With a mass of \(1.898 \times 10^{27}\) kg, over 318 times that of Earth, Jupiter dominates the Solar System through its gravity and orbital influence. Its composition, approximately 70% hydrogen and 20% helium, is similar to that of the Sun, earning it the nickname "failed star." However, the pressure and temperature at its core, although immense, are insufficient to trigger thermonuclear fusion.
| Constituent | Mass proportion | Main location | Remarks |
|---|---|---|---|
| Hydrogen (H₂) | ≈ 71 – 74% | Outer envelope and metallic layer | Main gas of Jupiter; becomes metallic under pressure > 3 Mbar |
| Helium (He) | ≈ 23 – 25% | Atmosphere and inner layers | Depleted in the upper atmosphere due to sedimentation toward the core |
| Heavy elements (O, C, N, Si, Fe, Mg, S, etc.) | ≈ 3 – 6% | Rocky and icy core | Represents 10 to 20 Earth masses; formed before gas accretion |
Source: NASA – Juno Mission (2024), ESA – JUICE Mission.
For a celestial body to initiate hydrogen fusion, it must reach a critical mass of about 75 times that of Jupiter. Otherwise, the internal pressure remains insufficient for protons to overcome their electrostatic repulsion, a necessary condition for the reaction \(\mathrm{H + H \rightarrow He}\). Thus, Jupiter remained a gas giant, with a rocky or icy core of about 10 to 20 Earth masses, covered by a vast envelope of metallic hydrogen. Inside Jupiter, this phase appears at depths of about 15,000 to 20,000 km below the visible surface. Metallic hydrogen plays a crucial role in generating the internal magnetic field, through the dynamo effect. Its partial transition between molecular and metallic states also contributes to the release of gravitational and thermal energy, which powers the planet's infrared radiation.
N.B.:
Metallic hydrogen is an exotic state of matter predicted by Eugene Wigner (1902-1991) and Hillard Bell Huntington (1903-1989) in 1935. Under pressures greater than 3 million atmospheres (≈ 3 Mbar), hydrogen atoms lose their valence electrons, forming a lattice of H⁺ ions immersed in a "free electron gas." This behavior gives hydrogen metallic properties: high electrical conductivity and optical reflectivity comparable to that of a liquid metal.
Jupiter emits about 1.7 times more energy than it receives from the Sun. This energy comes from the slow gravitational contraction of the planet, a phenomenon known as the Kelvin-Helmholtz mechanism. By contracting very slowly, Jupiter converts part of its gravitational energy into internal heat, which it radiates in the infrared.
Jupiter's atmosphere consists of cloud layers arranged in bands parallel to the equator. These bands alternate between light and dark zones, called zones and belts, respectively. Winds reaching over 500 km/h blow permanently there. The famous Great Red Spot is a giant anticyclone with a diameter of 12,000 km (≈12,756 km for Earth), active for over 300 years according to observations by Giovanni Cassini (1625-1712).
The Jovian system now has over 95 natural satellites, with the four largest—Io, Europa, Ganymede, and Callisto—discovered in 1610 by Galileo Galilei (1564-1642). These Galilean moons form a dynamic ensemble that resembles a miniature planetary system. Their observation allowed Galileo to demonstrate that not all celestial bodies orbit the Earth, supporting the validity of the heliocentric model of Copernicus (1473-1543).
| Moon Name | Radius (km) | Average Distance from Jupiter (km) | Main Characteristics | Mythological Character |
|---|---|---|---|---|
| Io | 1,821 | 421,800 | Most volcanic moon in the Solar System, young and sulfurous surface | Nymph loved by Zeus (Jupiter), transformed into a heifer to escape Hera |
| Europa | 1,561 | 671,100 | Smooth icy surface, internal ocean beneath the ice, candidate for microbial life | Phoenician princess abducted by Zeus in the form of a white bull |
| Ganymede | 2,634 | 1,070,400 | Largest moon in the Solar System, has a magnetic field and an internal ocean | Young Trojan prince abducted by Zeus to become the cupbearer of the gods |
| Callisto | 2,410 | 1,882,700 | Ancient and cratered moon, poorly differentiated core, possible deep ocean | Nymph of Artemis seduced by Zeus, transformed into a bear and then a constellation |
| Amalthea | 83 | 181,400 | Irregular reddish moon, very close to Jupiter, intense thermal illumination | Goat that nursed Zeus in his infancy, symbol of abundance |
| Himalia | 85 | 11,480,000 | Irregular moon of the Himalia group, inclined prograde orbit | Nymph who bore three sons to Zeus |
| Elara | 43 | 11,740,000 | Irregular moon, probably a captured fragment | Nymph loved by Zeus, mother of the giant Tityos |
| Pasiphae | 30 | 23,500,000 | Retrograde moon of the Pasiphae group, likely a captured body | Wife of Minos and mother of the Minotaur, daughter of the Sun god Helios |
| Sinope | 19 | 23,860,000 | Small retrograde moon, irregular shape, Pasiphae group | Princess whom Zeus wanted to seduce, but she tricked him to remain a virgin |
| Lysithea | 18 | 11,720,000 | Irregular moon, slightly inclined orbit | Nymph loved by Zeus, mother of Dionysus in some versions |
Beyond these giant moons, many irregular satellites orbit farther away, often captured by gravity. Their complex distribution provides insights into the accretion and planetary migration processes that occurred over 4.5 billion years ago.
Jupiter's colossal mass, equivalent to 318 times that of Earth, gives the planet a fundamental role in the gravitational stability of the solar system. Its immense gravitational field acts as a natural shield for the inner planets, especially Earth, by deflecting or capturing many potentially dangerous comets and asteroids. This phenomenon, called gravitational scattering, alters the dynamics of small bodies from the asteroid belt and the Oort cloud.
Orbital simulations show that without Jupiter, the flux of interplanetary projectiles toward Earth would increase by a factor of 10 to 100, depending on the models (Horner and Jones, 2010). The planet also acts as a dynamic stabilizer: it limits the orbital perturbations of Mars and the main belt by absorbing some of the gravitational resonances.
However, its influence is not solely protective. Some of its Lindblad resonances and mean motion resonances can destabilize asteroids, ejecting them toward the inner system. Thus, Jupiter acts both as a guardian and architect of the solar system, constantly shaping the distribution of planetary orbits and small bodies.
N.B.:
Jupiter's protective role remains debated. While the giant planet reduces the frequency of catastrophic impacts, it can also redirect some objects toward the inner planets. Its overall effect on impact probability strongly depends on dynamic epochs and the distribution of long-period comets.
To understand the fundamental difference between Jupiter and a star like the Sun, it is useful to compare their essential physical parameters.
| Characteristic | Jupiter | Sun | Comment |
|---|---|---|---|
| Mass | \(1.898 \times 10^{27}\) kg | \(1.989 \times 10^{30}\) kg | 75 Jupiters would be needed to reach the minimum mass of a red dwarf star |
| Radius | 71,492 km | 696,340 km | The Sun is almost ten times larger |
| Core temperature | ≈ 20,000 K | ≈ 15,000,000 K | Nuclear fusion requires > 4 × 106 K |
| Energy source | Gravitational contraction | Hydrogen fusion | The Sun converts hydrogen into helium, Jupiter does not |
N.B.:
If the protosolar nebula had been slightly more massive in its outer region, Jupiter's core could have reached the critical threshold of about 13 Jovian masses. However, the gas available in the circumsolar disk, already depleted by the formation of the Sun, did not allow complete gravitational collapse. Jupiter is thus the product of an unfinished star, born from a local reservoir too sparse to ignite.
Phantom Super-Earths: The Lost Worlds of the Solar System 
Perihelion and Aphelion: The Apsides in the Solar System 
Winds of the solar system and giant planets
3D Simulator: Revolutions of the Planets
Mercury: The Planet with Two Faces
Venus: Earth's Infernal Twin
Earth, the Blue Planet: A Fragile Jewel in the Universe
Mars: The Conquests and First Human Steps on the Red Planet
Jupiter: A Failed Star at the Heart of Our System
Saturn: The Harmony of Orbital Resonances
Uranus: The Planet Tipped on the Horizon of the Solar System
Neptune: The Blue Giant of the Solar System
Roche limit
Remarkable characteristics of the planet Mercury
Remarkable characteristics of the planet Venus
Remarkable characteristics of the planet Jupiter
Remarkable characteristics of the planet Saturn
Characteristics of Planets: What Makes Them Unique
Venus, Veiled Planet: A Puzzle in our Solar System
Live on Mars
The seven wonders of the World
Planet 9 is still nowhere in sight
Axial Tilt of Planets: An Unstable Dance Through the Ages
Comparative sizes of planets and stars
The Journey of the Planets Along the Ecliptic: A Cosmic Choreography
Declination and right ascension
Saturn's Rings: A Ring System in Jeopardy