- Jupiter is the largest planet in the Solar System, with distinct bands and four very large satellites.
- Saturn is the second largest planet, with a large and bright ring system.
- Uranus was discovered in 1781. It is tilted almost onto the plane of its orbit.
- Neptune was discovered in 1846 as a result of its perturbations of Uranus.
The outer planets all have ring systems, although all but Saturn's are faint.
Another aspect common to the gas giants is their many natural satellites, two of which are larger than the planet Mercury (Jupiter's Ganymede and Saturn's Titan). That pair and Io, Callisto, Europa, and Triton, are larger than Pluto and Eris.
In situ exploration by spacecraft includes Pioneer 10, Pioneer 11, Voyager 1, Voyager 2, Ulysses, Galileo, Cassini–Huygens, and New Horizons. Planned missions include Juno Jupiter Orbiter and possibly the Outer Planet Flagship Missions; there are various proposals too, such as Uranus orbiter and probe. Ongoing missions for the outer planets as of 2011 include Cassini probe, orbiting Saturn, New Horizons, headed for Pluto, and Juno, headed for Jupiter. Cassini and New Horizons also visited Jupiter with a flyby.
One of the breakthroughs that made exploration of the outer planets much easier, was the concept of the planetary gravity assist. Discovered in the 1960s, a spacecraft approaches a planet like Jupiter in such a way as to be accelerated to a higher speed. This allows a much smaller rocket to be used for a given launch.
Mission concepts 
- Jupiter Europa Orbiter Study
- Europa Flyby Study
- Europa Lander Study
- Exploration of Jovian Atmosphere Using Nuclear Ramjet Flyer
- Io Observer Concept Study
- Ganymede Orbiter Concept Study
- Trojan Tour Concept Study
- Titan Saturn System Mission
- Europa Orbiter
- Europa Flyby
- Europa Lander
- Saturn Atmospheric Entry Probe Study
- Saturn Ring Observer Concept Study
- Enceladus Flyby & Sample Return Concept Studies
- Journey to Enceladus and Titan (JET)
- Life Investigation For Enceladus (LIFE) - flyby sample return for Enceladus and Saturn's E-ring.
- Enceladus Orbiter Concept Study
- Titan Lake Probe Concept Study
- Titan Airplane
- Titan Rover
- Chiron Orbiter Mission Concept Study
- Uranus and Neptune Orbiter and Probe Concept Studies
- Uranus orbiter and probe
- Uranus Pathfinder
- Neptune-Triton-Kuiper Belt Objects Mission Concept Study
- Argo - Jupiter, Saturn, Neptune, Kuiper belt flyby
- Basics of Spaceflight - Chapter 4. Interplanetary Trajectories
- Planetary Science Decadal Survey Mission & Technology Studies
- G. Maise - Exploration of Jovian Atmosphere Using Nuclear Ramjet Flyer
- March 2012 OPAG Meeting
- JET - Cost Capped Titan-Enceladus Proposal
- Tsou, et al - LIFE link courtesy of Future Planetary Exploration
- ARPS Enabled Titan Rover Concept with Inflatable Wheels
- Uranus Pathfinder
- Argo - A Voyage Through the Outer Solar System
Further reading 
- Solar Power for Outer Planets Study (2007) – NASA Glenn Research Center
- Human Outer Planet Exploration (2003) – NASA Langley Research Center and Princeton University
- Future Exploration of the Outer Planet Satellites: A Decadal Perspective (2009)
See also 
- List of missions to the outer planets
- Discovery and exploration of the Solar System
- Jupiter: Moons - Rings - Exploration
- Saturn: Moons - Rings - Exploration
- Uranus:Moons - Rings - Exploration
- Neptune:Moons - Rings - Exploration
- Kuiper belt
- Oort Cloud
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