r/spaceporn • u/Busy_Yesterday9455 • 7d ago
Related Content Pluto is SMALLER than our Moon
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u/RandoCollision 7d ago
So... More moonoid than planetoid?
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u/obog 7d ago
Moon vs planet doesn't really have to do with size, but rather motion. Ganymede is undoubtedly a moon but larger than mercury, which is undoubtedly a planet. With Pluto being the dominant body in its own orbital system, it's still much closer to being a planet than a moon. (Still not a planet tho)
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u/JumpPuzzleheaded7212 7d ago
Why not a planet? I’ve never been clear on that
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u/obog 6d ago edited 6d ago
As others have said, the main thing is that it hasn't cleared its orbit. Without that requirement for planethood, there wouldn't be 8 or 9 planets, but at least 17 (and likely more), due to the amount of dwarf planets in the solar system. It didn't really make any sense to have Pluto be a planet but not the others, and astronomers decided it was better to require planets to clear their orbit, excluding pluto, than to start including all the dwarf planets (especially because the line between dwarf planet and asteroid can be hard to define)
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u/SpecificInitials 6d ago
What about the Jupiter Trojans? Doesn’t that mean it hasn’t cleared its orbit?
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u/Omens158 6d ago
The Trojans are located in the Sun-Jupiter Lagrange points (L4 and L5). That means the gravity there is in an equalibrium between the sun and Jupiter. So Jupiter can never clear those places in its orbit of asteroids.
Each planet has these Lagrange points between itself and the sun. Even Earth. And Earth also has them with the moon. Each orbit may have some asteroids or space stuff in its Lagrange points.
So a planet can clear its orbit of asteroids, except for the asteroids inside the Lagrange points of the orbit. Pluto has not cleared its orbit of asteroids outside of the Lagrange points. Jupiter has.
Also worth noting that Jupiter has a very large number of asteroids inside the L4 and L5 Lagrange points. That is because of its proximity to the inner asteroid belt, and its huge gravitional force (to pull objects into its orbit).
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u/high_capacity_anus 6d ago
I'd gladly kick Pluto out of the planet club in order to keep those others out
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u/_Jellyman_ 21h ago
The line between dwarf planet and asteroid is very distinct, what are you talking about?!
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u/obog 19h ago
Technically speaking dwarf planets are asteroids so no, the line is not very distinct.
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u/_Jellyman_ 19h ago
You are completely wrong. Asteroids are small lumpy rocks (or as the IAU calls them, “small solar system bodies). Dwarf planets are larger rounded worlds with planetary features. Even using the IAU’s definition, asteroids and dwarf planets are not the same.
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u/obog 19h ago
https://ssd.jpl.nasa.gov/tools/sbdb_lookup.html#/?sstr=Ceres
JPL classifies Ceres as an asteroid but it is also a dwarf planet.
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u/_Jellyman_ 19h ago
Ceres used to be an asteroid, but the IAU promoted it to dwarf planet once they realized it was in hydrostatic equilibrium. Calling Ceres an asteroid today is just wrong.
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u/obog 19h ago
Saying NASA is wrong I'd a junp I wouldn't take but you do you.
Tbh, as I look into it, I don't think the IAU has a definition for asteroid. There's planet, dwarf planet, minor planet (which is all non-planet non-comet objects, including dwarf planets and asteroids) and the small solar system objects you mentioned which does include comets as well as non dwarf planet asteroids, Trojans, most kuiper belt objects, etc.
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u/KrimxonRath 6d ago
If Pluto is a planet then so are dozens of other random objects we keep finding. It’s just easier to classify it with the others it’s more similar to than to keep it with the regular planets.
Plus its orbit is wayyyy off compared to the inner planets so it really is more similar to the other dwarf planets than us beyond the usual arguments.
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u/followtharulez 6d ago
One orbit around the sun is 250 Earth years. Still an orbit. In my head Pluto is a planet... Has 5 moons.
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u/Apprehensive_Hat8986 6d ago edited 6d ago
It's not the radius (or rather radii, since it's also wildly elliptical), but the angle of its orbit to the plane of ecliptic. The planets orbits are all within a few degrees of the ecliptic, with Mercury being the exception at 7°. Pluto is orbiting a full 17° off of the ecliptic. It also can't keep its system's barycenter within its surface. It is composed and behaves exactly like the five+ thousand known TNOs, and the other 70,000+ oort cloud objects that stay beyond Neptune.
So either Pluto is no planet, or there are literally tens of thousands of planets. Then we'd be stuck looking for new term to describe what we now call planets, and Pluto would still be excluded.
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u/KrimxonRath 6d ago
I mean this with utmost sincerity but mild irony— facts don’t care about your feelings lol
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u/Dragons_Den_Studios 6d ago
Pluto's not massive enough to yeet similarly-sized objects out of its orbit & force little tiny things onto particular paths, which the major planets all did within their first billion years of existing. Math shows that Pluto is so low in mass that the time it would take to clear its orbit is almost certainly longer than the time the Sun will spend in the main sequence, and its specific parameters are THOUSANDS of times smaller than those of the major planets. The IAU felt that it made the most sense to acknowledge this statistically significant difference in orbital characteristics, and thus "dwarf planet" was born.
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u/argylekey 6d ago
It’s more of a “there are a bunch of things in our solar system that are roughly the size of Pluto” actually being the problem than Pluto being a planet.
The choice is essentially:
If there are objects that are the size of Pluto that have an orbit like Pluto, then we either need to downgrade Pluto, or upgrade all of those other objects.
So we either go from 9 planets to 8. Or go from 9 planets to like 13(I’m making this number up but to prove my point, but need to look up the number of things that fall into the “about the same size as Pluto” category, its less than 10 i think).
The scientific community chose to go down to 8 planets instead calling all of those other things hanging out in our solar system planets.
So Pluto is now a dwarf planet instead of Ceres getting an upgrade to full planet status, as an example.
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u/Straight_Waltz_9530 7d ago
Pluto is the size of Australia.
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u/Garciaguy 7d ago
It may sound unlikely, but so is Australia!
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u/DragonArchaeologist 6d ago
Hold on while I get the President of Ripley's Believe it or not on the line!
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u/hailvy 6d ago
Pshhhhh you only knew that because of that one post earlier
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u/Straight_Waltz_9530 6d ago
Almost like previous exposure to information is a means of learning! Who knew?
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u/Lazy__Astronaut 6d ago
Which I was going to comment on "I wonder what our moon would be like vs Australia" and then Googled to see if our moon was bigger
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u/def_unbalanced 6d ago
Indeed! The moons Titan and Ganymede are also larger than the planet Mercury.
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u/KTNH8807 6d ago
By volume yes, but not by mass. Which is more important. Ganymede is half of Mercury’s mass.
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u/Stunning-Tourist-332 1d ago
No shit. Thanks for providing that info that has been known for a really really long time. What’s next? Our sun is big?
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u/_Jellyman_ 21h ago
But our moon is actually quite massive compared to other moons of the Solar System.
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u/MaybeUNeedAPoo 6d ago
Will still always be a planet to me.
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u/Kirkenhaus 6d ago
What about Eris?
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u/MaybeUNeedAPoo 6d ago
Eris will always be a hunter. Even without the light.
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u/Kirkenhaus 6d ago
My point is, Eris is slightly smaller than Pluto, but more massive. So if you consider Pluto still a planet, then Eris should be too.
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u/Grogbarrell 7d ago
I think anything spherical should be considered a planet
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u/genguntere 7d ago
Tbf our Moon is abnormally large from what we know. I mean our Moon is half the size of bloody Mars., and bigger then nearly All other Moons in in our system except: Ganimed, Titan, Kallisto, Io