But how round is round enough? What about the millions of asteroids floating in the asteroid belt, many of those are spherical. Should they be considered planets? No, of course not. We can’t just call everything that looks like a sphere a planet. That’s ridiculous.
It was decided that planets 1. should have a stable orbit around a star 2. have enough mass to become spherical (that’s your point) 3. massive enough to clear its orbit, which in our Solar System means there are 8 planets. Pluto is surrounded by millions of Pluto-like objects in the Kuiper belt. Pluto, as well as its buddies Eris, Makemake, etc. are classified as dwarf planets because they are not massive enough to have cleared their orbits.
Dwarf planets are cool too, they might even have life in subsurface oceans under all that ice :0
Moons are not planets because they don’t orbit a star. Stars are pretty well-defined, objects where there is enough mass for nuclear fusion to occur. Planets are definitely not stars, so moons are not planets.
You’re just repeating the definition that I’ve already made clear I disagree with. This definition isn’t science, it’s taxonomy. Taxonomy is just a tool; we group similar entities together so we can characterize them. A body’s orbit and neighbors aren’t as important for that purpose as other attributes like size and composition.
Should they be considered planets? No, of course not.
Why not?
how round is round enough?
I’m sure the IAU can come up with a suitable boundary. The lines are always fuzzy.
The IAU did come up with a boundary: having enough mass to clear (most of) its orbit. That’s because simply mass and roundness are pretty arbitrary numbers that could be set to anything, while clearing the orbit is decently well-defined, at least mostly. It’s a good post to set as the lower limit for the mass of planets. That’s why Ceres is no longer considered a planet as well, there’s millions of asteroids in the Asteroid belt and it’s not massive enough to get rid of them. Similar reasoning goes for Pluto, Eris, Makemake, and their friends in the Kuiper belt. They’re not massive enough to clear their orbits of all those asteroids and other small objects.
Well orbits and neighbours and such I would argue is very important in astronomy. If you were all alone in the void of space and there is nothing else that exist, there wouldn’t be anything to compare against. The idea of relative size, mass, rotation, position, and even time wouldn’t really exist. The whole idea of moons is that it orbits planets, no matter its size, composition, or mass as long as it was naturally formed (hence why the ISS is not considered a moon, it’s an artificial satellite). Planets, by definition, are objects that orbit stars. Moons don’t orbit stars and hence cannot be planets.
I’m not very good with analogies, but here me out. Imagine the hands of a clock, all alone with no clock for the hands to tick. They may as well be pointy bits of metal, they are not hands without the clock. Just as moons are but rocks when without a planet, or how planets are not so without a star.
But how round is round enough? What about the millions of asteroids floating in the asteroid belt, many of those are spherical. Should they be considered planets? No, of course not. We can’t just call everything that looks like a sphere a planet. That’s ridiculous.
It was decided that planets 1. should have a stable orbit around a star 2. have enough mass to become spherical (that’s your point) 3. massive enough to clear its orbit, which in our Solar System means there are 8 planets. Pluto is surrounded by millions of Pluto-like objects in the Kuiper belt. Pluto, as well as its buddies Eris, Makemake, etc. are classified as dwarf planets because they are not massive enough to have cleared their orbits.
Dwarf planets are cool too, they might even have life in subsurface oceans under all that ice :0
Moons are not planets because they don’t orbit a star. Stars are pretty well-defined, objects where there is enough mass for nuclear fusion to occur. Planets are definitely not stars, so moons are not planets.
You’re just repeating the definition that I’ve already made clear I disagree with. This definition isn’t science, it’s taxonomy. Taxonomy is just a tool; we group similar entities together so we can characterize them. A body’s orbit and neighbors aren’t as important for that purpose as other attributes like size and composition.
Why not?
I’m sure the IAU can come up with a suitable boundary. The lines are always fuzzy.
The IAU did come up with a boundary: having enough mass to clear (most of) its orbit. That’s because simply mass and roundness are pretty arbitrary numbers that could be set to anything, while clearing the orbit is decently well-defined, at least mostly. It’s a good post to set as the lower limit for the mass of planets. That’s why Ceres is no longer considered a planet as well, there’s millions of asteroids in the Asteroid belt and it’s not massive enough to get rid of them. Similar reasoning goes for Pluto, Eris, Makemake, and their friends in the Kuiper belt. They’re not massive enough to clear their orbits of all those asteroids and other small objects.
Well orbits and neighbours and such I would argue is very important in astronomy. If you were all alone in the void of space and there is nothing else that exist, there wouldn’t be anything to compare against. The idea of relative size, mass, rotation, position, and even time wouldn’t really exist. The whole idea of moons is that it orbits planets, no matter its size, composition, or mass as long as it was naturally formed (hence why the ISS is not considered a moon, it’s an artificial satellite). Planets, by definition, are objects that orbit stars. Moons don’t orbit stars and hence cannot be planets.
I’m not very good with analogies, but here me out. Imagine the hands of a clock, all alone with no clock for the hands to tick. They may as well be pointy bits of metal, they are not hands without the clock. Just as moons are but rocks when without a planet, or how planets are not so without a star.