For a long while, relatively little attention was paid in sf to the planets beyond Jupiter. Of them only Saturn was known to the ancients – Uranus was discovered in 1781, Neptune in 1846 and Pluto in 1930 – and it is therefore the only outer planet featured in Athanasius Kircher's and Emanuel Swedenborg's interplanetary tours. Uranus, however, is included in the anonymous Journeys into the Moon, Several Planets and the Sun: History of a Female Somnambulist (1837). The only object beyond Jupiter that has made significant appeal to speculative writers as a possible abode for life is Saturn's major moon Titan, though the fascinating rings have provoked a good deal of interest from interplanetary passers-by. For several decades Pluto came in for a certain amount of special attention as the apparent Ultima Thule of the solar system, although as much – if not more – interest was shown in the possibility of there being a tenth planet even further out. In the late twentieth century and early twenty-first century, the sf thrill of a unique tenth planet was dispersed by the discovery of further Pluto-like objects in what are now termed the Kuiper belt and (further out but partly overlapping) scattered disc regions at the rim of the solar system. Eris, a dwarf planet of the scattered disc which was discovered in 2003, is significantly larger than Pluto. In 2006 the International Astronomical Union defined the term "planet" in a way that relegates Pluto, Eris and the Asteroid Ceres to the new category "dwarf planet", thus making obsolete the traditional sf tally of nine planets.
Saturn was visited, en route to Earth, by Voltaire's tourist from Sirius in Micromégas (1750; 1952), and a Saturnian accompanied him on his sightseeing trip. Sir Humphry Davy's far-ranging traveller describes Saturn and its giant inhabitants in Consolations in Travel (1830). It was one of the major worlds featured in J B Fayette's anonymously published The Experiences of Eon and Eona (1886); and in John Jacob Astor's A Journey in Other Worlds (1894) it is the home of the spirits, who confirm the truth of the theological beliefs of travellers from a future Earth. Roy Rockwood's series of juvenile interplanetary novels extended thus far in By Spaceship to Saturn (1935), but relatively few Pulp-magazine writers followed suit. Arthur K Barnes's Interplanetary Hunter (stories June 1937-Winter 1946 Thrilling Wonder; fixup 1956) ventured beyond Jupiter on two occasions, but Stanley G Weinbaum was the only early pulp writer of any real significance to explore the outer planets, in "Flight on Titan" (January 1935 Astounding; vt "A Man, a Maid, and Saturn's Temptation" in Avon Fantasy Reader, anth 1951), "The Planet of Doubt" (October 1935 Astounding) – one of the rare stories set on Uranus – and "The Red Peri" (November 1935 Astounding), a Space Opera set partly on Pluto. Other pulp stories set in the outer reaches include J M Walsh's "The Vanguard to Neptune" (Spring 1932 Wonder Stories Quarterly), Wallace West's "En Route to Pluto" (August 1936 Astounding), Raymond Z Gallun's "The Raiders of Saturn's Ring" (Fall 1941 Planet Stories) and Murray Leinster's "Pipeline to Pluto" (August 1945 Astounding). One of Stanton A Coblentz's Satires, Into Plutonian Depths (Spring 1931 Wonder Stories Quarterly; 1950), delved there, and Clifford D Simak's Cosmic Engineers (February-April 1939 Astounding; rev 1950) begins near Pluto. By far and away the most significant role allotted to an outer planet in the speculative fiction of the pre-World War Two period was, however, that given to Neptune by Olaf Stapledon in Last and First Men (1930) and Last Men in London (1932): in the very Far Future, the ultimate members of the human race are forced to make a new home there following the expansion of the Sun. Neptune is subjected to gravitic implosion and dramatically employed as a World Ship in Piers Anthony's Macroscope (1969; cut 1972).
In the post-World War Two period the outer planets occasionally featured in more serious speculative fictions. The rings of Saturn play a key part in Isaac Asimov's "The Martian Way" (November 1952 Galaxy), and Asimov returned to the same locale in his juvenile Lucky Starr and the Rings of Saturn (1958) as by Paul French. Another notable juvenile in which Saturn is an abode of life is Philip Latham's Missing Men of Saturn (1953). An offbeat religious cult is engaged in painting the Saturnian "Ring Beta" red in John Varley's "Equinoctial" (in Ascents of Wonder, anth 1977, ed David Gerrold). Saturn's rings also clinch the identification of our long-lost (and often thought mythical) solar system in Isaac Asimov's Foundation and Earth (1986).
Elsewhere, Titan features much more prominently than its parent world. Alan E Nourse's Trouble on Titan (1954) is a juvenile novel about Colonization of the satellite, the climactic scenes of Kurt Vonnegut Jr's The Sirens of Titan (1959) take place there, and Titan is the location of huge Alien machines in Ben Bova's As on a Darkling Plain (1972). A more fully described Titan colony is featured in Arthur C Clarke's Imperial Earth (1976), and it is the home of the strange lifeform that provides the climax of Gregory Benford's and Gordon Eklund's If the Stars are Gods (fixup 1977). An artificial world hidden among the satellites of Saturn is the main locale of John Varley's Gaean trilogy begun with Titan (1979). Stephen Baxter's novel of the immense journey from Earth to Saturn's moon is also called Titan (1997), and Ben Bova's later Tales of the Grand Tour sequence visits Saturn and Titan in novels imaginatively titled Saturn (2003) and Titan (2006). Another moon of Saturn, Iapetus (discovered 1671), is of interest for its possession of a bright and a dark hemisphere: it features in Theodore Sturgeon's "The Comedian's Children" (May 1958 Venture) and – under its alternate spelling Japetus – Arthur C Clarke's 2001: A Space Odyssey (1968).
Pluto figures prominently in Algis Budrys's Man of Earth (1958), and is the destination of the characters in Wilson Tucker's To the Tombaugh Station (1960). It is the setting of Kim Stanley Robinson's mysterious artefact in Icehenge (1984), and the starting-point of the interplanetary tour featured in the same author's The Memory of Whiteness (1985), which zooms past Uranus and Neptune at considerable narrative pace. Neptune's moon Triton is the setting of Margaret St Clair's "The Pillows" (June 1950 Thrilling Wonder) and Samuel R Delany's "ambiguous heterotopia" in Triton (1976). The "Outer Satellites" conduct a war against the Inner Planets in Alfred Bester's Tiger! Tiger! (October 1956-January 1957 Galaxy as "The Stars My Destination"; 1956; rev vt The Stars My Destination 1957; rev 1996), but the reader never gets to visit them; a much more detailed conflict takes place in Cecelia Holland's Floating Worlds (1976), in which the cities of the title float above Saturn and Uranus. Few of those space operas whose action is partly set in the more remote regions of the solar system pause to take in much of the scenery, but notable recent exceptions include Colin Greenland's Take Back Plenty (1990) and Roger McBride Allen's The Ring of Charon (1990), both of which are partly set on Pluto's large moon (or binary twin) Charon.
It was long held in some quarters that a tenth planet is necessary to account for the orbital perturbations of Uranus, even after Neptune and Pluto are taken into account, and sf writers have occasionally dealt with the possibility. The protagonists of John W Campbell Jr's The Planeteers (stories December 1936-October 1938 Thrilling Wonder; coll of linked stories 1966) ultimately make their way there, and it is the setting for Henry Kuttner's "We Guard the Black Planet!" (November 1942 Super Science Stories). In Philip K Dick's Solar Lottery (1955; vt World of Chance) members of an esoteric cult flee Earth in the hope of finding such a world. Edmund Cooper's The Tenth Planet (1973) plants an advanced civilization there. Contrastingly, in Lucifer's Hammer (1977) by Larry Niven and Jerry Pournelle it is a much more remote Gas Giant, whose gravity perturbs the orbit of a comet, deflecting it towards Earth. Perhaps more intriguing than the notion of a 10th planet is speculation about the solar system's diffuse cometary "halo", the Oort cloud. An extravagant sf version of this is developed in The Reefs of Space (1964) by Frederik Pohl and Jack Williamson, which features a particularly imaginative reef life-system. Clarke's Imperial Earth makes much of the possibility of life existing beyond Pluto, and Williamson made further use of the locale in Lifeburst (1984).
More recently, there has been discussion among astronomers of the possibility that the cause of the orbital perturbations among the outer planets might instead be another star a couple of light years away; i.e., that the Sun might be not a singleton star but one element of a widely spaced binary (most stars are multiple rather than solitary), the other component being a dwarf star, a Neutron Star or even a Black Hole. Even a dwarf star would, at such a distance, be insignificant enough in our skies to make identification difficult. Or the cause might be a yet undetected nearby star heading in our direction, as suggested in Asimov's Nemesis (1989). [BS/DRL]
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