The IAU definition clearly makes the distinction between planets and satellites. So even if it is huge and bigger than another planet, if it clearly orbits another planet then it's not a planet. It's a moon or satellite.Wow if they get their 'spherical' definition through then the planet count will shoot up as many of the gas giant moons are, I believe, spherical. Isn't Ceres spherical as well? And then what would be the distinction between planet and dwarf planet?
Also I wonder how strict the spherical definition would be as the Earth is not spherical.
Well they're just labels. It's not like it actually changes much. I've never really understood the fuss.The IAU definition clearly makes the distinction between planets and satellites. So even if it is huge and bigger than another planet, if it clearly orbits another planet then it's not a planet. It's a moon or satellite.
So what if Galileo said Titan was a planet! Yeah at the time it was another 'wanderer' if we use the original meaning of the term, but we've moved on a bit since then. Note also that Galileo got comets completely wrong.
Quite happy to keep Pluto as a dwarf/minor planet. It's clearly just one of a large number of trans-Neptunian objects. Hell, if required, I'd drop Mercury in as dwarf planet if that made more sense . Maybe we should add that a major planet should be able to hold onto an atmosphere (of sorts - at least Martian levels?) and have climate and weather???
I think the problem is that the definition of planet covers a great deal of different circumstances - gas giants, ice giants and rocky inner planets make up our current 'main list' of planets - but definitions work best if they held a bunch of objects that are very similar.
Maybe we should add that a major planet should be able to hold onto an atmosphere (of sorts - at least Martian levels?) and have climate and weather???
Well, depends what you mean by 'core'. A Jovian core is radically different from a Terran core. Plus there's a big distinction in rocky worlds between an active geologic core and one that's 'switched off'. Therefore I don't think that's a good distinction for deciding what makes a 'planet'. (I mean I just point at the centre of any round astronomical object and it's got a core by definition - even our Moon's is actually still quite toasty, just that it doesn't do anything now )I don't think an atmosphere is required, there could be activity of sorts inside the planet itself that never gets past the surface. Something like Europa but much larger. There is water under the surface but the harsh surface conditions disrupt our normal idea of what an atmosphere should be. A moon going round a giant Europa could trigger tides under the ice the same our Moon triggers tides on the surface. Some people think that movement is needed for life to form, some kind of outside disturbance that rocks the boat. Supposedly the water vapor that makes up the thin atmosphere of Europa is generated by charged particles hitting the ice. That's a harsh way to do it. We think in terms of what we know, our atmosphere is the product of temperature, gravity, and a magnetic field. If the temperature of a huge planet is way below zero, the atmosphere is going to be radically different from something that is near triple points of substances like water. At the same time, if there was enough gas on the surface and the parameters were right, there could be oceans of super cooled liquid like ocean bodies covering the surface of an extremely cold planet. It could be liquid, flowing, held within huge valleys so for all purposes it would act like oceans and maybe could even create a cold vapor state that acted just like air floating above liquid oceans. Liquid, gas, and solid is a common triple point we know for water at temperatures and pressures we are familiar with. If you crank up the temperature and pressures you enter into a new set of performance parameters where there triple points are nothing like the simple triple point we know for water. A thick atmosphere that consisted of charged particles held in place by magnetic fields would look spectacular but probably not be too healthy for our form of life.
The core of a planet has a lot to do with how a planet works. I'm not sure if a core is required for an object to be a planet but it does shape what it looks like. Our core might be very unusual, powered by magnetism and heat generated by the decay of radioactive substances. How different would our planet be if the ratios of the core materials were different, same materials, just different mix of which there are billions of combinations. A planet could stay hot long after it formed if it had a radioactive core without magnetic materials to create a magnetic field.
Wouldn't one problem with an atmosphere based definition be that it would also be dependant upon its proximity to it's local star. I'd have thought a smaller planet farther away could sustain more atmosphere than a somewhat larger one that is close.Well, depends what you mean by 'core'. A Jovian core is radically different from a Terran core. Plus there's a big distinction in rocky worlds between an active geologic core and one that's 'switched off'. Therefore I don't think that's a good distinction for deciding what makes a 'planet'. (I mean I just point at the centre of any round astronomical object and it's got a core by definition - even our Moon's is actually still quite toasty, just that it doesn't do anything now )
The reason I thought briefly that an atmosphere might be a valid, relatively simple requisite to define a major planet from a minor planet is because it is chiefly dependent on it's mass. A bit like the distinction between brown dwarfs and Jupiter-like objects: At about 14 Masses x Jupiter it is believed that a form of nuclear fusion would be initiated in the object. Below that it can't. That seems a nice physical cut-off that tells us something about the object.
So a more exact distinction that might make the cut-off for a planet is 'can this planet sustain any sort of atmosphere at all times?' The exact composition of the atmosphere will depend on a number of factors of course - but all the major planets have an atmosphere - Venus is CO2, Earth is NO2/O2 and the Gas/Ice giants largely Hydrogen/Helium. Thus Mercury is kicked out to become a minor planet...and Mars is close to the edge .
Take the example you said. A Europan-type world with a giant moon skimming by it. If the Europan world was too small, it wouldn't matter in the long-term that the friction caused by the orbiting moon actually caused an atmosphere - it would be continually leaking gas into space all the time.
Magnetic field may be relevant it is useful at slowing the solar wind strip the atmosphere away. But the process of atmosphere stripping is, as far as I can see, continuous and will happen even for planets like us (I can't see an atmosphere being permanently charged forever as you mused, most big objects formed must be broadly electrically neutral, given that the universe is, we believe, electrically neutral. But hey, the universe is a big place, perhaps there is some way of doing that...) Again my thinking is that, if the mass is big enough, then the planet will have an atmosphere ('cause it starts with lots of it) so that it lasts for main sequence lifetime of the star it orbits. (Again Mars is very close to the edge )
True temperature, or really, the distance the planet is from it's star will have a big impact on the composition of the planet's atmosphere. Something the size of Earth orbiting at the same distance to Neptune will have gases such as Oxygen, Carbon dioxide and (quite a lot of its) Nitrogen frozen out, but actually at the temperatures present there, it would be easy to form and hold quite a dense atmosphere of Hydrogen and Helium.
Anyway I wasn't being too serious, as Vertigo says, this is about labels. Every object in the universe is interesting in it's own way.
Well it was just a throwaway thought, however...Wouldn't one problem with an atmosphere based definition be that it would also be dependant upon its proximity to it's local star. I'd have thought a smaller planet farther away could sustain more atmosphere than a somewhat larger one that is close.
A large planet could have sufficient quantities of gases that can't be frozen out. Triton isn't big enough to hold onto H/He, however it's parent, Neptune, has plenty of the stuff going around it in huge winds.Also what about a planet that is significantly large but sufficiently far from any external influences (heat or gravity) and/or sufficiently old that all it's atmosphere is frozen so no longer strictly speaking an atmosphere.