There is no objective criteria for what a planet is and isn’t.
There is, though, or rather there should be another one.
The official definition says it’s a planet if it’s big enough to be round, which IMHO is a bullshit definition because nobody cares whether your object’s round, as in, for practical settlement purposes.
What’s important though is that it’s large enough to hold an atmosphere (at least if it had one). That’s only the case if the gravitational field is strong enough, which is the case roughly for objects of mass starting at around 10^23 kg. That definition fits surprisingly well the current actual classification of what is a planet and what isn’t, though.
Edit: I want to elaborate a bit more on this. Basically, if you consider a planet that has an atmosphere, like Earth, you see that the atmospheric density/pressure decreases exponentially with height. The concept of Scale Height discusses this: The atmosphere decreases exponentially, but if you take the total mass of the atmosphere and divide it by the density of the atmosphere at sea level, you get a height. That means, if the atmosphere had constant density up to that limit height and then cut off to zero, it would have the same mass as the actual atmosphere has. For Earth, that atmospheric scale height is about 8 km, about as high as the highest mountains on Earth btw.
The same concept of a scale height also exists for the gravitational field. Planets have a gravitational potential, which is formally the integral of the gravitational acceleration from ground to infinitely far-away. But you can simplified imagine it as: If the gravitational field would be constant up to a limit height and then would cut off to zero, that’s the scale height. For Earth, that gravitational scale height is about 8000 km, or about 1000x the atmospheric scale height.
The consequence of that is that Earth can hold an atmosphere neatly. Because for every gas molecule in the atmosphere, it is affected by the field of gravity strongly enough to be certainly bound to Earth. We take that as a granted, but consider this:
If the atmospheric scale height of another, fictional planet, was also 8 km but its gravitational scale height was only 4 km, then that would mean that a large part of the atmosphere would be exposed to being above-the-cutoff-height for gravity, so it would be effectively un-affected by gravity and would float away freely from the planet. This would actually not only imply that the planet would lose half of its atmosphere, but all of it. This is because, when the planet loses half its atmosphere, the atmospheric scale height actually doesn’t decrease at all. This is because it’s not like the atmosphere becomes less high, instead it just becomes half as thick everywhere. That also includes the ground level. So you have half the total mass of the atmosphere, but also half the thickness on ground level, so if you divide this, it’s still the same atmospheric scale height (!). This would mean that again, half of it would be above the gravity field and would escape again, and this process would repeat indefinitely until the planet has lost practically all of its atmosphere. Thus the planet could not hold an atmosphere.
That’s why there’s an important relationship between the gravitational potential of a planet and the fact whether the planet can hold an atmosphere at all. This isn’t just about how big the atmosphere can be in total, but whether there’s any atmosphere at all. Below a certain minimum planet mass, that’s completely impossible. Above, it’s possible.
And of course, scientists often just use the non-dimensional number characterizing this; gravitational scale height divided by the atmospheric scale height is the Gandolfi number (Gf). :-)
The official definition says it’s a planet if it’s big enough to be round, which IMHO is a bullshit definition because nobody cares whether your object’s round, as in, for practical settlement purposes.
As to why it’s not bullshit - the roundness is a byproduct of the object achieving hydrostatic equilibrium (which is the actual criterion, not roundness).
What’s important though is that it’s large enough to hold an atmosphere (at least if it had one).
Define an atmosphere. Because there’s multiple asteroids that technically have one, albeit extremely thin ones. And be careful about being too nitpicky, as Mercury’s atmosphere is just it’s rock being vaporized due to its proximity to the sun
There is, though, or rather there should be another one.
The official definition says it’s a planet if it’s big enough to be round, which IMHO is a bullshit definition because nobody cares whether your object’s round, as in, for practical settlement purposes.
What’s important though is that it’s large enough to hold an atmosphere (at least if it had one). That’s only the case if the gravitational field is strong enough, which is the case roughly for objects of mass starting at around 10^23 kg. That definition fits surprisingly well the current actual classification of what is a planet and what isn’t, though.
Edit: I want to elaborate a bit more on this. Basically, if you consider a planet that has an atmosphere, like Earth, you see that the atmospheric density/pressure decreases exponentially with height. The concept of Scale Height discusses this: The atmosphere decreases exponentially, but if you take the total mass of the atmosphere and divide it by the density of the atmosphere at sea level, you get a height. That means, if the atmosphere had constant density up to that limit height and then cut off to zero, it would have the same mass as the actual atmosphere has. For Earth, that atmospheric scale height is about 8 km, about as high as the highest mountains on Earth btw.
The same concept of a scale height also exists for the gravitational field. Planets have a gravitational potential, which is formally the integral of the gravitational acceleration from ground to infinitely far-away. But you can simplified imagine it as: If the gravitational field would be constant up to a limit height and then would cut off to zero, that’s the scale height. For Earth, that gravitational scale height is about 8000 km, or about 1000x the atmospheric scale height.
The consequence of that is that Earth can hold an atmosphere neatly. Because for every gas molecule in the atmosphere, it is affected by the field of gravity strongly enough to be certainly bound to Earth. We take that as a granted, but consider this:
If the atmospheric scale height of another, fictional planet, was also 8 km but its gravitational scale height was only 4 km, then that would mean that a large part of the atmosphere would be exposed to being above-the-cutoff-height for gravity, so it would be effectively un-affected by gravity and would float away freely from the planet. This would actually not only imply that the planet would lose half of its atmosphere, but all of it. This is because, when the planet loses half its atmosphere, the atmospheric scale height actually doesn’t decrease at all. This is because it’s not like the atmosphere becomes less high, instead it just becomes half as thick everywhere. That also includes the ground level. So you have half the total mass of the atmosphere, but also half the thickness on ground level, so if you divide this, it’s still the same atmospheric scale height (!). This would mean that again, half of it would be above the gravity field and would escape again, and this process would repeat indefinitely until the planet has lost practically all of its atmosphere. Thus the planet could not hold an atmosphere.
That’s why there’s an important relationship between the gravitational potential of a planet and the fact whether the planet can hold an atmosphere at all. This isn’t just about how big the atmosphere can be in total, but whether there’s any atmosphere at all. Below a certain minimum planet mass, that’s completely impossible. Above, it’s possible.
And of course, scientists often just use the non-dimensional number characterizing this; gravitational scale height divided by the atmospheric scale height is the Gandolfi number (Gf). :-)
wait, fr?
That’s the second out of the three points of the definition.
As to why it’s not bullshit - the roundness is a byproduct of the object achieving hydrostatic equilibrium (which is the actual criterion, not roundness).
Define an atmosphere. Because there’s multiple asteroids that technically have one, albeit extremely thin ones. And be careful about being too nitpicky, as Mercury’s atmosphere is just it’s rock being vaporized due to its proximity to the sun
can you please link an example?
https://en.wikipedia.org/wiki/101955_Bennu
oh wait maybe you’re talking about something like this:
yeah, no, i don’t consider that an atmosphere. it’s individual molecules bouncing around, but they don’t fulfill the “statistical” character of gases.
on that site i found this:
but no other mention of air or atmosphere on that object.