Jupiter

• 1. Introduction

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  Jupiter is the fifth planet from the Sun and the largest in the Solar System. Its mass is nearly 2.5 times that of all the other planets combined. Yet it remains slightly less than one-thousandth the mass of the Sun. Picture a world so wide that 1,321 Earths could fit inside it. Picture a single storm, the Great Red Spot, that is itself larger than our entire planet.

  Humans have watched this light since prehistoric times. It is the third-brightest natural object in the night sky, after the Moon and Venus. Babylonian astronomers tracked it in the 7th and 8th centuries BC. The ancient Chinese called it the sui star. The Romans named it for the chief deity of their religion.

  But why was Jupiter the first planet to form? How does a world rain helium, and possibly diamonds, far below its clouds? What sends radio signals from Jupiter that can exceed the radio output of the Sun? And what happened in July 1994, when a comet struck this giant while observatories around the world watched? The answers begin deep in the early Solar System.

• 2. The Firstborn World

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  Jupiter is believed to be the oldest planet in the Solar System. It formed just one million years after the Sun, and roughly 50 million years before Earth. Current models suggest it took shape at or beyond the snow line, the distance from the early Sun where it was cold enough for water to condense into solids. A solid core came first. Only then did the planet gather its gaseous atmosphere around it.

  During its formation, Jupiter's mass climbed until it reached 20 times the mass of Earth. About half of that was silicates, ices, and other heavy-element material. Once the proto-planet passed 50 Earth masses, it carved a gap in the solar nebula. From there it reached its final mass in three to four million years.

  The grand tack hypothesis describes a restless youth. Jupiter began forming at roughly 3.5 AU from the Sun, then migrated inward as it interacted with the surrounding gas disk and felt orbital resonances from Saturn. This upset the orbits of several super-Earths nearer the Sun and sent them into destructive collisions. Saturn then caught up, and the two became locked in a 3:2 mean motion resonance at about 1.5 AU. That changed the direction of travel, pulling both planets back outward to where they sit today.

  The grand tack is not the only story. The Nice model proposes that infalling proto-Kuiper belt objects, over the first 600 million years, drove Jupiter and Saturn into a 1:2 resonance. That shift lifted Saturn into a higher orbit, disrupted Uranus and Neptune, depleted the Kuiper belt, and triggered the Late Heavy Bombardment. The jumping-Jupiter scenario goes further still, suggesting Jupiter's migration may have flung a fifth gas giant out of the Solar System entirely, leaving the four humans observe today.

  Some researchers argue from Jupiter's composition that it first formed far beyond the molecular nitrogen snow line, estimated at 20 to 30 AU, and perhaps beyond the argon snow line at 40 AU. In that picture the planet migrated inward over roughly 700,000 years to reach its present home. Whichever model holds, one consequence is shared. Jupiter's departure from the inner Solar System left the rubble from which Earth and the other inner planets formed.

• 3. A Planet Of Gas And Metal

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  Jupiter is a gas giant, which in planetary geology means its chemical makeup is primarily hydrogen and helium, not that those materials are gases in state. Its atmosphere is approximately 76% hydrogen and 24% helium by mass. By volume the upper atmosphere is about 90% hydrogen and 10% helium. The low share of helium follows from a simple fact: a helium atom is more massive than hydrogen in its diatomic molecular form. Trace amounts of carbon, oxygen, sulfur, and neon ride along, with ammonia, water vapour, phosphine, hydrogen sulfide, and hydrocarbons like methane, ethane, and benzene.

  The planet's atmospheric proportions sit close to the theoretical composition of the primordial solar nebula. Neon in the upper atmosphere measures 20 parts per million by mass, about a tenth as abundant as in the Sun. Jupiter's helium abundance runs near 80% that of the Sun, because these elements precipitate as helium-rich droplets deep inside. Saturn is thought to share a similar composition, while Uranus and Neptune carry relatively less hydrogen and helium and more oxygen, carbon, nitrogen, and sulfur, earning them the name ice giants.

  The giant radiates more heat than it receives from the Sun, driven by the Kelvin-Helmholtz mechanism in its contracting interior. That contraction shrinks the planet by about 1 mm per year. At its formation it was hotter and about twice its current diameter. Theory suggests that if Jupiter had over 40% more mass, its interior would compress so much that its volume would actually decrease. To fuse hydrogen and become a star, it would need to be about 75 times more massive. That gap between planet and star is the boundary the next layers of Jupiter help define.

• 4. Inside The Diffuse Core

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  Data from the Juno mission rewrote the textbook picture of Jupiter's heart. Rather than a sharp dense core, Juno showed a diffuse core that mixes into the mantle, extending across 30 to 50% of the planet's radius. It holds heavy elements with a combined mass of 7 to 25 times the Earth. The mixing may date to formation, as the planet swept up solids and gases. Or it may trace to an impact by a body of about ten Earth masses, a few million years after Jupiter formed, which could have shattered an originally compact core.

  Outside the metallic hydrogen lies a transparent atmosphere of hydrogen where pressure and temperature exceed molecular hydrogen's critical pressure of 1.3 MPa and critical temperature of 33 K. In that regime there are no separate liquid and gas phases. Hydrogen becomes a supercritical fluid, possibly resembling an ocean of liquid hydrogen growing hotter and denser with depth.

  Rain-like droplets of helium and neon fall through the lower atmosphere, stripping those elements from the upper layers. Calculations place the separation of helium drops at a radius of 60,000 km, about 11,000 km below the cloud tops, with the drops merging again near 50,000 km. Rainfalls of diamonds have been suggested here, as on Saturn and the ice giants.

  The heat of planetary formation can only escape by convection, so temperature and pressure climb steadily inward. Where the pressure reaches about one standard Earth atmosphere, the temperature is around 165 K. The transition from molecular to metallic fluid spans pressures of 500,000 to 4,000,000 bar at temperatures of 5,000 to 8,400 K. Deeper still, the diluted core may reach 20,000 K. That trapped heat helps power everything churning in the clouds above.

• 5. Bands, Storms, And The Great Red Spot

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  Jupiter is perpetually wrapped in clouds of ammonia crystals, possibly mixed with ammonium hydrosulfide. They sit in the tropopause and organize into bands at different latitudes, split into lighter zones and darker belts. Where these circulation patterns clash, storms and turbulence erupt. Wind speeds of 100 m/s are common in the zonal jet streams. The orange and brown hues come from upwelling compounds, called chromophores, that change colour under ultraviolet light. Their exact makeup is uncertain, thought to involve phosphorus, sulfur, or hydrocarbons.

  The cloud layer runs about 50 km deep, with at least two decks of ammonia clouds. Beneath them may lie water clouds, hinted at by lightning in the atmosphere. These discharges can be up to a thousand times more powerful than lightning on Earth. Juno revealed shallow lightning from ammonia-water clouds high up, carrying mushballs of water-ammonia slush coated in ice down into the depths. Higher still, brief flashes lasting around 1.4 milliseconds, known as elves or sprites, glow blue or pink from the hydrogen.

  The Great Red Spot is an anticyclonic storm sitting 22 degrees south of the equator. It was first observed in 1831, and possibly as early as 1665. It rotates counterclockwise with a period of about six days, rising about 8 km above the surrounding cloud tops. Its red colour may come from photodissociated ammonia reacting with acetylene. The storm is larger than Earth, yet it has shrunk over time. Early observations in the late 1800s put it at roughly 25,500 miles across; later measurements found about 10,250 by 6,800 miles, narrowing by about 580 miles per year. In October 2021, a Juno flyby measured its depth at around 300 to 500 km.

  Juno also found cyclone groups at the poles. The northern group holds nine cyclones, one central and eight around it. The southern group has a central vortex ringed by five large storms and one smaller one, seven in all. In 2000, a smaller cousin appeared in the southern hemisphere, formed when three white ovals born in 1939 and 1940 merged. Named Oval BA, it later reddened, earning the nickname the Little Red Spot. In April 2017 came another oddity, a Great Cold Spot in the thermosphere at the north pole. It stretches 24,000 km across and runs 200 degrees cooler than its surroundings, holding its position for more than 15 years.

• 6. The Strongest Magnetic Field

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  Jupiter's magnetic field is the strongest of any planet in the Solar System, with a dipole moment of 4.170 G tilted 10.31 degrees from the rotation pole. Surface strength ranges from 2 G up to 20 G. Eddy currents in the fluid, metallic hydrogen core generate it. About 75 Jupiter radii out, the magnetosphere meets the solar wind and forms a bow shock. The solar wind stretches the field on Jupiter's lee side until it nearly reaches Saturn's orbit. The four largest moons orbit within this shield, protected from the solar wind.

  The volcanoes of Io feed the system. They emit vast amounts of sulfur dioxide, forming a gas torus along Io's orbit. Ionized in the magnetosphere, this becomes sulfur and oxygen ions that join hydrogen ions from Jupiter's atmosphere to build a plasma sheet in the equatorial plane. The plasma co-rotates with the planet, bending the dipole field into a magnetodisk.

  Electrons in the plasma sheet broadcast a strong radio signature, short bursts in the range of 0.6 to 30 MHz, detectable from Earth with consumer-grade shortwave receivers. As Io crosses the torus, it generates Alfven waves that carry ionized matter into Jupiter's polar regions. Radio waves then pour out along a cone-shaped surface through a cyclotron maser mechanism. When Earth crosses that cone, the radio emissions from Jupiter can exceed the radio output of the Sun. That hidden broadcast was caught first by Bernard Burke and Kenneth Franklin in 1955, who heard bursts at 22.2 MHz.

• 7. Rings, Moons, And Resonance

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  Jupiter wears a faint ring system of three main segments: an inner torus called the halo, a brighter main ring, and an outer gossamer ring. These rings are made of dust, reddish in visible and near-infrared light, unlike Saturn's icy rings. The main ring likely comes from material ejected off the satellites Adrastea and Metis, drawn in by the planet's gravity. The moons Thebe and Amalthea are thought to feed the two parts of the gossamer ring. A possible fourth ring may be collisional debris from Amalthea, strung along its orbit.

  At least 115 known moons circle the planet, though only 16 exceed 10 km in diameter. The four largest, the Galilean moons, are Ganymede, Callisto, Io, and Europa in order of decreasing size, visible from Earth with binoculars. Ganymede, the largest, is bigger than the planet Mercury. The orbits of Io, Europa, and Ganymede lock into a Laplace resonance: for every four orbits Io makes, Europa makes exactly two and Ganymede exactly one. The repeated tug pulls their orbits into ellipses, while Jupiter's tidal force works to round them out.

  That tug-of-war flexes the moons and heats their interiors. The effect is most violent on Io, which faces the strongest tidal forces and erupts in volcanoes. It shows more gently in Europa, whose young surface points to recent resurfacing.

  Beyond the Galileans, the moons fall into groups by shared orbital traits, complicated by many small outer moons found since 1999. Two, Themisto and Valetudo, belong to no group at all. The eight innermost regular moons, on near-circular equatorial orbits, likely formed alongside Jupiter. The rest are irregular, thought to be captured asteroids or their fragments, gathered into clusters like the Himalia, Ananke, Carme, and Pasiphae groups.

• 8. The Vacuum Cleaner Of The Solar System

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  Jupiter has been called the Solar System's vacuum cleaner, for its immense gravity well near the inner system. More objects strike it than any other planet, about 200 times more asteroid and comet impacts than Earth. Whether this protects the inner planets remains contested. Computer simulations in 2008 suggested Jupiter does not lower the net number of comets passing inward, since its gravity nudges them inward roughly as often as it ejects them. Some argue it draws comets toward Earth from the Kuiper belt, others that it guards Earth from the Oort cloud.

  In July 1994, Comet Shoemaker-Levy 9 collided with Jupiter. Observatories around the world watched, including the Hubble Space Telescope and the Galileo spacecraft, and the media covered it widely. The first spacecraft to orbit Jupiter, Galileo, reached the planet on the 7th of December 1995 and stayed over seven years. A 340-kilogram titanium probe plunged into the atmosphere, parachuting through 150 km at about 2,575 kph and gathering data for 57.6 minutes before destruction. It recorded temperatures above 300 degrees and winds over 644 km/h. To avoid contaminating Europa, NASA steered the orbiter into Jupiter on the 21st of September 2003.

  NASA's Juno mission arrived on the 4th of July 2016 to study the planet from a polar orbit. On the 27th of August 2016 it sent back the first-ever images of Jupiter's north pole. Extended through September 2025, its plan includes flybys of Ganymede, Europa, and Io before a controlled deorbit into the atmosphere. The exploration continues outward. The European Space Agency's Jupiter Icy Moon Explorer launched on the 14th of April 2023, and NASA's Europa Clipper followed on the 14th of October 2024, both bound for the icy moons that may hide subsurface oceans.