Gas giant

A gas giant is a planet made mostly of hydrogen and helium, the two lightest elements, with no solid surface to stand on. Jupiter and Saturn are the Solar System's gas giants; Uranus and Neptune, which are mostly heavier material, are classed separately as ice giants. Gas giants range from about a tenth of Jupiter's mass up to about 13 Jupiter masses, where objects become massive enough to fuse deuterium and are called brown dwarfs. They were the first planets found around Sun-like stars, and they shape the systems they live in: their gravity steers asteroids and comets, and their migration can clear or scatter everything nearby.

What makes a gas giant

A gas giant has roughly the composition of the gas its star formed from. Jupiter's upper atmosphere is about 90% molecular hydrogen and 10% helium by volume, with methane, ammonia, water and hydrogen sulfide as trace gases ^[1]. The heavier elements are enriched relative to the Sun: the Galileo probe, which fell into Jupiter in 1995, measured carbon, nitrogen and sulfur at about three times the solar proportion ^[2]. That enrichment is a clue to how the planet formed, since it means Jupiter swallowed solids as well as gas.

The name is slightly misleading. Only the outer few per cent of a gas giant's radius behaves like a gas. Deeper, the pressure squeezes hydrogen into a dense fluid, and beyond about a million times the pressure at Earth's surface the hydrogen's electrons break free and it conducts electricity like a liquid metal. Currents in this metallic hydrogen generate the planet's magnetic field ^[3].

Gas giants are also oddly uniform in size. Adding mass to a giant squeezes its interior harder, and above about half a Jupiter mass the extra compression nearly cancels the extra material. An old, cool giant of 0.5 Jupiter masses and one of 10 both have radii close to Jupiter's own, and the curve stays nearly flat into the brown dwarf range ^{[4] [5]}. Saturn, at 0.30 Jupiter masses, is on the rising part of the curve at 0.84 Jupiter radii, and its mean density of 0.687 g/cm³ is lower than that of water ^[6]. The main exceptions are the hot Jupiters, many of which are inflated well beyond Jupiter's size by their stars' heat.

Inside a gas giant

NASA's Juno spacecraft, orbiting Jupiter since 2016, has mapped the planet's gravity field closely enough to test models of its interior. The result was unexpected: Jupiter's heavy elements are not packed into a small, dense core but spread through a large "dilute core" that blends into the hydrogen envelope around it ^{[7] [8]}. Saturn has a similar diffuse core, revealed by the way oscillations inside the planet disturb the waves in its rings ^[9].

Helium is separating out of both planets. At the pressures and temperatures deep in a cooling giant, helium stops mixing with metallic hydrogen and condenses into droplets that sink, a process called helium rain. The Galileo probe found Jupiter's upper atmosphere depleted in helium, with a mass fraction of 0.234 against about 0.27 in the gas the Sun formed from ^[10]. Neon, which dissolves into the helium droplets and is carried down with them, is depleted about tenfold, the signature helium rain predicts ^[11]. Saturn, smaller and colder, has taken the process further ^[12].

Image pendingFile:Giant_planet_interior_diagram.svgCutaway of Jupiter and Saturn showing cloud decks, a molecular hydrogen envelope, metallic hydrogen, falling helium droplets and a dilute core that blends outward

Gas giants also shine by their own heat. Jupiter radiates 7.5 watts per square metre from its interior on top of the sunlight it re-emits, so it gives off slightly more than twice the energy it absorbs from the Sun ^[13]. The heat is left over from the planet's formation and is released as the planet slowly cools and contracts; helium rain adds to it. Internal heat drives the convection that stirs the atmosphere and powers the magnetic field.

Clouds, colours and temperature

Rapid rotation (Jupiter turns in 9.93 hours ^[1]) organises a gas giant's weather into bands of alternating east and west winds: the familiar belts and zones, with long-lived storms like the Great Red Spot riding between them. Which clouds form, and so what colour the bands are, depends on temperature. A cold giant like Jupiter has ammonia ice clouds on top; a warmer one would have water clouds; warmer still, no clouds form high in the atmosphere at all; and the hottest have clouds of rock and metal. The Sudarsky classification turns that sequence into five classes.

Astronomers usually sort giant exoplanets by orbital period or equilibrium temperature. Hot Jupiters have periods of a few days; warm Jupiters orbit in roughly 10 to 200 days; cold Jupiters orbit beyond about 1 AU, the region where giants are most common. The sim uses equilibrium temperature for the same split, with three types: cold (below 150 K), temperate (150 to 800 K) and hot (above 800 K).

Formation

Two ideas compete to explain how gas giants form. In core accretion, the favoured model for most giants, a core of rock and ice grows in the disc around a young star until, at about ten Earth masses, its gravity can pull in the surrounding gas faster and faster. The planet must finish before the disc disperses, within a few million years, which favours the region just beyond the snow line, where ice adds to the solid material available ^[14]. In disc instability, a massive, cold disc fragments directly into giant planets under its own gravity, far faster than a core can grow ^[15]; this route may explain some massive giants on very wide orbits.

Giants need not stay where they form. Interactions with the gas disc, with other planets or with a distant companion star can move them inward, and a giant pushed close to its star becomes a hot Jupiter ^[16].

Core accretion predicts that stars richer in heavy elements should form giants more easily, and they do. The chance that a Sun-like star has a giant planet rises roughly with the square of its iron abundance, so a star with twice the Sun's iron is about four times as likely to host one ^[17].

How common are they?

Radial-velocity surveys, which detect the wobble a planet induces in its star, find about 14 giant planets per 100 Sun-like stars with orbits of 2 to 8 AU, and about 9 per 100 at 8 to 32 AU. Giants are about four times more common beyond 1 AU than inside it ^[18]. Hot Jupiters are rare: they orbit about 0.4% of the stars Kepler watched ^[19] and about 1.2% of nearby Sun-like stars in radial-velocity surveys ^[20]. They were found first because they are the easiest planets to detect.

How we know

• Spacecraft have visited all four giants of the Solar System. The Galileo probe sampled Jupiter's atmosphere directly in 1995; Cassini measured Saturn's gravity, rings and heat balance and refined Jupiter's ^[13]; Juno is mapping Jupiter's gravity and magnetic field.
• Radial velocities found the first gas giant around a Sun-like star, 51 Pegasi b, in 1995 ^[21].
• Transits give radii and, combined with radial velocities, densities; HD 209458 b in 1999 was the first planet seen to transit ^[22].
• Spectroscopy during transits and eclipses reads the atmospheres. JWST made the first clear detection of carbon dioxide in an exoplanet, the Saturn-mass WASP-39 b, in 2022 ^[23].
• Direct imaging separates the light of young or wide-orbit giants from their stars' glare ^{[24] [25]}.

Notable examples

See also

• Cold gas giant
• Temperate gas giant
• Hot Jupiter
• Sudarsky classification
• Ice giant
• Brown dwarf
• Planet classification
• Sol

References

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