Editing 3063: Planet Definitions

{{comic
| number    = 3063
| date      = March 14, 2025
| title     = Planet Definitions
| image     = planet_definitions_2x.png
| imagesize = 653x1435px
| noexpand  = true
| titletext = Under the 'has cleared its orbital neighborhood' and 'fuses hydrogen into helium' definitions, thanks to human activities Earth technically no longer qualifies as a planet but DOES count as a star.
}}

==Explanation==
This comic addresses the {{w|IAU definition of planet|controversy of whether of Pluto is a planet}} and explores many definitions, most of them humorous and nonsensical, of what a planet could be.
{| class="wikitable sortable" style="margin:auto"
!colspan="2"|Definition !! # of planets !! Explanation
|-
! Traditionalist
| {{w|Pluto}} is a planet 
| align=center | 9 
| In modern times, there was {{w|IAU definition of planet#Background|no formal definition of a "planet"}} prior to 2006.  However, it was generally accepted as a colloquialism that there were nine planets around the {{w|Sun}}, Pluto included. This view started primarily with Pluto's discovery in 1930, based upon that time's scientific consensus that there ought to be another planet to account for peculiarities in the the orbits of the other outer planets. This ties back to [[988: Tradition]] which discusses how events and beliefs that were popular in the 1940s and 1950s are considered "traditional" in the United States.
As more sophisticated methods of mapping the {{w|Solar System}} were developed, and {{w|Eris (dwarf planet)|Eris}} was discovered and found to be even more massive than Pluto (which may not have been as significant as the theories that led to its discovery suggested), it became clear to astronomers that a more standardized definition was needed.

In 2006 the {{w|International Astronomical Union}} (IAU) published their formal redefinition of a "planet" to require a planet to be gravitationally dominant within its orbit, clearing other objects that aren't moons. This disqualified Pluto and Eris, which are now considered "{{w|dwarf planets}}". This has been subject to pushback from nostalgic laypeople dissatisfied with Pluto being "demoted" or otherwise relegated, when schoolchildren and adults alike have 'known' that there are nine planets for the most part of the last century.

Ironically, some of the latest study of the outer Solar System includes the possibility of yet ''another'' {{w|Planet Nine}}, but only time will tell if such an object exists and whether it would cross the IAU's current threshold or even require the threshold itself to be reassessed once more.
|-
! Modern
| Pluto is not a planet 
| align=center | 8 
| When the IAU redefined what a planet is in 2006, Pluto no longer qualified as a planet since it wasn't able to clear its neighborhood around its orbit.
Using the modern, and recently official, definition of a planet, only eight celestial objects qualified: {{w|Mercury (planet)|Mercury}}, {{w|Venus (planet)|Venus}}, {{w|Earth (planet)|Earth}}, {{w|Mars (planet)|Mars}}, {{w|Jupiter (planet)|Jupiter}}, {{w|Saturn (planet)|Saturn}}, {{w|Uranus (planet)|Uranus}} and {{w|Neptune (planet)|Neptune}}.
|-
! Expansive
| Dwarf planets are planets 
| align=center | 17+ 
| This category also includes nine other bodies that aren't dominant within their orbits, including the ones that are considered to have compacted into fully solid bodies {{w|Dwarf planet#Most likely dwarf planets|as defined by Grundy ''et al.''}}: {{w|Ceres (dwarf planet)|Ceres}}, Pluto, Eris, {{w|Makemake}}, {{w|Haumea}}, {{w|Gonggong (dwarf planet)|Gonggong}}, {{w|Quaoar}}, {{w|Orcus (dwarf planet)|Orcus}} and {{w|Sedna (dwarf planet)|Sedna}}.
The basis for this viewpoint is the possible alternative re-evaluation that the IAU could have adopted, in that all newly discovered things ''like'' Pluto (being considered a planet at the time) should therefore be considered a planet. Indeed, Ceres had been observed some time before Pluto and had been called a planet (or a "minor planet") within both scientific and public realms.
|-
! Ultratraditionalist
| Only the classical planets are planets 
| align=center | 5 
| The {{w|classical planets}} are objects found and considered by the Greek astronomers in classical antiquity to be considered planets. Their definition of "planet" considered visible objects that move across the sky relative to the fixed stars, the original word itself being translated as "wanderer".
There are seven classical planets, but this included the Sun and Moon. If one considers only the ones that also fall under either the IAU's definition of a planet (and so ''less'' traditional) or the convention before that, then there would be only five. Being mostly true to the spirit of the historic naming convention, this would be a conservative but 'valid' version of the criterion.

Notably, Earth itself is not considered a planet by these criteria as, from the perspective of anyone who might even consider such things, it is not wandering the heavens. Or even in the night skies at all, but always underfoot.
|-
! Condescending
| Only giant planets are planets; the rest are big {{w|asteroid}}s 
| align=center | 4 
| This definition may refer to the {{w|giant planets}}, planets much larger than the {{w|Earth}}. Only the four outer (IAU-defined) planets fall under this definition. Relegation of anything smaller, including our own planet, is an extreme attitude.
Incidentally, most of the initial [[:Category:Exoplanets|exoplanets]] discovered were, by practical necessity in their detection, also only of the "giant planet" kind.
|-
! Simplistic
| Anything gravitationally round is a planet 
| align=center | 37+ 
| The Wikipedia {{w|list of gravitationally rounded objects of the Solar System}} has thirty-seven objects. It includes the Sun, eight planets, nine dwarf planets, nineteen {{w|Natural satellite|moon}}s, but falls short of also highlighting all of the smallest visible objects (per Universalist, below).
This definition is essentially ''part'' of the actual current definition of a planet, leaving out the main factor that disqualifies Pluto, orbital dominance.
|-
! Grounded
| Only objects a spaceship has landed on are planets 
| align=center | 10 
| This list includes objects in the Solar System that a spacecraft has {{w|List of landings on extraterrestrial bodies|performed a soft landing on}}. The list includes {{w|Venus}}, Earth, {{w|Mars}}, the Moon, {{w|Titan (moon)|Titan}}, the comet {{w|Churyumov-Gerasimenko}} plus the asteroids {{w|433 Eros|Eros}}, {{w|25143 Itokawa|Itokawa}}, {{w|162173 Ryugu|Ryugu}} and {{w|101955 Bennu|Bennu}}.
The justification for this seems to be that we must 'touch' the object before we consider it as worthy of being classified as more than a mere blob (or dot) in space.

It could be argued that Jupiter and Saturn also count, due to the {{w|Galileo (spacecraft)|Galileo}} and {{w|Cassini–Huygens|Cassini}} spacecraft respectively, which plunged into the atmospheres of those planets.
|-
! Regolithic
| Anything covered in dirt and ice and stuff is a planet 
| align=center | Infinite 
| This list excludes the {{w|gas giant}}s and {{w|ice giant}}s. The list would likely include dwarf planets, asteroids, moons and the comets and {{w|Oort cloud#Structure and composition|trillions of other objects}} in the {{w|Oort cloud}}.
This is effectively the opposite of the "condescending" definition: every object in the Solar System except the Sun is included in one definition or the other.
This is also an extension on the "Grounded" classification. In this case we ''could'' meaningfully touch the object, with predominantly atmospheric bodies being not considered so.
|-
! Lunar
| You can't be a planet if you don't have a moon 
| align=center | 12+ 
| Only some objects in the solar system have known moons orbiting them. The value given may be {{w|List of natural satellites|the number of planets and dwarf planets}} that have moons, excluding {{w|Haumea}} for not {{w|hydrostatic equilibrium|being spherical}} despite having moons.
Adopting this definition would suggest that a planetary body is not worthy of the name if it doesn't demonstrably dominate its orbit by having at least one satellite of its own.

If this statement were "You can't be a planet if you don't have a Moon", ''only'' the Earth would qualify.
|-
! Solipsistic
| Earth is the only planet 
| align=center | 1 
| {{w|Solipsism}} is the idea that only one's own mind is sure to exist. Randall extrapolated this idea to mean that only one's own planet that they are standing on is sure to exist.
This relies on a more philosophical and/or semiotic assessment than any scientific one.
|-
! Judgemental
| Only the prettiest ones are planets 
| align=center | 6 
| This list is likely formulated from Randall's own perception of the prettiest planets in the Solar System. Seven objects are highlighted: Earth, Jupiter, one of Jupiter's moons (likely {{w|Europa (moon)|Europa}}, based on [[1547: Solar System Questions]]), Saturn, one of Saturn's moons (possibly Iapetus or Phoebe), {{w|Triton (moon)|Triton}} and Pluto.
The subjectivity of this version of the definition makes it unlikely that a consensus of this form could be established.
|-
! Empiricist
| Only worlds that I, author of this table, have personally seen are planets 
| align=center | 12 
| This list may refer to the celestial objects in the Solar System that have been visible at night for the author, probably going so far as using an optical telescope (which could be a hobbyist one, perhaps Randall's own, or from time granted on a major institutional installation) but not any more indirect method that uses a camera/screen or historic images of any kind. Apparently Randall has seen Uranus, which technically [https://www.skyatnightmagazine.com/advice/skills/how-see-uranus-in-night-sky ''is'' visible to the naked eye] under the very best viewing conditions, but these conditions are rare and it requires knowing exactly where to look. Jupiter's {{w|Galilean moons|four largest moons}} are [https://web.archive.org/web/20201112024151/http://denisdutton.com/jupiter_moons.htm technically visible to the naked eye] but hard to distinguish due to Jupiter's brightness, while Neptune is considered too faint to see even if you know where to look. It appears that Randall has never used a telescope to see Neptune.
As a different form of subjectivity, the value of this grouping's criteria is questionable, but not uncommon in other 'softer' sciences.
|-
! Marine biologist
| Only objects with oceans are planets 
| align=center | 6+ 
| This list includes Earth, {{w|Europa (moon)|Europa}}, {{w|Ganymede (moon)|Ganymede}}, {{w|Callisto (moon)|Callisto}}, Titan and {{w|Enceladus}}. These have had the presence of significant liquid identified from measurements of their magnetic/electric fields, but see the "Maritime" entry.
There is a resemblance, here, to a loose understanding of what a "world" is, i.e., one that possesses various distinct 'terrains' beyond mere dry (and possibly considered featureless) rock. A marine biologist would, of course consider a marine (if not pelagic or bathyspheric) environment to be an essential element of any world.
|-
! Maritime
| Only objects with ''surface'' oceans are planets 
| align=center | 2 
| In the comic, only Earth and Titan are highlighted. Earth is the only body known in the solar system to have liquid water on the surface significant enough to be called an ocean. Titan's cold and dense atmosphere notably maintains surface 'seas' of methane and nitrogen, while other moons (given as additional in the prior item) seem to have their liquid water beneath either whole-surface ice caps or otherwise deep under the surface.
From the narrower point of view of a sailor, for example, there is no benefit in considering water hidden away far beneath the surface, and it might as well not be there. In contrast, it's possible that a well-prepared mariner could sail the strange seas of Titan, as easily as (or easier than) {{w|Dragonfly (Titan space probe)|an aircraft}} might fly through {{what if|30|its skies}}.
|-
! Universalist
| They're all planets 
| align=center | Infinite 
| This list claims that all objects are planets, with all drawn items (also presumably all undrawn/undrawable items) being marked as such, including the Sun. Giving up on any thought of exclusivity, this unconventional view willingly inducts all objects into consideration.
|-
! Existentialist
| What if {{w|outer space|space}} ''itself'' is a planet??? 
| align=center | ''Duude'' 
| This list is different from the list above as it claims that all of space, rather than only the objects existing in space, are planets. The interjection ''Duude'' expresses one's amazement at this 'revelation' and replaces the number count— and is sometimes used to imply [https://www.urbandictionary.com/define.php?term=duuuude the speaker is high] on marijuana or other mind-altering drugs.
The strange stretch of imagination, as prompted by some narcotic or other, abandons all pretense at sensibly sorting everything into "planet" or "not planet", as not only is everything a planet, but so is the nothing ''between'' these titular planets. However, the more serious subject of {{w|black hole cosmology}} holds the view that the observable universe is the interior of a black hole.
|-
! Spiteful
| ''Only'' Pluto is a planet 
| align=center | 1 
| This list is a malicious play on the demotion of Pluto by demoting all other planets except Pluto instead, leaving Pluto as the only planet in the solar system. 
This is the taxonomic equivalent of refusing to play and taking your ball home to spite those who you think don't deserve to enjoy themselves.
|-
! {{w|Star}} (title text)
| Earth is a star 
| align=center | 2 stars 
| In May 1934, Mark Oliphant, Paul Harteck and Ernest Rutherford at the Cavendish Laboratory published an intentional deuterium fusion experiment and made the discovery of both tritium and helium-3. This is widely considered the first experimental demonstration of fusion. Randall considers that this and subsequent human-induced fusion makes Earth fall into the category of a star, and hence not a planet. Also, the IAU definition of a planet requires that the planet has cleared its "orbital neighborhood" of other objects — objects must either be captured as moons or have their orbits disrupted such that they are flung away.
Under this definition, one could humorously argue that recent human activities, launching into space ''new'' non-orbiting objects like the James Webb Space Telescope, technically disqualify Earth from being a planet, as the orbital neighborhood is no longer completely clear. By changing not only the definition, but the term being defined, this drifts yet further from any consensus view on the original question and into a typical punchline absurdity.
|}
The paper "[https://arxiv.org/abs/2110.15285 Moons are planets: Scientific usefulness versus cultural teleology in the taxonomy of planetary science]" reviews the historical (and astrological) history of what is considered to be a planet within the folk taxonomy, lambasts the IAU definition as "rushed" before sorting out vital issues, recognises the history of science recognising moons as planets and suggests a definition of planets of any orbital state based on their unique complexity between a pile of rubble and an undifferentiated orb of fusing hydrogen and helium.

The 2006 redefinition of Pluto as a dwarf planet is a common theme on xkcd, occurring also in [[473: Still Raw]], [[482: Height]], [[1020: Orion Nebula]], [[1093: Forget]], [[1458: Small Moon]], [[1551: Pluto]] and [[1555: Exoplanet Names 2]].

==Transcript==
{{incomplete transcript|Do NOT delete this tag too soon.}}
:[A table with 3 columns, and 17 rows below the the header row, labelled "Definition", "# of planets" and "Solar system".]
:[In each row, the first column has a single word, in bold, then a descriptive sentence. The second column has a digit or other 'value'. The third column is a not-to-scale drawing of the Solar system, featuring the Sun, various 'planetary' bodies and an apparently selective sample of moons and asteroids, as follows: The Sun, Mercury, Venus, Earth with the Moon, Mars with its two moons (Phobos and Deimos), a small selection of some asteroid belt bodies (Ceres in the midst of other, smaller, examples), Jupiter and four of its moons (likely the Galilean moons: Io, Europa, Ganymede and Callisto), a ringed Saturn and usually one of its moons (probably Titan) or two (possibly Enceladus or Iapetus, as required), Uranus and four or five of its moons (likely to be Miranda, Ariel, Umbriel, Titania and Oberon, but one of these (shown upon the face of Uranus) only appears in some iterations of the base image), Neptune and one of its moons (probably Triton), Pluto and one of its moons (Charon), four more plutoid or Kuiper Belt objects (too little context to identify, but possibly Haumea, Makemake, Eris, and maybe Sedna in distance order), the first two of them with distinct moons indicated (entirely dependent upon which main objects they are).]
:[Each row's illustrated solar system has individual combinations of green highlights applied to the otherwise repeated diagram.]
:[Row 1: Definition:] Traditionalist: Pluto is a planet [Number:] 9 [Highlighted: Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, Neptune and Pluto]
:[Row 2: Definition:] Modern: Pluto is not a planet [Number:] 8 [Highlighted: Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus and Neptune]
:[Row 3: Definition:] Expansive: Dwarf planets are planets [Number:] 17+ [Highlighted: Mercury, Venus, Earth, Mars, Ceres (in Asteroid Belt), Jupiter, Saturn, Uranus, Neptune, Pluto and the further main bodies]
:[Row 4: Definition:] Ultratraditionalist: Only the classical planets are planets [Number:] 5 [Highlighted: Mercury, Venus, Mars, Jupiter, Saturn]
:[Row 5: Definition:] Condescending: Only giant planets are planets; the rest are big asteroids. [Number:] 4 [Highlighted: Jupiter, Saturn, Uranus, Neptune]
:[Row 6: Definition:] Simplistic: Anything gravitationally round is a planet [Number:] 37+ [Highlighted: The Sun, Mercury, Venus, Earth, The Moon, Mars, Ceres (without other asteroids), Jupiter + moons, Saturn with Titan, Uranus and its moons, Neptune with its moon, Pluto and the four further dwarf planets, your mom]
:[Row 7: Definition:] Grounded: Only objects a spaceship has landed on are planets [Number:] 10 [Highlighted: Venus, Earth, The Moon, Mars, five (non-Cererian) asteroids and Titan]
:[Row 8: Definition:] Regolithic: Anything covered in dirt and ice and stuff is a planet [Number:] [infinity symbol] [Highlighted: Mercury, Venus, Earth, The Moon, Mars, Ceres with all other asteroids depicted in the Asteroid Belt, the moons of Jupiter, the sole moon! of Saturn, the moons of Uranus, the moon of Neptune, Pluto with  Charon, and all remaining dwarf planets with their moons]
:[Row 9: Definition:] Lunar: You can't be a planet if you don't have a moon [Number:] 12+ [Highlighted: Earth, Mars, Jupiter, Saturn, Uranus, Neptune, Pluto and three of the other dwarf planets in the Kuiper belt, including one with no obviously drawn moon]
:[Row 10: Definition:] Solipsistic: Earth is the only planet [Number:] 1 [Highlighted: The Earth]
:[Row 11: Definition:] Judgemental: Only the prettiest ones are planets [Number:] 6 [Highlighted: The Earth, Jupiter with one of its moons (not identified), Saturn, one of ''two'' Saturnian moons in this image and Pluto]
:[Row 12: Definition:] Empiricist: Only worlds that I, author of this table, have personally seen are planets [Number:] 12 [Highlighted: Mercury, Venus, The Earth, The Moon, Mars, Jupiter with its four moons, Saturn and Uranus]
:[Row 13: Definition:] Marine biologist: Only objects with oceans are planets [Number:] 6+ [Highlighted: The Earth, three Jovian moons, the two illustrated Saturnian moons]
:[Row 14: Definition:] Maritime: Only objects with [next word in italics] surface oceans are planets [Number:] 2 [Highlighted: The Earth and Titan]
:[Row 15: Definition:] Universalist: They're all planets [Number:] [infinity symbol] [Highlighted: All drawn objects, including The Sun and all other objects including all the moons/asteroids]
:[Row 16: Definition:] Existentialist: What if space [next word in italics] itself is a planet??? [Word:] ''Duude'' [Highlighted: The whole third column cell]
:[Row 17: Definition:] Spiteful: [next word in italics] Only Pluto is a planet [Number:] 1 [Highlighted: Pluto]

==Trivia==
*In the [https://www.explainxkcd.com/wiki/images/archive/6/66/20250314195557%21planet_definitions_2x.png original version of the comic], there were two errors that would later be fixed. The "Traditionalist" definition highlighted Neptune's satellite {{w|Triton (moon)|Triton}} instead of Pluto. The images of the Solar System for the "Traditionalist" and "Modern" definitions were swapped, resulting in Pluto being incorrectly highlighted in "Modern" and omitted in "Traditionalist".

*The "Judgemental" definition has seven colored objects instead of the stated six. This mistake has not yet been fixed.

{{comic discussion}}

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