Jupiter

Jupiter is the largest planet in the solar system, with a diameter of 139,822 km — more than 11 times Earth's. It is a gas giant composed primarily of hydrogen and helium, with no solid surface. Jupiter's mass (1.898 × 10²⁷ kg) is 2.5 times that of all other planets combined. The planet is famous for its vivid cloud bands — alternating light zones and dark belts of ammonia and ammonium hydrosulfide clouds driven by powerful jet streams reaching 360 km/h. The Great Red Spot, a massive anticyclonic storm larger than Earth, has been raging for at least 350 years. Jupiter has 95 known moons, including the four large Galilean moons: Io, Europa, Ganymede, and Callisto.

Jupiter is one of the most rewarding planets for amateur telescopes. Even at low power, the disk shows obvious cloud bands and the four Galilean moons appear as bright points that change position nightly — tracking their dance is endlessly engaging. A 4-inch telescope at 150-200x reveals the two main equatorial belts, the Great Red Spot (when it faces Earth — it rotates into view roughly every 10 hours), and shadow transits of the Galilean moons across Jupiter's face. An 8-inch or larger telescope shows festoons, barges, and turbulence in the belts, plus fine details in the Great Red Spot. A blue filter (#80A) enhances the Great Red Spot, while a green filter (#58) improves belt contrast. Jupiter reaches opposition annually.

Known since prehistory, Jupiter was named after the king of the Roman gods. Galileo's discovery of its four largest moons in January 1610 was revolutionary — they were the first objects found orbiting another planet, providing direct evidence against the geocentric model. Ole Rømer used the timing of Io's eclipses by Jupiter in 1676 to make the first measurement of the speed of light. The Pioneer and Voyager spacecraft flew past in the 1970s, and the Galileo orbiter studied the system from 1995 to 2003. NASA's Juno mission has been orbiting Jupiter since 2016, revealing details of its interior structure and polar cyclones.