{"id":94308,"date":"2026-07-23T01:15:55","date_gmt":"2026-07-23T04:15:55","guid":{"rendered":"https:\/\/service.codeus.ca\/index.php\/2026\/07\/23\/atlantic-skies-how-the-moon-charon-dethroned-pluto-as-a-planet\/"},"modified":"2026-07-23T01:15:55","modified_gmt":"2026-07-23T04:15:55","slug":"atlantic-skies-how-the-moon-charon-dethroned-pluto-as-a-planet","status":"publish","type":"post","link":"https:\/\/service.codeus.ca\/index.php\/2026\/07\/23\/atlantic-skies-how-the-moon-charon-dethroned-pluto-as-a-planet\/","title":{"rendered":"ATLANTIC SKIES: How the moon Charon dethroned Pluto as a planet"},"content":{"rendered":"<div>\n<p>Pluto\u2019s moon Charon was discovered in 1978, and ultimately led to the decision more than 30 years later that Pluto wasn\u2019t a planetPublished Jul 22, 2026Last updated 12hours ago8 minute readEnhanced color global view of Pluto, taken when NASA\u2019s New Horizons spacecraft was 450,000 kilometres away. NASAArticle contentMuch to the dismay of many professional and amateur astronomers, the 1978 discovery of Pluto\u2019s largest moon Charon ultimately led to Pluto being demoted from the status of a planet to that of a dwarf planet by the International Astronomical Union (IAU) in 2006.THIS CONTENT IS RESERVED FOR SUBSCRIBERS ONLY.Subscribe now to access this story and more:Unlimited access to the website and appExclusive access to premium content, newsletters and podcastsFull access to the e-Edition app, an electronic replica of the print edition that you can share, download and comment onEnjoy insights and behind-the-scenes analysis from our award-winning journalistsSupport local journalists and the next generation of journalistsSUBSCRIBE TO UNLOCK MORE ARTICLES.Subscribe or sign in to your account to continue your reading experience.Unlimited access to the website and appExclusive access to premium content, newsletters and podcastsFull access to the e-Edition app, an electronic replica of the print edition that you can share, download and comment onEnjoy insights and behind-the-scenes analysis from our award-winning journalistsSupport local journalists and the next generation of journalistsRegister to unlock more articles.Create an account or sign in to continue your reading experience.Access additional stories every monthShare your thoughts and join the conversation in our commenting communityGet email updates from your favourite authorsSign In or Create an AccountorArticle contentHow was Charon discovered?Article contentArticle contentIn June 1978, American astronomer James Christie, working at the US Naval Observatory (USNO) in Washington, DC, USA, was examining photographic plates of Pluto, taken with the 1.55-meter Kaj Strand Reflector Telescope in Arizona, USA, to refine calculations of the planet\u2019s orbit around the sun.Article contentArticle contentWhile examining the photographic plates, which were not of particularly good quality, Christie noticed a strangely distinct bulge on one side of Pluto. His background in photographing double stars enabled him to deduce that what he was seeing was something unusual. Although the images of Pluto (and the bulge) on the plates were fuzzy, the background stars appeared as perfectly sharp points of light.Article contentResearching photographs of Pluto in the USNO\u2019s archives, Christie discovered the same planetary bulge. This indicated to him that the apparent bulge was not the result of either an improper telescope alignment or atmospheric turbulence. The archive photos also showed that the bulge systematically moved around Pluto over time.Article contentWith his colleague Robert Harrington, Christie tracked the position of the bulge across the archived images of Pluto as far back as 1965, and calculated that the bulge circled Pluto once every 6.4 days, which just happened to match Pluto\u2019s own known rotation period.Article contentArticle contentThis indicated to Christie and Harrington that the bulge was, in fact, an unknown, massive companion moon to Pluto that was locked in a synchronous orbit with Pluto. The newly discovered moon orbiting Pluto at a distance of approximately 19,400 km was provisionally labelled S 1978 P1.Article content A true-colour image of Pluto\u2019s large moon Charon taken from 46,091 miles away by the New Horizons spacecraft. NASA, Johns Hopkins University AArticle contentArticle contentHow did Charon get its name?Article contentAs the discoverer of the new moon, Christie had the right to name it.Article contentHe chose to name it after his wife Charlene, whose nickname happened to be \u201cChar\u201d. Due to his interest in physics, where particles such as protons and electrons end in \u201con\u201d, Christie added this suffix to his wife\u2019s nickname, thereby creating the name \u201cCharon\u201d.Article contentBy a serendipitous coincidence, unknown to Christie at the time, Charon was the name of the ferryman in Greek mythology who transported the souls of the dead across the river Acheron into the underworld, the realm of Pluto, the Greek god of the dead.Article contentThe IAU officially adopted the name of \u201cCharon\u201d on Jan. 3, 1986.Article content Natural-colour view of Pluto and its large moon Charon, compiled from images taken by NASA\u2019s New Horizons spacecraft on July 13 and 14, 2015. NASAArticle contentHow did Charon form?Article contentFor many years, it was thought that Charon formed when a huge Kuiper Belt object slammed into a proto-Pluto in a cataclysmic collision that threw up an enormous cloud of debris into orbit, which eventually coalesced to form Charon.Article contentHowever, a new theory regarding Charon\u2019s formation has been brought forward. It proposes that Pluto and Charon were not molten bodies of rock, but, rather, small rigid bodies of rock and water ice, which would react differently under the stress of a violent collision than would molten bodies.Article contentThe new hypothesis suggests that, rather than a direct collision, it was more a glancing blow than a planetary-shattering one, with the two bodies colliding gently enough that they stuck together and spun as a single huge, snowman-shaped object for several hours before tidal forces finally separated them. However, they didn\u2019t drift away from one another altogether, but remained bound by gravity as a binary system.Article contentThis theory possibly explains how both bodies survived essentially intact, and how much of their individual internal structure and composition was preserved.Article content A high-resolution view of Pluto\u2019s Sputnik Planitia, captured by NASA\u2019s New Horizons spacecraft. The bright expanse is the western lobe of Pluto\u2019s famous \u201cheart,\u201d which is rich in nitrogen, carbon monoxide and methane ices. NASAArticle contentHow did Charon\u2019s discovery result in Pluto\u2019s demotion?Article contentIt all has to do with the IAU\u2019s definition of a planet, plus a number of factors regarding the dynamics of the Pluto-Charon system.Article contentArticle contentThe tracking of Charon\u2019s orbit around Pluto allowed astronomers to accurately calculate the mass of the Pluto-Charon system by applying the three laws of planetary motion formulated by the German astronomer Johannes Kepler (1571-1630).Article contentThe application of these laws ultimately led to the discovery that Pluto was not a small terrestrial-type planet as had been previously theorized, but, instead, a rather lightweight body with a mass of approximately one-sixth that of Earth\u2019s moon.Article contentCharon was found to have approximately 12 per cent of Pluto\u2019s mass, a ratio much larger than any other major moon in the solar system. Our moon is approximately only 1.2 per cent of Earth\u2019s mass.Article contentThe size parity of Pluto and Charon sets them apart from every other planet-moon system in the solar system. The closeness in mass of Pluto and Charon means that their barycentre \u2014 the centre of their combined mass \u2014 lies in the empty space approximately 900 kms above Pluto\u2019s surface.Article contentArticle contentBoth Charon and Pluto always present the same hemispheres towards each other, a phenomenon known as mutual tidal locking.Article contentThe external barycentre and the mutual tidal lock led astronomers to suggest that the Pluto-Charon system should be considered a double planet system, rather than a planet-moon system. In 2006, the IAU considered a formal proposal to classify the Pluto-Charon system a double or binary planet system; however, the proposal was eventually set aside and not adopted.Article contentHowever, the ambiguity of the Pluto-Charon system would eventually result in Pluto being demoted and reclassified as a dwarf planet.Article contentWhat makes an object a planet?Article contentBefore 2006, the definition of what constituted a planet was rooted in traditions that predated the modern understanding of the solar system.Article contentMore and more objects were being discovered beyond the orbit of Neptune \u2014 including Quawa, Sedna, Makemake, and Eris.Article contentThe question for the IAU then was, if Pluto was to remain classified as a planet, should these newly-discovered objects, some of which were more massive, though slightly smaller, than Pluto, also be classified as planets?Article contentIn response to this question, the IAU proposed a new definition, based on three criteria, of what constituted a planet. According to the IAU\u2019s new definition, a planet has to:Article content1. Orbit the Sun (or a star in the case of an exoplanet)Article content2. Have sufficient mass for its own gravity to form it into a roughly round shapeArticle content3. Clear the neighbourhood along its orbital path, excluding any satellites that orbit the planet itself.Article contentUnfortunately for Pluto, it was the third criterion that sealed its fate and its subsequent demotion from planet to dwarf planet. Pluto\u2019s small mass was insufficient enough to clear its orbit of thousands of other large objects, such as the Plutinos, Orcas, and Lempo.Article content A true-colour image of Pluto from 22,025 miles away during the approach of the New Horizons spacecraft. NASA, Johns Hopkins University AArticle contentCould Pluto be classified as a planet again someday?Article contentThere may yet be hope that Pluto will one day be returned to its celestial status as a full planet, despite its strange relationship with Charon.Article contentIf Pluto was demoted due to its failure to meet the requirements of the IAU\u2019s third criterion (failure to clear other objects from its orbital path around the sun), then what about our planet Earth, and the planet Jupiter?Article contentArticle contentEarth currently has approximately 10,000 asteroids orbiting with it around the sun, while our solar system\u2019s fifth planet, Jupiter, has about 100,000 Trojan asteroids along its orbital path. Should Earth and Jupiter therefore not likewise be subject to the IAU\u2019s third criterion, and lose their status as planets?Article contentMany planetary scientists are now proposing taking a body\u2019s geological processes into consideration when attempting to decide whether it is or is not a planet, and that this should, in fact, take precedence over the orbital-clearing requirement.Article contentWe can only hope that the IAU will one day revisit the question of what constitutes a planet, and that someday Pluto might be returned to its rightful place as our solar system\u2019s ninth planet.Article contentThis week\u2019s skyArticle contentMercury (magnitude +3.5, in Gemini \u2013 the Twins) is not readily observable this coming week, as it is no higher than two degrees above the eastern horizon at dawn. Our solar system\u2019s smallest planet will reach its greatest elongation (the apparent angular separation of two bodies, as measured from Earth) west of the sun on Aug. 2 at 10:54 a.m. ADT.Article contentArticle contentVenus (mag. -4.2, in Leo \u2013 the Lion) becomes visible around 9:10 p.m. ADT, 13 degrees above the western horizon as dusk gives way to darkness, before sinking towards the horizon and setting about 10:30 p.m. ADT.Article contentMars (mag. +1.3, in Taurus \u2013 the Bull) rises in the eastern, predawn sky around 2:15 a.m. ADT, reaching 23 degrees above the eastern horizon before fading from view around 4:50 a.m. ADT.Article contentJupiter (mag. -1.8, in Cancer \u2013 the Crab) is not readily observable this week, as it is too close to the sun (less than one degree separation) to be seen. On July 29, at 9:06 a.m. ADT, Jupiter will reach superior solar conjunction. On July 30, Jupiter will reach its farthest point from Earth (apogee) in its orbit around the sun, reaching a distance of 6.3 AU or approximately 937,440,000 km from the sun.Article contentSaturn (mag. +0.5, in Pisces \u2013 the Fish) is visible in the eastern pre-dawn sky, rising around 11:30 p.m. ADT, reaching 45 degrees above the southern horizon before fading from view around 5 a.m. ADT.Article contentUranus (mag. +5.8, in Taurus), having recently passed behind the sun, rises around 1:25 a.m. ADT, reaching 27 degrees above the eastern horizon before fading from view by about 4:15 a.m. ADT.Article contentNeptune (mag. +7.9 in Pisces), visible in the eastern pre-dawn sky, rises shortly after 11 p.m. ADT, reaching 42 degrees above the southern horizon before fading from view around 4:15 a.m. ADT.Article contentThe moon will reach its farthest point from the sun (aphelion) in its combined Earth-Moon orbit around the sun on July 28 at 6:53 a.m. ADT, reaching a distance of 1.018 AU or approximately 151,478,400 km.Article contentThe Full Moon on July 29 was often referre d to as the \u201cBuck Moon\u201d by the Indigenous peoples of eastern and central North America, as July is typically the month when male deer (bucks) begin to grow new antlers to replace those they shed early in the spring.Article contentUntil next week, clear skies.Article contentEvents:Article contentJuly 28 \u2013 Moon at aphelion (farthest point from sun) at 6:53 a.m. ADTArticle contentJuly 29 \u2013 Jupiter at superior solar conjunction at 9:06 a.m. ADT; Full \u201cBuck Moon\u201d at 11:35 a.m. ADTArticle contentJuly 30 \u2013 Jupiter at apogee (farthest point from Earth) at 12:07 p.m. ADTArticle contentAug. 2 \u2013 Mercury at greatest elongation west of the sun at 10:54 a.m. ADTArticle content<\/p>\n<\/p><\/div>\n","protected":false},"excerpt":{"rendered":"<p>Pluto\u2019s moon Charon was discovered in 1978, and ultimately led to the decision more than 30 years later that Pluto wasn\u2019t a planetPublished Jul 22, 2026Last updated 12hours ago8 minute readEnhanced color global view of Pluto, taken when NASA\u2019s New Horizons spacecraft was 450,000 kilometres away. NASAArticle contentMuch to the dismay of many professional and [&hellip;]<\/p>\n","protected":false},"author":207,"featured_media":94309,"comment_status":"closed","ping_status":"closed","sticky":false,"template":"","format":"standard","meta":{"footnotes":""},"categories":[118,590,1],"tags":[116,589],"class_list":["post-94308","post","type-post","status-publish","format-standard","has-post-thumbnail","category-indigenous","category-newfoundland-and-labrador","category-uncategorized","tag-indigenous","tag-newfoundland-and-labrador"],"_links":{"self":[{"href":"https:\/\/service.codeus.ca\/index.php\/wp-json\/wp\/v2\/posts\/94308","targetHints":{"allow":["GET"]}}],"collection":[{"href":"https:\/\/service.codeus.ca\/index.php\/wp-json\/wp\/v2\/posts"}],"about":[{"href":"https:\/\/service.codeus.ca\/index.php\/wp-json\/wp\/v2\/types\/post"}],"author":[{"embeddable":true,"href":"https:\/\/service.codeus.ca\/index.php\/wp-json\/wp\/v2\/users\/207"}],"replies":[{"embeddable":true,"href":"https:\/\/service.codeus.ca\/index.php\/wp-json\/wp\/v2\/comments?post=94308"}],"version-history":[{"count":0,"href":"https:\/\/service.codeus.ca\/index.php\/wp-json\/wp\/v2\/posts\/94308\/revisions"}],"wp:featuredmedia":[{"embeddable":true,"href":"https:\/\/service.codeus.ca\/index.php\/wp-json\/wp\/v2\/media\/94309"}],"wp:attachment":[{"href":"https:\/\/service.codeus.ca\/index.php\/wp-json\/wp\/v2\/media?parent=94308"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"https:\/\/service.codeus.ca\/index.php\/wp-json\/wp\/v2\/categories?post=94308"},{"taxonomy":"post_tag","embeddable":true,"href":"https:\/\/service.codeus.ca\/index.php\/wp-json\/wp\/v2\/tags?post=94308"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}