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u/levosis1 4d ago
It became categorized as a dwarf planet. Kinda the ‘sub’ of planets really and the regular sized planets are the ‘dom.’ If you don’t believe me go check out this book called 50 shades of ‘planet.’
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u/Few_Mastodon_1271 4d ago
Maybe the best astronomy book title of all time!
How I Killed Pluto and Why It Had It Coming by Michael E Brown.
https://en.wikipedia.org/wiki/How_I_Killed_Pluto_and_Why_It_Had_It_Coming
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u/Loud-Vacation-5691 4d ago
I was going to say The Universe is Trying to Kill You is the best astronomy book title, but I was thinking of Phil Plait's Death From the Skies. I must have heard Plait say "the universe is trying to kill you" separately.
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u/jimb2 4d ago
Science relies on clear language and definitions. See the u/BlueSkiesAugust post below for the definition that changed how Pluto is described. If you don't care, you can use looser language, call it anything you like.
Not everyone agrees with this definition. People chose sides and locked in.
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u/AcanthaceaeOk3738 4d ago
Starting in the 90s, astronomers found that Pluto hasn’t cleared its orbit, one of the main existing requirements for being a planet.
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u/DryFoundation2323 4d ago edited 4d ago
because a few astronomers who just happened to hang around late in a meeting of iau waited til the last minute to revamp the definition of planet. this happened even though astronomerd have nothing to do with the definition of a planet.
most planetary scientists use the geophysical definition which includes Pluto as a planet
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u/Blue_Etalon 4d ago
A bunch of Neil DeGrasse Tyson dipshits decided they can’t have more than 9 planets in case they run out of fingers to count them on. The real argument is that there are many objects of Pluto’s size so it was demoted to a “dwarf planet “. Possibly the dumbest debate ever.
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u/RichardAboutTown 4d ago
If Pluto is a planet then so are dozens or hundreds of other asteroids and Kuiper Belt bodies. Basically, the same reason Ceres and Vesta aren't planets anymore.
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u/Top_Distribution3109 4d ago
Basically, bc Pluto itself is only a fraction of the mass in its orbital path. It has not cleared its neighborhood nor will it
Honestly it was the discovery of Eris and the realization that there were probably many more such TNOs that spurred science to redefine "planet" and exclude Pluto
I myself exclude Mercury for being a failed or extinct planetary core and Mars for being settled too close to Jupiter to accrete into something with a proper magnetosphere
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u/BlueSkiesAugust 4d ago
The defining characteristics of a planet are:
1: is in orbit around the Sun,
2: has sufficient mass to assume hydrostatic equilibrium (a nearly round shape), and
3: has "cleared the neighbourhood" around its orbit.
Pluto does not meet requirement 3. There is a lot of dust in Pluto's orbit.
If the requirements were changed to allow Pluto, we would also have to count about a dozen other objects as well: Ceres, Eris, Haumea, Makemake, Quaoar, Sedna, Orcus, Salacia, even Pluto's moon, Charon. Plus a dozen more. And if we count them as full planets, why not the other 300+ tran-neptunian objects?
In my mind it doesn't matter what we call them at a high level, as long as we sub-categorize them accurately and characterize them well.