Comment on Tell me the truth.
sbeak@sopuli.xyz 1 day agoI’m pretty sure Pluto doesn’t orbit a planet, so it’s not a moon. And the Moon, not only is it named a moon, but also orbits a planet, so therefore is a moon. One is a moon and the other is not a moon. Moon, not moon.
Pluto and Charon orbit each other. The barycentre (the center of mass they both orbit) is far outside of Pluto. The Earth-Moon barycentre is still inside Earth, though this could be changed by moving the Moon further out.
Either way, Earth, the largest rocky planet, could be made into a moon by sending it to Jupiter, so I don’t think being a moon should disqualify a celestial body from being a planet.
One of the main criteria for a planet is that it orbits a star. Moons don’t orbit stars and hence not planets. If Earth was orbiting Jupiter, it would be a moon but not a planet. Moons could harbour life too! Titan (which orbits Saturn) has an atmosphere, and Europa could have subsurface oceans under all that ice.
That definition means a planet has nothing to do with physical state, and everything to do with the proximity of your neighbors. We could promote the Moon to a planet by pushing it further away, or demote Earth from being a planet by slinging it a bit closer to it’s hungry uncle Jupiter. We could demote all planets by extinguishing the Sun! Then the entire system stops working and it’s all just asteroid or something.
That arbitrarily chosen definition doesn’t describe the object, only it’s place in the malleable hierarchy. With this, the title of planet tells us nothing about the object itself, except that it’s orbit is only dominated by a star.
Even worse, the IAU definition is extra arbitrary, as it only counts objects that orbit specifically the Sun, so the vast majority of bodies in hydrostatic equilibrium that don’t fuse hydrogen aren’t planets. They also play very lose with hydrostatic equilibrium, as Mercury isn’t in hydrostatic equilibrium, yet is explicitly classified as a planet. And “clearing it’s orbit” is also rather indistinct, with no method to determine this is given. It’s up to argument if Neptune is a planet, as many plutoids intersect it’s orbit.
Even more worse, the barycentre of our solar system is sometimes outside of the sun! That means sometimes the Sun is co-orbiting with the rest of the solar system bodies, and therefore by this definition nothing is a planet! It’s a definition so arbitrary that it periodically stops existing!
I’m not just saying I disagree with the IAU here, but that their definitely is objectively poor, and poorly used. I agree that Pluto, Eris, Ceres, and many others should be in a different category from Jupiter, but make some categories that make sense, please!
traches@sh.itjust.works 1 day ago
Why does the definition involve location? Intrinsic properties make more sense. Who cares what it orbits or what else is is in a similar orbit?
sbeak@sopuli.xyz 1 day ago
Moons are defined as naturally-formed objects that orbit a planet. Natural satellites, basically. What’s wrong with that definition? The Moon is a moon, but Pluto is not. Moons don’t have to be a fixed size, Earth’s moon is relatively big compared to the planet it orbits, Ganymede is larger than Mercury, and some moons are teeny tiny. If you tried to classify them based on size, you’d have a million different categories.
traches@sh.itjust.works 1 day ago
I’m saying that (most) moons are planets too. Anything big enough to be round, but not big enough to burn hydrogen, should be a planet regardless of where it orbits.
sbeak@sopuli.xyz 1 day ago
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.