{"id":228850,"date":"2023-03-01T06:16:56","date_gmt":"2023-03-01T04:16:56","guid":{"rendered":"https:\/\/osr.org\/?p=228850"},"modified":"2023-07-24T14:04:34","modified_gmt":"2023-07-24T12:04:34","slug":"why-is-pluto-not-a-planet","status":"publish","type":"post","link":"https:\/\/osr.org\/en-uk\/blog\/astronomy-uk\/why-is-pluto-not-a-planet\/","title":{"rendered":"Why Is Pluto Not a Planet?"},"content":{"rendered":"

Our technological abilities are constantly increasing, leading to incredible new tools like the James Webb Space Telescope. Because of this, we’re finding more and more planets each year – but it’s not very often that we lose one! Yet in 2006, the outermost planet of our Solar System, diminutive Pluto, was downgraded from a planet to a dwarf planet. But why is Pluto not a planet anymore?<\/p>\n

What Is Pluto?<\/span><\/h2>\n

Pluto<\/a> is a dwarf planet located in the Kuiper Belt<\/a>, a doughnut-shaped ring of icy bodies on the outskirts of our Solar System. With a diameter of about 2,400 km, it’s only 70% the size of the Moon and just 18% of the diameter of Earth<\/a>. Pluto also has a more oval-shaped and tilted orbit<\/a> than the planets of our Solar System. This means that while the dwarf planet is usually further away than all of the official planets, sometimes it’s closer to the Sun than Neptune. It also takes Pluto 248 years to make it all the way around the Sun \u2013 so we still haven’t seen one Plutonian year since its discovery!<\/p>\n

Pluto’s distance from the Sun also makes it a chilly world, with average temperatures of -387 \u00b0C and only 1\/900th of the sunlight we get here on Earth. If you’ve ever wondered what daylight looks like on Pluto, the people at NASA have created a fun tool<\/a> that tells you when the light in your hometown will be the same as the Midday sun on the dwarf planet!<\/p>\n

Pluto has five moons, of which the biggest and most famous is Charon<\/a>. Charon is less than 20,000 km away from Pluto (for comparison, the Moon is almost 385,000 km away from Earth. Charon is also nearly half the size of Pluto, meaning that the two bodies are technically a double-planetary system – the only one of its kind in our Solar System.<\/p>\n

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National Aeronautics and Space Administration<\/a>, CC0, via Wikimedia Commons<\/p><\/figure> National Aeronautics and Space Administration<\/a>, CC0, via Wikimedia Commons<\/p><\/div>\n

When Was Pluto Discovered?<\/span><\/h2>\n

While Clyde Tombaugh<\/a> officially discovered Pluto in 1930, evidence for its existence dates back to the 19th<\/sup> century. French astronomer Urbain Le Verrier<\/a> first predicted the presence and position of Neptune based on discrepancies in the expected orbit of Uranus. Neptune was later discovered by Johan Gottfried Galle<\/a>, almost precisely where Le Verrier had predicted. Following the discovery of Neptune, astronomers still felt that the new planet didn’t fully explain the orbit of Uranus, so searches for more new objects continued.<\/p>\n

Most of the detective work in the discovery of Pluto took place at the Lowell Observatory<\/a> in Flagstaff, Arizona. After Percival Lowell<\/a>, the observatory’s founder, laid much of the groundwork, Clyde Tombaugh picked up the mantle in 1929 after Lowell’s death more than ten years earlier. On February 18th<\/sup> 1930, Tombaugh found photographic evidence confirming Pluto’s existence in photos he had taken a month earlier using the observatory’s telescope.<\/p>\n

Why Is Pluto Not a Planet?<\/span><\/h2>\n

Shortly after the turn of the millennium, astronomers were making exciting discoveries in the Solar System’s outer reaches. The findings of objects such as Sedna<\/a>, Eris<\/a>, and the ringed Quaoar<\/a> were fascinating, but they did beg the question: if Pluto is a planet, shouldn’t these objects be planets too?<\/p>\n

After several debates, in 2006, the International Astronomical Union defined a dwarf planet<\/a> as an object that:<\/p>\n