Outer planets

From top to bottom: Neptune, Uranus, Saturn, and Jupiter to approximate scale and color.

The outer planets are those planets in the Solar System beyond the asteroid belt, which typically refers to these gas giant planets in order of their distance from the Sun:

• 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.

Size comparison of Earth and Saturn

The gas giant 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 either Pluto or Eris.

This region of space is also occupied by centaurs, various fields of trojans, and many comets.

Pluto had been considered as an outer planet from its discovery in 1930 until its reclassification as a dwarf planet in 2006 (see also Kuiper belt).

Exploration

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.

Galileo Probe diagram. The probe successfully entered Jupiter in 1995, returning data. It was carried to the Jovian system by the Galileo spacecraft.

Spacecraft Exploration Summary

Spacecraft	Launch Year	Jupiter	Saturn	Uranus	Neptune	End Year
Pioneer 10	1972	Flyby				2003
Pioneer 11	1973	Flyby	Flyby			1995
Voyager 1	1977	Flyby	Flyby
Voyager 2	1977	Flyby	Flyby	Flyby	Flyby
Galileo	1989	Orbiter				2003
Galileo Probe	1989	Entry				1995
Ulysses	1990	Flyby				2009
Cassini	1997	Flyby	Orbiter
New Horizons	2006	Flyby
Juno	2011	Orbiter

See also

• Solar System
• Moons of Jupiter - Rings of Jupiter - Exploration of Jupiter
• Moons of Saturn - Rings of Saturn - Exploration of Saturn
• Moons of Uranus - Rings of Uranus - Exploration of Uranus
• Moons of Neptune - Rings of Neptune - Exploration of Neptune

Further reading

• Scott W. Benson – Solar Power for Outer Planets Study (2007) – NASA Glenn Research Center
• Trautman and Bethke – Human Outer Planet Exploration (2003) – NASA Langley Research Center and Princeton University

External links

• Outer Planets Assessment Group (NASA)
• Planetary Science Decadal Survey Mission & Technology Studies
• Dwayne Day – Exploration of the Outer Planets – U.S. Centennial of Flight Commission ~2003