Saturn

Saturn is the second-largest planet and unmistakably the most photogenic. Its bright ring system — visible in any backyard telescope — spans about 282,000 km across yet is on average just tens of metres thick. Saturn is the least dense major planet, with a mean density of 0.687 g/cm³, lower than water. A gas giant like Jupiter, it has no solid surface, a hydrogen-rich atmosphere, and a turbulent interior. Its moon family of 274 (and still climbing) is the largest in the solar system, including Titan and the icy ocean world Enceladus.

About 96% hydrogen and 3% helium, similar in bulk to Jupiter but at lower density. Internal pressures compress hydrogen into a liquid and eventually a metallic-hydrogen layer surrounding a possibly diffuse rocky-icy core. Like Jupiter, Saturn radiates more energy than it receives — partly from continued gravitational contraction, partly from helium "rain" releasing energy as it sinks.

A hydrogen-helium atmosphere banded into pale yellow and gold belts, less contrasting than Jupiter's. A persistent hexagonal jet stream rings the north pole — a six-sided polar vortex thousands of kilometres across, first imaged by Voyager and revisited by Cassini. Equatorial winds reach roughly 1,800 km/h, among the fastest in the solar system.

Pioneer 11 made the first flyby in 1979, followed by Voyager 1 and 2 in 1980-81. The defining mission was NASA-ESA-ASI's Cassini-Huygens, which orbited from 2004 to 2017, deployed the Huygens probe to land on Titan in 2005, and ended in a planned dive into Saturn's atmosphere. NASA's Dragonfly rotorcraft, launching in 2028, will fly across Titan starting in 2034.