a rocky sea floor — one of the strongest candidate sites for extra-terrestrial life. Ganymede is the largest moon in the solar system, larger than Mercury, and the only moon known to generate its own intrinsic magnetic field; it also has a deep sub-surface ocean, roughly 100 km thick beneath 130–150 km of ice. Callisto, the outermost, is the most heavily cratered body in the solar system, its surface effectively a fossil from the era of late heavy bombardment. Together the four Galileans are a planetary system in miniature — and much of Jupiter exploration for the next decade is aimed squarely at them.
The rings, and the fleet that watched
Jupiter has rings — thin, dark, and easy to miss until Voyager 1 imaged them in March 1979. Nothing like Saturn's spectacle. But they carry a curious archaeological record.
There are four ring components: a faint Halo ring closest to the planet; a narrow, bright Main ring; and two very faint outer Gossamer rings — one shepherded by the moon Amalthea, the other by Thebe. All are dust, with lifetimes of only a few centuries: Poynting-Robertson drag and Jupiter's magnetic field spiral the small grains inward continuously, so the rings must be constantly replenished. They are, by meteoroid impacts knocking fresh crustal material off the four inner moonlets — Metis, Adrastea, Amalthea, and Thebe.
Both Galileo and New Horizons imaged something unexpected in the main ring: two sets of spiral corrugations, vertical ripples in the ring plane. By backwards-extrapolating how tightly they had wound, the prominent set was dated to July 1994 — when the dust cloud from the disintegrating Comet Shoemaker-Levy 9 slammed into the ring system on its way to hit Jupiter, tilting the ring out of the equatorial plane by roughly 2 km. The rings have carried the record of that impact ever since.
Watching all of this has been the work of a small parade of spacecraft. Pioneer 10 and 11 opened the account in 1973 and 1974 with the first flybys and the first measurements of the radiation belts. Voyager 1 and 2 in 1979 discovered the rings, imaged Io's volcanic eruptions, and detected lightning on Jupiter's night side. Ulysses used Jupiter's gravity for a slingshot into a solar polar orbit. Cassini captured 26,000 images on its way to Saturn. New Horizons did the same en route to Pluto. And two dedicated orbiters — Galileo (1995–2003) and Juno (2016–present) — did the deep science. The next wave has already launched: ESA's JUICE, on its way to enter orbit around Ganymede in 2031, and NASA's Europa Clipper, arriving in 2030 to make dozens of close flybys of Europa.
The oldest planet in the solar system is also its most active — and still, after eight spacecraft, mostly unexplored.