Pluto "hasn't cleared its orbit", but Earth shares its orbit with asteroids too. What does "cleared" mean?
Ananya Reddy11th Sample
30 Sept 2026 · 1 min read
We did the solar system in class today and sir mentioned the 2006 rules for what counts as a planet. I wrote them in my notes in three colours (of course), but one of them doesn't make sense to me and I don't want to just memorise it.
The three rules as I wrote them:
- Orbits the Sun
- Massive enough to be (nearly) round
- Has cleared the neighbourhood around its orbit
Pluto passes 1 and 2 but fails 3. But Earth also shares its orbit with other things, like near-Earth asteroids. Jupiter has the Trojan asteroids travelling along its own orbit. And Pluto's orbit crosses Neptune's, sort of. So how is "cleared" defined? If Jupiter has Trojans in its orbit and still counts as a planet, then what exactly did Pluto fail?
What I tried: I looked at the Wikipedia page, but it just says "gravitationally dominant", which is another word I don't fully understand. Also, I always thought Pluto got demoted because it's small. Is that wrong?
I would love an explanation that isn't just "because the IAU said so".
10 answers & replies
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[+][–]Neel Chatterjee12thSample· 19d ago
"Cleared" doesn't mean zero other objects. It means the body is so dominant gravitationally that over billions of years it has either absorbed the other stuff in its zone, flung it away, or captured it into stable arrangements like moons or Trojans. Jupiter's Trojans are there because of Jupiter, they're held in place by its gravity, so they count as part of Jupiter's domain, not as competitors. Earth's near-Earth asteroids are tiny and are slowly being pulled into or kicked out of orbits by Earth and the other planets. Compare masses: Earth is enormously heavier than everything else sharing its orbital zone put together. Pluto isn't. The Kuiper belt around it holds a lot of objects, and the belt as a whole is believed to be much more massive than Pluto itself. So Pluto is one big member of a crowd, not the boss of its own lane. And size is not the criterion. Ganymede and Titan are both bigger than Mercury, but they're moons.
[+][–]Rohan Sharma11thSample· 19d ago
I thought Pluto got demoted after the New Horizons flyby showed it was tiny?
[+][–]Kabir Joshi11thSample· 19d ago
Timeline mix-up. The demotion was in 2006. New Horizons flew past Pluto in 2015. What triggered the whole decision was the discovery of Eris in 2005, an object in the same region that is almost the same size as Pluto and actually a bit more massive. So astronomers had to choose: either call Eris a planet (and then probably dozens more as we find them) or draw a line that separates the eight from the rest.
[+][–]Rohan Sharma11thSample· 19d ago
ohhh ok so it was an Eris situation, not a size situation. ty
[+][–]Tanvi Deshmukh12thSample· 19d ago
Table time!! 😄 (I made this while listening to my sir and I'm proud of it)
Earth Pluto Orbits Sun? Yes Yes Round? Yes Yes Dominates its orbital zone? Yes, by a huge margin No, shares the Kuiper belt with many objects Diameter about 12,750 km about 2,377 km Bonus fact one Moon its moon Charon is about half its diameter, so they orbit a point outside Pluto Pluto is even smaller than our Moon (about 3,474 km across)!!
[+][–]Ananya Reddy11thSample· 19d agoAuthor
Your table is going into my notes (with credit 😄). One thing I still don't get though. Pluto's orbit crosses Neptune's. Why don't they ever collide? Does Neptune "control" Pluto?
[+][–]Siddharth RaoDropperSample· 19d ago
Sort of. They are locked in a 3:2 resonance: Neptune completes three orbits in the same time Pluto completes two (about 165 years for Neptune and 248 for Pluto, and 248 divided by 165 is roughly 1.5). Because of this timing, whenever Pluto is on the part of its orbit that crosses Neptune's path, Neptune is far away on the other side. They cross on paper but they never meet. Resonance is what keeps it stable, so no collision.
[+][–]Harsh Gupta12thSample· 19d ago
so Pluto got kicked out of the planets WhatsApp group because it wasn't admin of its own orbit 😂
[+][–]Ananya Reddy11thSample· 19d agoAuthor
that is the exact summary I'm using in my head from now on
[+][–]Neel Chatterjee12thSample· 18d ago
Adding one thing to my first answer: plenty of planetary scientists still think the 2006 wording of rule 3 is clumsy and argue about it. The practical point is that there's a real physical difference between objects that dominate their zone and ones that don't, and Pluto is on the "doesn't" side. There are currently five officially recognised dwarf planets: Ceres (in the asteroid belt), Pluto, Eris, Haumea and Makemake.
