Nasa<\/a> administrator, Jared Isaacman, telling a congressional committee in April that he was in favour of a rethink. \u201cI am very much in the camp of Make Pluto a Planet Again,\u201d he said.<\/p>\nThe event that sparked the IAU\u2019s 2006 vote was the discovery of a distant celestial body that would come to be called Eris. Eris was out in the Kuiper Belt, beyond Neptune \u2013 where Pluto also resides. It was roughly the same size as Pluto (less volume, but more mass). If Pluto was a planet, the argument went, then so was Eris. And if Eris wasn\u2019t a planet, how could Pluto continue to be one?<\/p>\n
Philip Metzger, director of the Stephen W Hawking Centre for Microgravity Research and Education. Photograph: Florida Space Institute<\/p>\n
Eris and Pluto were by no means alone in the Kuiper Belt. Other smaller bodies were discovered around the same time, including Haumea, Makemake and Sedna. Were these also planets? There were likely many more similar objects still to be found. How would schoolchildren manage a planetary roster that might number in the hundreds?<\/p>\n
In search of a solution, the IAU came up with a three-point definition of a planet: it must orbit the Sun; it must have sufficient mass to pull itself into a round shape; and it must have \u201ccleared its neighbourhood\u201d \u2013 meaning it must gravitationally dominate the area in which it orbits. Nothing beyond Neptune that we knew about qualified; Pluto was out.<\/p>\n
It was the last point in particular that upset a lot of Pluto advocates, because a body\u2019s ability to clear its neighbourhood depends a lot on the neighbourhood. \u201cOne of the biggest flaws with the IAU definition is that you can have the same object in two different orbits,\u201d says Kornfeld. \u201cIn one, it\u2019s considered a planet, and in the other, it\u2019s not considered a planet.\u201d If the Earth were towed out to Pluto\u2019s position, the planetary scientist Alan Stern argued, it wouldn\u2019t be able to clear its zone either, and would no longer meet the IAU definition of a planet.<\/p>\n
\u201cYou could even have small asteroids, that are too small to be round, clearing an orbit if they\u2019re close to a star,\u201d says Metzger. \u201cIt just goes to show that orbit clearing isn\u2019t really that meaningful of a category.\u201d<\/p>\n
To its detractors, the IAU definition seemed crafted with a specific intent. \u201cA reason was invented to exclude Pluto,\u201d says Kornfeld.<\/p>\n
\u2018A reason was invented to exclude Pluto\u2019 \u2026 the IAU vote on the resolution for planet definition in 2006. Photograph: Michal \u010c\u00ed\u017eek\/AFP\/Getty Images<\/p>\n
Pluto\u2019s demotion created two opposing scientific teams: the so-called geophysical camp, which believes the physical properties of a celestial body should count toward its planethood; and the dynamicists, who are more concerned with the position a body occupies, its surroundings and its direction of travel. You could characterise the split as planetary scientists v astronomers. Many planetary scientists, including Metzger, simply do not use the IAU definition.<\/p>\n
When the New Horizons spacecraft made its flyby of Pluto in 2015, the geophysical camp seized what it saw as an advantage. Pluto definitely had the geophysical makings of a planet, they maintained. It had mountains, evidence of recent geological activity, and water ice hills floating on frozen nitrogen. It might even be able to support life. How could that not be a planet?<\/p>\n
Those agitating for Pluto\u2019s planetary status do not, by and large, recognise any problems stemming from reinstatement. They welcome a definition broad enough to take in hundreds of new planets, as taxing as that might be for children and their wallcharts. \u201cThe more the merrier,\u201d says Paul Byrne, associate professor of earth, environmental and planetary sciences at Washington University in St Louis. \u201cThere are 1,025 official Pok\u00e9mon species, and I\u2019ve never heard a kid say that\u2019s too many.\u201d<\/p>\n
Those against Pluto\u2019s reinstatement say the problem isn\u2019t with a larger number of planets, per se \u2013 but with a primarily geophysical definition that would encompass, among other bodies, planetary moons, including our moon.<\/p>\n
Underdog story \u2026 the surface of Pluto pictured from Nasa\u2019s New Horizons spacecraft. Photograph: AFP\/Getty Images<\/p>\n
\u201cI have no problem with having a larger number of planets,\u201d says the astronomer Mike Brown, over email. \u201cMy argument is simply that we decided long ago that the moon is not a planet and that also precludes Pluto.\u201d Brown is often described as The Man Who Killed Pluto, because he led the team that discovered Eris back in 2005, setting the whole de-planetisation in motion. It\u2019s a title he leans into: he even wrote a book called How I Killed Pluto and Why It Had It Coming.<\/p>\n
Brown often says that the moon is bigger than Pluto; it\u2019s round and has a complex geophysical history, and would certainly meet the criteria for a planet if you excluded any requirement concerning orbit. What he\u2019s saying is: if Pluto\u2019s a planet, so is the moon.<\/p>\n
\u201cI don\u2019t have an issue with calling the moon a planet,\u201d says Byrne. \u201cBecause I can be a brother and a husband at the same time, and it doesn\u2019t require that I be one or the other. And so I don\u2019t see why the moon can\u2019t be a satellite and a planet.\u201d<\/p>\n
For the outsider, this debate can often seem mired in political, even procedural questions, with Plutonians contending that the IAU vote was incorrectly conducted and therefore void. \u201cYou could make the argument that they never officially voted on a planet because they didn\u2019t follow the procedure that was required,\u201d says Metzger.<\/p>\n
Brown denies there was anything political about the decision. \u201cThe only people who complain about the voting procedure were the ones who didn\u2019t like the outcome.\u201d<\/p>\n
So the question remains: why does any of this matter?<\/p>\n
\u201cIt was almost like a coup,\u201d says Kornfeld. \u201cAnd as a result, kids are learning less. They\u2019re learning eight planets, and that\u2019s it.\u201d<\/p>\n
Cosmic controversy \u2026 astronomer Gerard P Kuiper claimed that Pluto is actually one of Neptune\u2019s satellites. Photograph: Bettmann Archive<\/p>\n
For Metzger, the planetary definition excluding Pluto is based on an outmoded notion of the solar system. \u201cThey were creating a reactionary view towards an orderly cosmos, rather than the more modern scientific view where it\u2019s chaotic,\u201d he says. \u201cThe second problem was that I think they misread the room. The public of today is not the public of the 1800s. They\u2019re perfectly happy understanding a dynamic, chaotic cosmos, and we missed the opportunity to teach that.\u201d<\/p>\n
Pluto\u2019s nature has always been elusive. In 1915, the astronomer Percival Lowell published calculations based on peculiarities in the orbits of Uranus and Neptune, predicting the existence of a new, ninth planet, which he called Planet X, with a mass somewhere between Earth\u2019s and Neptune\u2019s. In 1930, the long search for Planet X bore fruit: the new body, discovered at the observatory that bore Lowell\u2019s name, was christened Pluto.<\/p>\n
Gradually, however, it became clear that whatever Pluto was, it wasn\u2019t Planet X. It didn\u2019t seem anywhere near large enough to account for irregularities in Neptune\u2019s orbit. Scientists offered theories to explain why Pluto looked so small, but the truth was, it was small \u2013 even smaller than the moon. Only in 1992 did we learn that Lowell\u2019s calculations were based on an overestimation of Neptune\u2019s mass. There was no Planet X.<\/p>\n
Pluto\u2019s reputation as an underdog, the picked-on kid, may stem from the 2006 vote, but one could argue it was punching above its weight for years. How would they have voted on Pluto\u2019s planetary status if they had known the truth in 1930? And will it take another vote to satisfy Pluto\u2019s defenders?<\/p>\n
\u201cPeople have locked in,\u201d says Metzger. \u201cWe would have a less useful vote today than we did back in the 2006 timeframe. So we don\u2019t want another vote. What we want is for people to recognise that voting was a mistake, and we\u2019re not going to do that any more. We\u2019re going to go back to the scientific method.\u201d<\/p>\n","protected":false},"excerpt":{"rendered":"Twenty years ago, Pluto got demoted. After a vote by the International Astronomical Union (IAU) in August 2006,…\n","protected":false},"author":2,"featured_media":1178429,"comment_status":"","ping_status":"","sticky":false,"template":"","format":"standard","meta":{"footnotes":"","_share_on_mastodon":"0"},"categories":[3844],"tags":[70,413,16,15],"class_list":["post-1178428","post","type-post","status-publish","format-standard","has-post-thumbnail","category-space","tag-science","tag-space","tag-uk","tag-united-kingdom"],"share_on_mastodon":{"url":"https:\/\/pubeurope.com\/@uk\/117188281150967469","error":""},"_links":{"self":[{"href":"https:\/\/www.europesays.com\/uk\/wp-json\/wp\/v2\/posts\/1178428","targetHints":{"allow":["GET"]}}],"collection":[{"href":"https:\/\/www.europesays.com\/uk\/wp-json\/wp\/v2\/posts"}],"about":[{"href":"https:\/\/www.europesays.com\/uk\/wp-json\/wp\/v2\/types\/post"}],"author":[{"embeddable":true,"href":"https:\/\/www.europesays.com\/uk\/wp-json\/wp\/v2\/users\/2"}],"replies":[{"embeddable":true,"href":"https:\/\/www.europesays.com\/uk\/wp-json\/wp\/v2\/comments?post=1178428"}],"version-history":[{"count":0,"href":"https:\/\/www.europesays.com\/uk\/wp-json\/wp\/v2\/posts\/1178428\/revisions"}],"wp:featuredmedia":[{"embeddable":true,"href":"https:\/\/www.europesays.com\/uk\/wp-json\/wp\/v2\/media\/1178429"}],"wp:attachment":[{"href":"https:\/\/www.europesays.com\/uk\/wp-json\/wp\/v2\/media?parent=1178428"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"https:\/\/www.europesays.com\/uk\/wp-json\/wp\/v2\/categories?post=1178428"},{"taxonomy":"post_tag","embeddable":true,"href":"https:\/\/www.europesays.com\/uk\/wp-json\/wp\/v2\/tags?post=1178428"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}