Mars, the fourth planet from the Sun, has appeared as a setting in works of fiction since at least the mid-1600s. Trends in the planet's portrayal have largely been influenced by advances in planetary science. It became the most popular celestial object in fiction in the late 1800s, when it became clear that there was no life on the Moon. The predominant genre depicting Mars at the time was utopian fiction. Around the same time, the mistaken belief that there are canals on Mars emerged and made its way into fiction, popularized by Percival Lowell's speculations of an ancient civilization having constructed them. The War of the Worlds, H. G. Wells's novel about an alien invasion of Earth by sinister Martians, was published in 1897 and went on to have a major influence on the science fiction genre.
The planet Venus has been used as a setting in fiction since before the 19th century. Its opaque cloud cover gave science fiction writers free rein to speculate on conditions at its surface—a "cosmic Rorschach test", in the words of science fiction author Stephen L. Gillett. The planet was often depicted as warmer than Earth but still habitable by humans. Depictions of Venus as a lush, verdant paradise, an oceanic planet, or fetid swampland, often inhabited by dinosaur-like beasts or other monsters, became common in early pulp science fiction, particularly between the 1930s and 1950s. Some other stories portrayed it as a desert, or invented more exotic settings. The absence of a common vision resulted in Venus not developing a coherent fictional mythology, in contrast to the image of Mars in fiction.
An extraterrestrial or alien is a lifeform that did not originate on Earth. The word extraterrestrial means "outside Earth". Extraterrestrials are a common theme in modern science-fiction, and also appeared in much earlier works such as the second-century parody True History by Lucian of Samosata.
The Moon has appeared in fiction as a setting since at least classical antiquity. Throughout most of literary history, a significant portion of works depicting lunar voyages has been satirical in nature. From the late 1800s onwards, science fiction has successively focused largely on the themes of life on the Moon, first Moon landings, and lunar colonization.
Fictional depictions of Mercury, the innermost planet of the Solar System, have gone through three distinct phases. Before much was known about the planet, it received scant attention. Later, when it was incorrectly believed that it was tidally locked with the Sun creating a permanent dayside and nightside, stories mainly focused on the conditions of the two sides and the narrow region of permanent twilight between. Since that misconception was dispelled in the 1960s, the planet has again received less attention from fiction writers, and stories have largely concentrated on the harsh environmental conditions that come from the planet's proximity to the Sun.
Terraforming is well represented in contemporary literature, usually in the form of science fiction, as well as in popular culture. While many stories involving interstellar travel feature planets already suited to habitation by humans and supporting their own indigenous life, some authors prefer to address the unlikeliness of such a concept by instead detailing the means by which humans have converted inhospitable worlds to ones capable of supporting life through artificial means.
Jupiter, the largest planet in the Solar System, has appeared in works of fiction across several centuries. The way the planet has been depicted has evolved as more has become known about its composition; it was initially portrayed as being entirely solid, later as having a high-pressure atmosphere with a solid surface underneath, and finally as being entirely gaseous. It was a popular setting during the pulp era of science fiction. Life on the planet has variously been depicted as identical to humans, larger versions of humans, and non-human. Non-human life on Jupiter has been portrayed as primitive in some works and more advanced than humans in others.
Saturn has made appearances in fiction since the 1752 novel Micromégas by Voltaire. In the earliest depictions, it was portrayed as having a solid surface rather than its actual gaseous composition. In many of these works, the planet is inhabited by aliens that are usually portrayed as being more advanced than humans. In modern science fiction, the Saturnian atmosphere sometimes hosts floating settlements. The planet is occasionally visited by humans and its rings are sometimes mined for resources.
Asteroids have appeared in fiction since at least the late 1800s, the first one—Ceres—having been discovered in 1801. They were initially only used infrequently as writers preferred the planets as settings. The once-popular Phaëton hypothesis, which states that the asteroid belt consists of the remnants of the former fifth planet that existed in an orbit between Mars and Jupiter before somehow being destroyed, has been a recurring theme with various explanations for the planet's destruction proposed. This hypothetical former planet is in science fiction often called "Bodia" in reference to Johann Elert Bode, for whom the since-discredited Titius–Bode law that predicts the planet's existence is named.
Neptune has appeared in fiction since shortly after its 1846 discovery, albeit infrequently. It initially made appearances indirectly—e.g. through its inhabitants—rather than as a setting. The earliest stories set on Neptune itself portrayed it as a rocky planet rather than as having its actual gaseous composition; later works rectified this error. Extraterrestrial life on Neptune is uncommon in fiction, though the exceptions have ranged from humanoids to gaseous lifeforms. Neptune's largest moon Triton has also appeared in fiction, especially in the late 20th century onwards.
Fictional planets of the Solar System have been depicted since the 1700s—often but not always corresponding to hypothetical planets that have at one point or another been seriously proposed by real-world astronomers, though commonly persisting in fiction long after the underlying scientific theories have been refuted. Vulcan was a planet hypothesized to exist inside the orbit of Mercury between 1859 and 1915 to explain anomalies in Mercury's orbit until Einstein's theory of general relativity resolved the matter; it continued to appear in fiction as late as the 1960s. Counter-Earth—a planet diametrically opposite Earth in its orbit around the Sun—was originally proposed by the ancient Greek philosopher Philolaus in the fifth century BCE, and has appeared in fiction since at least the late 1800s. It is sometimes depicted as very similar to Earth and other times very different, often used as a vehicle for satire, and frequently inhabited by counterparts of the people of Earth.
Planets outside of the Solar System have appeared in fiction since at least the 1850s, long before the first real ones were discovered in the 1990s. Most of these fictional planets do not differ significantly from the Earth, and serve only as settings for the narrative. The majority host native lifeforms, sometimes with humans integrated into the ecosystems. Fictional planets that are not Earth-like vary in many different ways. They may have significantly stronger or weaker gravity on their surfaces, or have a particularly hot or cold climate. Both desert planets and ocean planets appear, as do planets with unusual chemical conditions. Various peculiar planetary shapes have been depicted, including flattened, cubic, and toroidal. Some fictional planets exist in multiple-star systems where the orbital mechanics can lead to exotic day–night or seasonal cycles, while others do not orbit any star at all. More fancifully, planets are occasionally portrayed as having sentience, though this is less common than stars receiving the same treatment or a planet's lifeforms having a collective consciousness.
Stars outside of the Solar System have been featured as settings in works of fiction since at least the 1600s, though this did not become commonplace until the pulp era of science fiction. Stars themselves are rarely a point of focus in fiction, their most common role being an indirect one as hosts of planetary systems. In stories where stars nevertheless do get specific attention, they play a variety of roles. Their appearance as points of light in the sky is significant in several stories where there are too many, too few, or an unexpected arrangement of them; in fantasy, they often serve as omens. Stars also appear as sources of power, be it the heat and light of their emanating radiation or superpowers. Certain stages of stellar evolution have received particular attention: supernovae, neutron stars, and black holes. Stars being depicted as sentient beings—whether portrayed as supernatural entities, personified in human form, or simply anthropomorphized as having intelligence—is a recurring theme. Real stars occasionally make appearances in science fiction, especially the nearest: the Alpha Centauri system, often portrayed as the destination of the first interstellar voyage. Tau Ceti, a relatively-nearby star regarded as a plausible candidate for harbouring habitable planets, is also popular.
The Tunguska event—an enormous explosion in a remote region of Siberia on 30 June 1908—has appeared in many works of fiction.
Black holes, objects whose gravity is so strong that nothing—including light—can escape them, have been depicted in fiction since at least the pulp era of science fiction, before the term black hole was coined. A common portrayal at the time was of black holes as hazards to spacefarers, a motif that has also recurred in later works. The concept of black holes became popular in science and fiction alike in the 1960s. Authors quickly seized upon the relativistic effect of gravitational time dilation, whereby time passes more slowly closer to a black hole due to its immense gravitational field. Black holes also became a popular means of space travel in science fiction, especially when the notion of wormholes emerged as a relatively plausible way to achieve faster-than-light travel. In this concept, a black hole is connected to its theoretical opposite, a so-called white hole, and as such acts as a gateway to another point in space which might be very distant from the point of entry. More exotically, the point of emergence is occasionally portrayed as another point in time—thus enabling time travel—or even an entirely different universe.
The concepts of space stations and space habitats feature in science fiction. The difference between the two is that habitats are larger and more complex structures intended as permanent homes for substantial populations, but the line between the two is fuzzy with significant overlap and the term space station is sometimes used for both concepts. The first such artificial satellite in fiction was Edward Everett Hale's "The Brick Moon" in 1869, a sphere of bricks 61 meters across accidentally launched into orbit around the Earth with people still onboard.
Immortality is a common theme in fiction. The concept has been depicted since the Epic of Gilgamesh, the oldest known work of fiction. Originally appearing in the domain of mythology, it has later become a recurring element in the genres of horror, science fiction, and fantasy. For most of literary history, the dominant perspective has been that the desire for immortality is misguided, albeit strong; among the posited drawbacks are ennui, loneliness, and social stagnation. This view was challenged in the 20th century by writers such as George Bernard Shaw and Roger Zelazny. Immortality is commonly obtained either from supernatural entities or objects such as the Fountain of Youth or through biological or technological means such as brain transplants.
Comets have appeared in works of fiction since at least the 1830s. They primarily appear in science fiction as literal objects, but also make occasional symbolical appearances in other genres. In keeping with their traditional cultural associations as omens, they often threaten destruction to Earth. This commonly comes in the form of looming impact events, and occasionally through more novel means such as affecting Earth's atmosphere in different ways. In other stories, humans seek out and visit comets for purposes of research or resource extraction. Comets are inhabited by various forms of life ranging from microbes to vampires in different depictions, and are themselves living beings in some stories.
The far future has been used as a setting in many works of science fiction. The far future setting arose in the late 19th century, as earlier writers had little understanding of concepts such as deep time and its implications for the nature of humankind. Classic examples of this genre include works such as H.G. Wells' The Time Machine (1895) or Olaf Stapledon's Last and First Men (1930). Recurring themes include themes such as Utopias, eschatology or the ultimate fate of the universe. Many works also overlap with other genres such as space opera, science fantasy or apocalyptic and post-apocalyptic fiction.
The Sun has appeared as a setting in fiction at least since classical antiquity, but for a long time it received relatively sporadic attention. Many of the early depictions viewed it as an essentially Earth-like and thus potentially habitable body—a once-common belief about celestial objects in general known as the plurality of worlds—and depicted various kinds of solar inhabitants. As more became known about the Sun through advances in astronomy, in particular its temperature, solar inhabitants fell out of favour save for the occasional more exotic alien lifeforms. Instead, many stories focused on the eventual death of the Sun and the havoc it would wreak upon life on Earth. Before it was understood that the Sun is powered by nuclear fusion, the prevailing assumption among writers was that combustion was the source of its heat and light, and it was expected to run out of fuel relatively soon. Even after the true source of the Sun's energy was determined in the 1920s, the dimming or extinction of the Sun remained a recurring theme in disaster stories, with occasional attempts at averting disaster by reigniting the Sun. Another common way for the Sun to cause destruction is by exploding, and other mechanisms such as solar flares also appear on occasion.
