Dwarf planet Pluto<\/h3>\n
On August 24, 2006, the International Astronomical Union (IAU) announced it had re-classified Pluto as a dwarf planet. From 1930 until that day, Pluto had been considered a normal planet, and the outermost world of the solar system. As technology advanced, though, astronomers began to see fainter and fainter objects in our neighborhood. That led them to recognize Pluto as only the largest of many small bodies in the outer solar system. <\/p>\n
Now Pluto is known as a dwarf planet, and Neptune \u2013 8th world from the sun \u2013 is considered the outermost major planet. It all came about because the IAU had formulated a new definition of what it means to be a planet.<\/p>\n
The public and even some astronomers didn\u2019t initially take it lightly. Some declared they would still consider Pluto a planet. The word \u2018plutoed\u2018 \u2013 meaning to demote or devalue something \u2013 entered the global lexicon. Nowadays, though, it seems most have accepted the logic of Pluto\u2019s re-classification\u2026 or, at least, we don\u2019t hear as much of an uproar about it as we used to.<\/p>\n
Why did Pluto change status? <\/h3>\n
Prior to 2006, astronomers hadn\u2019t gotten around to establishing clear standards for categorizing a solar system \u201cplanet\u201d versus a \u201cdwarf planet.\u201d <\/p>\n
They began to see a need when many small bodies \u2013 such as Haumea and Makemake \u2013 began to be discovered in the outer solar system. Eris, also considered a dwarf planet, has even more mass than Pluto (though it\u2019s not quite as large). So if Pluto is a planet, why shouldn\u2019t Eris be granted planet status as well? That was the question the IAU asked itself, which led to its formation of a Planet Definition Committee and ultimately the 2006 decision.<\/p>\n
The committee\u00a0had a few possible roads to travel down. One would be to choose a size or mass limit that would make Pluto remain a planet. That would mean that Eris and Ceres \u2013 the largest body in the inner solar system\u2019s asteroid belt \u2013 would become planets, too. This was the solution of some IAU committee members, and it looked like it might be implemented for a while.<\/p>\n
Another option for the IAU might have been to define the concept of a planet without any specific logic: Earth is a planet, Pluto is a planet, but Eris is not just because<\/em>. But would that have been very scientific?<\/p>\n On August 24, 2006, the IAU announced its solution. It decided to create a scientific definition of what it means to be a planet. And that definition excluded Pluto from major planet status. Here\u2019s the definition:<\/p>\n A planet is a celestial body that It\u2019s \u201cc\u201d that causes Pluto to fail as a planet, according to the IAU. For an object to be a major planet, according to this definition, it must be the dominant gravitational object in its orbit. It must either sling\u00a0other objects\u00a0away or\u00a0merge with\u00a0them. <\/p>\n Pluto is only 0.07 times the mass of the objects in its orbit. Meanwhile, Earth is 1.7 million times the mass of the objects in its orbit.<\/p>\n On that fateful day \u2013 August 24, 2006 \u2013 the IAU also created a new category of celestial objects for Pluto and all Pluto-like objects:<\/p>\n A \u201cdwarf planet\u201d is a celestial body that How many dwarf planets are there? According to this page by planetary astronomer Mike Brown of CalTech, as of November 25, 2025, there are:<\/p>\n 10 objects which are nearly certainly dwarf planets,![\"Casually](\"https:\/\/earthsky.org\/upl\/2016\/08\/IAU-planet-definition-committee-e1471861409554.jpg\")
New planetary classifications<\/h3>\n
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(a) is in orbit around the Sun,
(b) has sufficient mass for its self-gravity to overcome rigid body forces so that it assumes a hydrostatic equilibrium (nearly round) shape, and
(c) has cleared the neighborhood around its orbit.<\/p>\n<\/blockquote>\n![\"Round](\"https:\/\/earthsky.org\/upl\/2015\/07\/pluto-heart-enhanced-e1471862884689.jpg\")
Pluto hasn\u2019t cleared its neighborhood<\/h3>\n
\n
(a) is in orbit around the sun,
(b) has sufficient mass for its self-gravity to overcome rigid body forces so that it assumes a hydrostatic equilibrium (nearly round) shape,
(c) has not cleared the neighborhood around its orbit, and
(d) is not a satellite.<\/p>\n<\/blockquote>\n\n
27 objects which are highly likely to be dwarf planets,
68 objects which are likely to be dwarf planets,
130 objects which are probably dwarf planets, and
741 objects which are possibly dwarf planets. <\/p>\n<\/blockquote>\n![\"Black](\"https:\/\/earthsky.org\/upl\/2016\/07\/pluto-mountains-new-horizons-cp-e1467414206728.jpg\")
Hundreds of dwarf planets?<\/h3>\n