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How did Pluto become a dwarf planet?

24 Aug 2019, 10:00 UTC
How did Pluto become a dwarf planet?
(200 words excerpt, click title or image to see full post)

Pluto backlit, via New Horizons spacecraft on July 14, 2015. Image via NASA/JHUAPL/SwRI.
August 24, 2006. Today is the 13th anniversary of the decision by the International Astronomical Union (IAU) to demote Pluto to dwarf planet status. Our solar system went from having nine major planets to having eight major planets. Pluto, once considered the outermost planet, became more widely known as the largest of a number of small bodies in the outer solar system. Neptune, the eighth large planet out from our sun, is now considered outermost major planet. The IAU formulated a new definition of what it means to be a planet. The IAU’s XXVIth General Assembly formalized the decision and announced it on August 24, 2006. The public and many astronomers didn’t take it lightly, with some declaring they would still consider Pluto a planet. The word plutoed – meaning to demote or devalue something – entered the global lexicon.
Why did it happen? We still get questions about this today. Prior to 2006, astronomers hadn’t gotten around to establishing clear standards – such as a minimum size or mass, or other considerations – by which an object might be categorized as a solar system “planet” ...

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