Jupiter

• Overall properties: The name comes from the king of the gods, Zeus / Jupiter; The planet's radius is 11.2 Earth radii, with a rotation period 10 hr, and the radius of its orbit is 5.20 AU [with a period 11.9 years]; It is more massive than all other planets put together.
• Missions: Flybys by Pioneer 10 and 11 in 1973-1974, and by Voyager 1 and 2 in 1979; Orbited by Galileo in 1995-2003 (then plunged!); The Juno spacecraft is currently on its way; Future ones will focus on its moons.
• Clouds: It is covered by circulating, global colored cloud bands, with turbulent changing patterns and a Great Red Spot (a thunderstorm larger than Earth, due to a changing high pressure region, at least 300 years old) and many smaller spots; Powered by internal heat and the Sun.
• Moons: It has the most complex system of moons known, with moons very different from each other, like a miniature solar system: Four Galilean moons, Io (volcanos), Europa (ice; water?), Ganymede (the largest in the Solar System, icy), and Callisto (similar); a total of 67 known, some as small as 1 km across, some retrograde, probably captured asteroids.
• Rings: Thin, made of rock fragments and dust, located inside the moons' orbits.

Saturn

• Overall properties: The name comes from the Greek god Cronos, son of Uranus and Gaia; A Titan, ruler of the world until he was dethroned by his own son Zeus; Its radius is 9.4 Earth radii, with a rotation period 10 hr; The least dense and most flattened planet; It is covered by thick turbulent cloud belts, less colorful than Jupiter; It orbits 9.45 AU from the Sun, with a 29.5-yr period.
• Observation: Galileo saw the rings and didn't understand, Huygens figured them out in 1659, Cassini studied them extensively in 1675. Pioneer 11 and Voyager 1 and 2 flew by in 1980-1981; Cassini is now in orbit since 2004, and is sending us a lot of images and information.
• Rings: It has the most extended set of rings by far, with a complex relationships with its moons; They are very bright, thin and changing, made of icy particles of all sizes; three main parts plus the Cassini Division, and dimmer ones like the F ring; Located inside the Roche limit, they may be remnants of a shattered body.
• Moons: A very complex set of at least 63; The largest one, Titan, has a thick atmosphere and is being examined for possible life.

  Uranus

• Overall properties: The name comes from the God of the Heavens, father of Cronus-Saturn [+ Cyclopes + Titans]; Almost 20 AU from the Sun, on an almost circular orbit [eccentricity 0.02, period 84 years]; The planet is a bluish gas ball, because of methane in the clouds, four times Earth's size; Differential rotation [faster at the poles, with a 17.2 hr period], tilted about 98° (possibly from a catastrophic impact billions of years ago), so on average the poles get the most sunlight!
• Viewing: It is barely visible to the eye; The first planet recognized in modern times, it had been recorded long ago, and mistaken for a star until William Herschel discovered that it moved [in 1781; at first, he thought it was a comet].
• Exploration: Visited only by Voyager 2 (in 1986), which now is more than 90 AU from the Sun, on its way out of the Solar System.
• Moons: So far, there are 27 known ones; The most special one is Miranda, which must have suffered a catastrophic impact and has 20-km deep cliffs.

Neptune

• Overall properties: The name comes from Neptune-Poseidon, God of the seas; One needs a telescope to see it; Galileo saw it in 1613 and thought it was a star (whose position somehow changed...); It was predicted from perturbations of Uranus' motion, and identified as a planet in 1846. The orbit has a radius of about 30 AU and is almost circular [eccentricity 0.01, period 165 years]; Sometimes actually more distant than Pluto. The planet is also a bluish gas ball, for the same reason as Uranus, about 4 times the Earth's size; But Neptune is a bit more massive, yet smaller!
• Exploration: It has only been visited by Voyager 2 in 1989.
• Surface: It has more cloud features than Uranus, (it is heated from below like Jupiter and Saturn); The Great Dark Spot (seen by Voyager, gone by 1994).
• Moons, in general: So far, there are 13 known ones; The most special moon is Triton, the largest one, on a retrograde orbit, with active ice volcanoes; It has an atmosphere, like Titan (no other cases are known).