Why Pluto is No Longer a Planet
Written by
Fraser Cain
This has got to be be one of the most heartbreaking questions I get asked, "Why Isn't
Pluto a Planet". And I get it a lot. I was expecting that a few years after the International Astronomical Union's controversial decision, the debate would have settled down and people would finally accept it. But no, it's still a sore point for many people -
Pluto is not a planet (let that sink in). In this article, I'll explain the events that led up to the decision, the current state of planetary definition, and any hope
Pluto has for the future. Let's find out why Pluto is no longer considered a planet.
Pluto was first discovered in 1930 by Clyde W. Tombaugh at the Lowell Observatory in Flagstaff Arizona. Astronomers had long predicted that there would be a ninth planet in the Solar System, which they called Planet X. Only 22 at the time, Tombaugh was given the laborious task of comparing photographic plates. These were two images of a region of the sky, taken two weeks apart. Any moving object, like an asteroid, comet or planet, would appear to jump from one photograph to the next.
After a year of observations, Tombaugh finally discovered an object in the right orbit, and declared that he had discovered Planet X. Because they had discovered it, the Lowell team were allowed to name it. They settled on Pluto, a name suggested by an 11-year old school girl in Oxford, England (no, it wasn't named after the Disney character, but the Roman god of the underworld).
The Solar System now had 9 planets.
Astronomers weren't sure about Pluto's mass until the discovery of its
largest Moon, Charon, in 1978. And by knowing its mass (0.0021 Earths), they could more accurately gauge its size. The most accurate measurement currently gives the
size of Pluto at 2,400 km (1,500 miles) across. Although this is small,
Mercury is only 4,880 km (3,032 miles) across. Pluto is tiny, but it was considered larger than anything else past the orbit of Neptune.
Over the last few decades, powerful new ground and space-based observatories have completely changed previous understanding of the outer Solar System. Instead of being the only planet in its region, like the rest of the Solar System,
Pluto and its moons are now known to be just a large example of a collection of objects called the Kuiper Belt. This region extends from the orbit of Neptune out to 55 astronomical units (55 times the distance of the
Earth to the Sun).
Astronomers estimate that there are at least 70,000 icy objects, with the same
composition as Pluto, that measure 100 km across or more in the Kuiper Belt. And according to the new rules, Pluto is not a planet. It's just another Kuiper Belt object.
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