Jupiter

Jupiter is the dominant planet of the solar system. It has more mass than all the other planets combined and contains about one 1047th of the Sun's mass. Through a telescope it shows the traits of a giant planet: there is no solid visible surface, only a deep gaseous atmosphere dominated by hydrogen and helium. Its composition resembles the Sun's, but Jupiter is not massive enough to ignite nuclear fusion.

Infrared observations show that Jupiter emits more energy than it receives from sunlight. The usual explanation is slow gravitational contraction: a tiny shrinkage of the planet, on the order of a millimetre per year, can release enough internal heat to account for the excess radiation.

Jupiter is the easiest planet for Earth-based observers to study in detail. Its day lasts less than ten Earth hours, producing a noticeable flattening and driving complex cloud bands that circle the planet at different speeds. The Great Red Spot is the best-known feature, a long-lived storm that has changed size and color over the centuries. Jupiter's moons also create visible events, including transits, shadows, eclipses, and occultations such as Io disappearing behind the planet.

In July 1994 Jupiter became the scene of a major impact event when fragments of comet Shoemaker-Levy 9 struck the planet after being torn apart by Jupiter's gravity. The impacts left dark marks and strong radiometric signatures that remained observable for months. Jupiter orbits beyond the asteroid belt and is often the brightest planet in the night sky; Venus is usually confined to twilight, and Mars exceeds Jupiter's brightness only briefly near favorable oppositions.