The exploration of other worlds is one of the most enduring themes of science fiction.
During the first decades of science fiction, Mars was the most common planet and the most romanticized of our solar system whose surface conditions seemed closest to being amenable to life. Percival Lowell's idea about canals of Mars was taken at face value then.
Currently Mars is depicted mainly as a target of terraforming. See Mars in fiction for more details on the red planet's numerous roles.
During the early-to-mid 20th century, Venus was also a popular subject. Venus is very similar to Earth in its size and surface gravity, and its surface is hidden by a thick cloud layer. Venus was usually depicted as a warm, wet, jungle- and marsh-covered world where life was plentiful, with often thinly-veiled allegories of the European colonization of Africa. Venus is in fact an inhospitable world — the clouds are sulfuric acid, the atmosphere is hundreds of times thicker than Earth's, and the surface temperature could melt lead. See Venus in fiction for more details and particular works.
Authors have created thousands of fictional planets.
Most of them are nearly indistinguishable from Earth, which is why Brian M. Stableford calls them "Earth-Clones".
In these, differences with Earth life are mostly social (like Barrayar in the science fiction of Lois McMaster Bujold).
More physically unusual planets have been in the hard science fiction books.
Unusual social environment
Typical examples are prison planets, primitive cultures, political or religious extremes and pseudo-medieval societies.
Pern — Anne McCaffrey's Dragonriders of Pern series (people ride genetically-engineered dragons)
Qom-Riyadh — Dan Simmons's Hyperion Cantos (Moslem)
Rimmerworld Arnold Rimmer of Red Dwarf spends 600 years on a planet by himself. He creates clones of himself (originally to make a girlfriend). The planet is populated by millions of clones who eventually imprison the original Rimmer.
Shora — Joan Slonczewski 's A Door into Ocean (waterbound culture)
Solaria — Isaac Asimov's Robot series. People grow up isolated, and eventually lead totally solitary lives, doing all their interactive via telepresence.
Ishtar — Poul Anderson's Fire Time (periods of intense heat)
Kashyyyk — A forest world caused by a terraforming accident where gigantic trees and furry, sentient Wookiees to maintain them evolved at an accelerated pace, Star Wars (particularly Knights of the Old Republic)
Kithrup — David Brin's Startide Rising (waterworld rich in heavy metals, which form part of the biochemical structure of its life. Mildly toxic to non-native life. also the "retirement" home of a neurotic race with enormous psi power)
Star One. A star with a single planet holding the Federation's main computers in Blakes Seven, situated between our galaxy and the Andromeda galaxy. Planet destroyed in an intergalactic war.
Pern — Anne McCaffrey's Dragonriders of Pern. Deadly spore capable of eating ANYTHING (except rock and metal) rains down on planet for fifty years every 200-400 years.
Zeelich, a planet in Little Big Adventure 2. It is covered by a thick layer of gas clouds and beneath lies a sea of lava. Vegetation and civilisation is current only on mountains above the cloud layer.
Kosmos — A planet in the Marvel Universe from which a criminal sludge-like alien escapes to hide on Earth where he kills The Wasp's father and fight Ant-Man
Lar Metaal — Planet which shifts location in space every 1,000 years. Homeworld of Queen Promethium, Maetel and possibly Emeraldas — Galaxy Express 999, Queen Millenia , Maetel Legend
Legis XV — location of Scott Westerfeld 's Succession Series
Nihil, Latin word for 'nothing', additional planet of Earth's solar system in the novel Beyond the Spectrum. Due to a flaw in space, the planet is invisible except at close range, although it can see most of the other planets. The inhabitants attempt to conquer Earth during the 30th century.
Qar'To — a planet established in the first season of War of the Worlds to be in the same system as that of the invading aliens (Mor-Tax) and has sent a synth to assassinate the Advocacy
Twinsun from the Little Big Adventure games, a planet which is lighted by two suns (which are fixated). It has three climates: the poles are hot and desertic, the equator is cold and artic (in opposite to planet Earth), and between them lie temperate lands.
