By Drew Lichtenstein, updated March 24, 2022
The Sun is a star, while Jupiter is a planet. The defining trait of a star is that its core reaches temperatures and pressures high enough to ignite nuclear fusion, converting hydrogen into helium and releasing vast amounts of energy. Jupiter, despite being the largest planet in the Solar System, does not reach these conditions; its core is too cool and its mass too small to sustain fusion.
Both the Sun and Jupiter are dominated by hydrogen and helium. The Sun’s core is so hot that hydrogen atoms ionize into free protons and electrons, enabling fusion. Jupiter’s core, on the other hand, consists of liquid metallic hydrogen under immense pressure. While their overall makeup is similar, the Sun’s sheer size gives it a vastly greater gravitational influence.
The Sun’s mass exceeds 99 % of the Solar System’s total mass, allowing it to anchor eight planets, countless comets, and other small bodies in stable orbits. Newton’s law of universal gravitation explains how such a massive object can keep these bodies bound at great distances.
Jupiter’s significant mass creates its own gravitational field, which captures numerous moons. The four largest—Io, Europa, Ganymede, and Callisto—were first observed by Galileo in 1610. A dozen smaller moons have since been discovered, and Voyager 1 revealed a faint ring system encircling the planet.
