The outer planets are those planets in the Solar System beyond the asteroid belt, and hence refers to the gas giants, which are 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.
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.
This region of space is also occupied by centaurs, various fields of trojans, and many comets.
Pluto was considered to be an outer planet from its discovery in 1930 until its reclassification as a dwarf planet in 2006 (see also Kuiper belt).
Read more about Outer Planets: Exploration, Mission Concepts
Famous quotes containing the words outer and/or planets:
“I know, it must have been my imagination, but it makes me realize how desperately alone the Earth is. Hanging in space like a speck of food floating in the ocean. Sooner or later to be swallowed up by some creature floating by.... Time will tell, Dr. Mason. We can only wait and wonder. Wonder how, wonder when.”
—Tom Graeff. Young astronomer, Teenagers from Outer Space, after just seeing the invading spaceship through his telescope, and dismissing it (1959)
“Why are all these dolls falling out of the sky?
Was there a father?
Or have the planets cut holes in their nets
and let our childhood out,
or are we the dolls themselves,
born but never fed?”
—Anne Sexton (1928–1974)