{"id":28654,"date":"2023-06-12T08:40:00","date_gmt":"2023-06-12T06:40:00","guid":{"rendered":"https:\/\/www.sciencefocus.com\/?p=101484"},"modified":"2023-06-12T09:38:28","modified_gmt":"2023-06-12T07:38:28","slug":"pluto-should-be-our-ninth-planet-a-planetary-scientist-explains-why","status":"publish","type":"rss_feed","link":"https:\/\/c01.purpledshub.com\/bbcsciencefocus\/rss_feed\/pluto-should-be-our-ninth-planet-a-planetary-scientist-explains-why\/","title":{"rendered":"Pluto should be our ninth planet. A planetary scientist explains why"},"content":{"rendered":"

Astronomers believe they\u2019re closing in on the so-called Planet Nine, but planetary scientist Paul Byrne argues our official definition of what is and isn\u2019t a planet is in need of a long-overdue shake up. <\/p>

By Paul Byrne\n \t\t<\/p>

Published: Monday, 12 June 2023 at 12:00 am<\/p>

\n\n

There are some topics that elicit strong opinions from people. Is Die Hard<\/em> a Christmas movie? (Yes.) Does pineapple belong on pizza? (Also yes.) Is Pluto a planet?<\/p>\n

I say that it is. But this isn\u2019t a view that\u2019s universally held.<\/p>\n

Those of us born in the latter part of the last century grew up learning that there are nine planets \u2013 Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, Neptune, and Pluto, in increasing distance from the Sun \u2013 and we had clever mnemonics to help.<\/p>\n

But in 2006, the International Astronomical Union, the entity charged with (among other things) deciding what to call stuff in space, held a vote that overnight reduced the number of planets in the Solar System from nine to eight.<\/p>\n

From its discovery in 1930 until the 1990s, Pluto was the largest known object in a distant region of the Solar System that came to be called the Kuiper Belt. Soon after Pluto was detected, astronomers conjectured that other, similarly sized objects might lurk out there in the distant reaches of our planetary system.<\/p>\n

But the first confirmed detection of another Kuiper Belt object wasn\u2019t until 1992, with the discovery of a body eventually called 15760 Albion. Since then, more than 2,000 bodies have been identified in this part of space, with the true quantity of worlds greater than 100km across perhaps numbering in the hundreds of thousands.<\/p>\n

Read more about Pluto:<\/strong><\/p>\n

Messages from the edge of the Solar System: What New Horizons is still revealing about Pluto and beyond<\/a><\/li>\n
Why Pluto may have a large ocean beneath its icy surface<\/a><\/li>\n
See every New Horizons picture of Pluto in one image<\/a><\/li>\n<\/ul>
And herein lies the core of the problem with Pluto\u2019s classification as a planet: if Pluto were to retain its status as a planet, then so too would everything else we find out in the Kuiper Belt \u2013and before we know it, we\u2019d have hundreds of planets! Crazy, right?<\/p>\n
(Never mind that we have hundreds of countries, thousands of languages, and 8.7 million<\/em> known species of animal.)<\/p>\n
So, to avoiding cluttering up children\u2019s bedroom walls with unreasonably huge posters, or something, the IAU held a vote at its General Assembly in Prague in August 2006 at which it was decreed that a planet must meet three criteria:<\/p>\n
that it orbits the Sun (so sorry, planets ejected from the Solar System or in orbit of other stars, you\u2019re out);<\/li>\n
that is has attained hydrostatic equilibrium (that is, its gravity has pulled it into a spherical shape, or close enough); and<\/li>\n
that it has \u201ccleared its neighbourhood\u201d.<\/li>\n<\/ol>
Whereupon Pluto stopped being a \u201cclassical planet\u201d and started being a \u201cdwarf planet\u201d.<\/p>\n
That last criterion states that a planet must be the gravitationally dominant object in the area of space in which it orbits. This rule makes sense for somewhere like, say, Earth, which is far more massive that the Moon and anything else along its orbital path. But out in the Kuiper Belt, where neighbouring bodies are far, far more distant than in the inner Solar System, Earth would not necessarily be able to clear its neighbourhood.<\/p>\n
In fact, if we were somehow able to tractor-beam Earth out past Neptune, our world could become gravitationally dominated by that icy giant and thus lose its own planethood.<\/p>\n
It\u2019s this idea of \u201cneighbourhood clearing\u201d that\u2019s the crux of the problem regarding the IAU\u2019s definition of a planet. And that\u2019s because it\u2019s a dynamic criterion: it\u2019s a function of where a body is in space, and pays no heed to the character<\/em> of a body beyond the fact that it\u2019s big enough to be a ball. This criterion does not allow for the geology<\/em> of a world to be considered.<\/p>\n
This argument predates the flyby of Pluto in July 2015 of NASA\u2019s New Horizons spacecraft, but the images returned by that spacecraft really help make the case: Pluto is an enigmatic world with towering ice mountains, vast glaciers of nitrogen ice, a tenuous atmosphere, a thick, outer icy carapace and a probably liquid water ocean below, all atop a huge rocky interior. By any geological measure \u2013 including the fact that there are surface processes acting on Pluto today \u2013 <\/em>Pluto is a planet.<\/p>\n
But it\u2019s the International Astronomical<\/em> Union, not the International Geophysical<\/em> Union. And the people who voted on the new planet classification were overwhelmingly astronomers, even if some proportion (most?) were planetary astronomers.<\/p>\n
What properties matter to some scientists \u2013 where an object is, how massive it is, even what its orbit looks like \u2013 might be far less important to others. And to those of us who study the surfaces and interiors of bodies across the Solar System, neighbourhood clearing just doesn\u2019t come close to being important in how we regard them.<\/p>\n
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