Planet

Planet (pronounced plan-it)

(1) In astronomy (now also as “major planet), any of the eight large spherical bodies revolving about the sun in elliptical orbits and shining by reflected light: Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, or Neptune, in the order of their proximity to the sun.

(2) A similar body revolving about a star other than the sun.

(3) A celestial body moving in the sky, as distinguished from (the apparently fixed) stars, applied also to the sun and moon (obsolete except in historic reference).

(4) In astrology, the sun, moon, Mercury, Venus, Mars, Jupiter, Saturn, Uranus, Neptune, or Pluto: regarded as sources of energy or consciousness in the interpretation of horoscopes.

1250-1300: From the Middle English planete, from the late Old English planete, from the Old French planete (which endures in Modern French as planète), from the Latin planeta & planetes (found only in plural planētae), from the Ancient Greek πλανήτης (planḗtēs) (wanderer), (ellipsis of πλάνητες ἀστέρες (plánētes astéres (literally “wandering stars”)), from the Ancient Greek πλανάω (planáō) (to wander about, to stray), from planasthai (to wander), of uncertain etymology.  It was cognate with the Latin pālor (to wander about, to stray), the Old Norse flana (to rush about), and the Norwegian flanta (“to wander about”); from here English ultimately gained flaunt.  The source may have been a nasalized form of the primitive Indo-European root pele- (flat; to spread) on the notion of "spread out" but it’s speculative, etymologists noting a similarity of meaning in the Greek plazein (to make devious, repel, dissuade from the right path, bewilder) the evidence simply doesn’t exist to permit a conclusion to be drawn.  Planets were originally so-called because, viewed by the astronomers from Antiquity, they display apparent motion, unlike the stars which seemed “fixed” in space, the word derived from the Ancient Greek phrase plánētes astéres (literally “wandering stars”), ultimately from planasthai (to wander).  Thus the earliest definitions of planets encompassed both the Moon and Sun but not the Earth.  The sense define by modern science of a “world which orbits a star" was first noted in English in the 1630s.  It wasn’t until the Copernican revolution that the Earth was recognized as a planet, and the Sun was seen to be fundamentally different.

The noun planetoid (one of the asteroids, or minor planets, revolving about the sun between the orbits of Mars and Jupiter) is from 1803, the adjectival form planetoidal adopted in 1809.  Strangely, there’s never been an accepted definition of planetoid.  Within astronomical circles, it was initially a synonym for asteroid, "asteroid" referring to the star-like image seen through a telescope while "planetoid" described the object’s planet-like orbit.  In the literature, by the early twentieth century “planetoid” and “asteroid” were both widely used but the latter had prevailed almost wholly by the late 1970s.  This decline in use as a synonym is because it had instead become handy as a word to describe a subset of the larger members of the asteroid community, used to mean “planet-like in form or geology”.  Improvements in observational capacity in the early twenty-first century saw a surge in use as so many more planetary bodies were discovered in the Kuiper belt and beyond.  Within the astronomical community, there was a consensus most were hardly asteroids and concomitant with doubts as to the appropriate definition of "planet", planetoid was the label of choice.

The noun protoplanet (a large, diffuse cloud of matter in the orbit of a young star, regarded as the preliminary state of a planet) dates from 1949, the construct being proto- + planet.  The proto- prefix was a learned borrowing from the Ancient Greek πρωτο- (prōto-), a combination form of πρῶτος (prôtos) (first), superlative of πρό (pró) (before).  The adjective planetary (of or pertaining to a planet) was from the 1590s, probably influenced by the Late Latin planetarius (pertaining to a planet or planets) although the Oxford English Dictionary (OED) notes the only attested meaning as a noun was "an astrologer".  The planetary nebula, so-called because of the shape as seen through a telescope, is from 1785.  1690s The adjective interplanetary (existing between planets) was used in the sense of "travel between planets” as early as 1897 although the English philosopher and physician John Locke (1632–1704) used intermundane in the same sense, the Roman Epicureans having had intermundia (neuter plural) for "spaces between the worlds", translating from the Ancient Greek metakosmia.  Mundane was from the Middle English mondeyne, from the Old French mondain, from the Late Latin mundanus, from the Classical Latin mundus (world).  The noun planetarium (orrery, astronomical machine which by the movements of its parts represents the motions and orbits of the planets) dates from 1734 and was a creation of Modern Latin, the construct being the Late Latin planeta + Latin -arium (a place for).  The modern meaning "device for projecting the night sky onto the interior of a dome" describe the device developed by the optical engineering company Zeiss in Germany; it was first demonstrated in Munich in 1923, the word planetarium adopted in English in 1929.

#plutoisaplanet: Pluto photographed on 14 July 2015 by the New Horizons interplanetary space probe, launched by NASA in 2006.

The Galileian satellites of Jupiter were initially called satellite planets but were later reclassified along with the Moon.  The first observed asteroids were also considered planets, but were reclassified when became apparent how many there were, crossing each other's orbits, in a zone where only a single planet had been expected.   Pluto was found where an outer planet had been expected, but doubts were soon raised about its status because (1) it was found to cross Neptune's orbit and (2) was much smaller than the expectation had suggested.  The debate about the status of Pluto went on for decades after its discovery in 1930.  The pro-planet faction may have become complacent, thinking that because Pluto had always been a planet, it would forever be thus but, after seventy-six years in the textbooks as a planet, the International Astronomical Union (IAU) in 2006 voted to re-classify Pluto as a dwarf planet on the basis that the icy orb failed to meet a set of criteria which the IAU claimed had been accepted for decades.

To be a planet, the IAU noted, the body must (1) orbit a sun, (2) be sufficiently massive to it pull itself into a sphere under its own gravity and (3), "clear its neighborhood" of debris and other celestial bodies, proving it has gravitational dominance in its corner of the solar system.  Pluto fails the third test.  Because it orbits in the Kuiper Belt (a massive ring of asteroids and planetoids that stretches beyond the orbit of Neptune), Pluto is surrounded by thousands of other celestial bodies and chunks of debris, each exerting its own gravity.  Pluto is thus not the gravitationally dominant object in its neighborhood and therefore, not a planet and but a dwarf, a sort of better class of asteroid.  The IAU’s action had been prompted by the discovery in the Kuiper Belt of a body larger than Pluto yet still not meeting the criteria for planethood.  Feeling the need to draw a line in the cosmos, the IAU dumped Pluto.

Lindsay Lohan in Planet Fitness commercial played during Super Bowl 2022.

However #plutoisaplanet is a thing and Pluto’s supporters have a website, arguing that while it’s universally accepted a planet should be spherical and orbit the Sun, the "clearing the neighborhood" rule is arbitrary, having appears only in a single paper published in 1801.  The history is certainly muddied, Galileo having described the moons of Jupiter as planets and there are plenty of other more recent precedents to suggest the definitional consensus has bounced around a bit and there are even extremists really to accept the implications of loosening the rules such as the moons of Earth, Jupiter and Saturn becoming planets.  Most however just want Pluto restored.

The most compelling argument however is probably just that the IAU are a bunch of humorless cosmic clerks, something like the Vogons ("...not actually evil, but bad-tempered, bureaucratic, officious and callous.") in Douglas Adams’ (1952–2001) Hitchhiker's Guide to the Galaxy and that Pluto should be restored to planethood because of the romance.  Although lacking the lovely rings of Saturn, Pluto is the most charming of all because it’s so far away; desolate, lonely and cold, it's the solar system’s emo.  If for no other reason, it should be a planet in tribute to the scientists who, for decades during the late nineteenth and early twentieth centuries, calculated possible positions and hunted for the elusive orb.  In an example of Donald Rumsfeld's (1932–2021; US secretary of defense 1975-1977 & 2001-2006) “unknown knowns”, the proof was actually obtained as early as 1915 but it wasn’t until 1930 that was realized.  In an indication of just how far away Pluto lies, since the 1840s when equations based on Newtonian mechanics were first used to predict the position of the then “undiscovered” planet, it has yet to complete even one orbit of the Sun, one Plutonian year being 247.68 years long.

The name Pluto is from the Roman god of the underworld, from the Classical Latin Pluto & Pluton, from the Ancient Greek Πλούτων (Ploútōn) (god of wealth) from ploutos (wealth; riches (probably originally "overflowing" from the primitive Indo-European pleu- (to flow).  It was the alternative Greek name or epithet of Hades in his function as the god of wealth (precious metals and gems, coming from beneath the earth, form part of his realm).