Etymology
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planet (n.)
Origin and meaning of planet

late Old English planete, in old astronomy, "star other than a fixed star; star revolving in an orbit," from Old French planete (Modern French planète) and directly from Late Latin planeta, from Greek planētēs, from (asteres) planētai "wandering (stars)," from planasthai "to wander," a word of uncertain etymology.

Perhaps it is from a nasalized form of PIE root *pele- (2) "flat; to spread," on the notion of "spread out," "but the semantics are highly problematic," according to Beekes, who notes the similarity of meaning to Greek plazein "to make devious, repel, dissuade from the right path, bewilder," but adds, "it is hard to think of a formal connection."

So called because they have apparent motion, unlike the "fixed" stars. Originally including also the moon and sun but not the Earth; modern scientific sense of "world that orbits a star" is from 1630s in English. The Greek word is an enlarged form of planes, planetos "who wanders around, wanderer," also "wandering star, planet," in medicine "unstable temperature."

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planetoid (n.)

"one of the asteroids, or minor planets, revolving about the sun between the orbits of Mars and Jupiter," 1803; see planet + -oid. Related: Planetoidal.

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protoplanet (n.)

"large, diffuse cloud of matter in the orbit of a young star, regarded as the preliminary state of a planet," 1949, from proto- + planet.

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planetary (adj.)

1590s, "of or pertaining to a planet;" see planet + -ary. Perhaps from or based on Late Latin planetarius "pertaining to a planet or planets," but according to OED this is attested only as a noun meaning "an astrologer." Planetary nebula, so called for its shape as seen through a telescope, attested from 1785.

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plano- 

alternative form of plani- "flat, level" (based on Latin planus), but an identical word-forming element is used in sciences as a combining form of Greek planos "wandering" (see planet).

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interplanetary (adj.)
1690s, "existing between planets," from inter- "between" + planet + -ary. In reference to travel between planets, attested from 1897. Boyle and Locke both used intermundane in the same sense; the Roman Epicureans had intermundia (neuter plural) for "spaces between the worlds" (translating Greek metakosmia).
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planetarium (n.)

1734, "orrery, astronomical machine which by the movements of its parts represents the motions and orbits of the planets," Modern Latin, from Late Latin planeta (see planet) + Latin -arium "a place for." Sense of "device for projecting the night sky onto the interior of a dome," developed by Zeiss in Germany, debuted in Munich in 1923 and is attested by the name in English from 1929.

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aeroplane (n.)

1866, originally in reference to surfaces such as shell casings of beetle wings, from French aéroplane (1855), from Greek-derived aero- "air" (see air (n.1)) + stem of French planer "to soar," from Latin planus "level, flat" (from PIE root *pele- (2) "flat; to spread").

The word was later extended to the wing of a heavier-than-air flying machine. The use of the word in reference to the machine itself is first attested 1873 and probably is an independent coinage in English. Also see airplane. Ancient Greek had a word aeroplanos, but it meant "wandering in the air," from planos "wandering" (see planet).

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*pele- (2)
*pelə-, Proto-Indo-European root meaning "flat; to spread."

It forms all or part of: airplane; dysplasia; ectoplasm; effleurage; esplanade; explain; explanation; feldspar; field; flaneur; floor; llano; palm (n.1) "flat of the hand;" palm (n.2) "tropical tree;" palmy; piano; pianoforte; plain; plan; planar; Planaria; plane (n.1) "flat surface;" plane (n.3) "tool for smoothing surfaces;" plane (v.2) "soar, glide on motionless wings;" planet; plani-; planisphere; plano-; -plasia; plasma; plasmid; plasm; -plasm; -plast; plaster; plastic; plastid; -plasty; Polack; Poland; Pole; polka; protoplasm; veldt.

It is the hypothetical source of/evidence for its existence is provided by: Greek plassein "to mold," plasma "something molded or created;" Latin planus "flat, level, even, plain, clear;" Lithuanian plonas "thin;" Celtic *lanon "plain;" Old Church Slavonic polje "flat land, field," Russian polyi "open;" Old English feld, Middle Dutch veld "field."
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Venusian (n.)
"(hypothetical) inhabitant of the second planet from the sun," 1866, from Venus + -ian. Middle English had Venerian "one under the influence of the planet Venus; a lover" (late 14c.).
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