Is there a general term for earth (or mars, or venus etc) sized planets?

[Bohandas]

Is there a general term for planets in the size range of the inner planets?

Wikipedia gives the terms "'terrestrial planet', 'telluric planet', or 'rocky planet'" but these are more specific than what I'm looking for (I think). These terms adequetely describe the actual planets of the inner solar system but not necessarily all possible planets in that size range, similarly to how "gas giant" is sufficient to describe Saturn but not Neptune; to describe both you need the general term "giant planet". What is the next step in size down from that? Is there a general name for the size of planet smaller than a giant planet but bigger than a dwarf planet or minor planet?

[factotum]

Um, why is "gas giant" not sufficient to describe Neptune? It's a giant planet formed mostly of gas--the only difference between it and Saturn is that its core contains a much larger proportion of various ices. All the planets of the inner solar system are rocky planets because they consist mostly of rock. The only other type of planet that might be relevant is an ice planet, which likely has a rocky core but where the surface and mantle are formed of ice--Pluto used to be classified as one back when it was a planet, but we don't have any in the Solar System anymore.

[Lvl 2 Expert]

I guess you could use the term macroplanets or mesoplanets to describe those objects in between dwarf planets and giant planets in size, but I don't think I've ever heard anyone do it.

Terrestrial planets is in essence about the composition of a planet, not its size. But since that composition tends to belong to planets in a pretty specific size range, it works pretty well in this scenario. Yeah, I guess you could have a planet with an mixed or undifferentiated rock/iron core of near earth size if there was very little metal in it (less heat produced by radioactive isotopes in the early stages of planetary formation) and a lot of water (which has a high heat capacity, it can take up a lot of heat preventing it from being used to melt the rock and have it differentiate from the metal). But that would be kind of an edge case, hardly worth overthinking the classifications for.

But who knows, a term might rise up as more research reveals new types of exoplanets.

(@Factotum: I've seen Neptune and Uranus described as ice giants, separating them from the larger gas giants. The idea seems to be that their interior should be mostly a mix of water, ammonia and methane, which form ice or maybe even fluid layers under these circumstances, while the makeup of the real gas giants is mostly hydrogen, which stays gassy at much more extreme conditions. I'm assuming that's what Bohandas is referring.)

[factotum]

(@Factotum: I've seen Neptune and Uranus described as ice giants, separating them from the larger gas giants. The idea seems to be that their interior should be mostly a mix of water, ammonia and methane, which form ice or maybe even fluid layers under these circumstances, while the makeup of the real gas giants is mostly hydrogen, which stays gassy at much more extreme conditions. I'm assuming that's what Bohandas is referring.)

Maybe, although Neptune and Uranus are still "mostly hydrogen" by any measure--they're more than 80% of the stuff. Yes, that's a somewhat lower proportion than Jupiter and Saturn (especially Saturn, which is around 96% hydrogen) but enough to make a major difference? Really not convinced there.

[King of Nowhere]

Terrestrial planets is in essence about the composition of a planet, not its size. But since that composition tends to belong to planets in a pretty specific size range, it works pretty well in this scenario.

to expand about it, terrestrial planets are too small and have too little gravity to retain large amounts of hydrogen or helium in their atmospheres. those are the most common elements in the universe, so if the planet cannot retain them, it won't have much gases and will be rocky.
A bigger planet that's large enough to keep hydrogen wiill, instead, be a gas giant, as hydrogen is so commonplace that if the planet can keep it, than it will be mostly made of it.

For those reasons, it's virtually impossible to have a rocky giant planet (it would have accumulated more hydrogen, and the rock material would have become the nucleus of a gas giant) or a terrestrial gas giant (technically possible with heavier gases than hydrogen or helium, but there aren't enough of those to make a gas giant. the most you can realistically get is a rocky planet with a thick, dense atmosphere, like venus. there may be such a planet somewhere, but it's bound to be extremely rare)

[halfeye]

Depending on the density, the size of a planet with surface gravity like Earth's can vary quite widely. Something with Mercury's density could be smaller, something much less dense could be bigger. Most of the planets being found out there are Jupiter mass or so, because it's easier to find big planets. Most of the "earthlike" planets being found are "super earths" with huge graivities which we are seriously not adapted to, or "mini neptunes" which are bigger still, and have even higher surface gravities, if they have surfaces.

to expand about it, terrestrial planets are too small and have too little gravity to retain large amounts of hydrogen or helium in their atmospheres. those are the most common elements in the universe, so if the planet cannot retain them, it won't have much gases and will be rocky.
A bigger planet that's large enough to keep hydrogen wiill, instead, be a gas giant, as hydrogen is so commonplace that if the planet can keep it, than it will be mostly made of it.

For those reasons, it's virtually impossible to have a rocky giant planet (it would have accumulated more hydrogen, and the rock material would have become the nucleus of a gas giant) or a terrestrial gas giant (technically possible with heavier gases than hydrogen or helium, but there aren't enough of those to make a gas giant. the most you can realistically get is a rocky planet with a thick, dense atmosphere, like venus. there may be such a planet somewhere, but it's bound to be extremely rare)

I think there can be rocky giants if they are near enough to their star that the hydrogen and helium were burnt off.

[Bohandas]

I think there can be rocky giants if they are near enough to their star that the hydrogen and helium were burnt off.

https://en.wikipedia.org/wiki/Chthonian_planet

[Peelee]

... Isn't "planet" a good term for it? The bigger ones are gas giants, the smaller ones are dwarf planets, so the bog standard planet seems fine.

[Yora]

There are plenty of gas dwarfs in other systems, but they are only small compared to our own gas giants. They are still much bigger than out terrestial planets, and I believe also bigger than pretty much all super-earths.

Planetary classification is really just a bunch of terms that were made up in the moment and somehow got stuck. There is no real system behind them. Even when they decided to make a proper definition for just "planet", they defined it in a way that excludes Jupiter (not cleared its orbit) and Saturn (not nearly spherical).

[factotum]

There are plenty of gas dwarfs in other systems, but they are only small compared to our own gas giants. They are still much bigger than out terrestial planets, and I believe also bigger than pretty much all super-earths.

Depends what you mean by "bigger". Kepler-138d is estimated to have the same mass as Earth but 60% larger radius, and the most likely cause of something like that would be a rocky planet with a very thick gas envelope, e.g. a "gas dwarf" as you call it. (For Kepler-138d to be entirely rocky with that mass and radius it would have to be even less dense than the Moon).

[Radar]

Depends what you mean by "bigger". Kepler-138d is estimated to have the same mass as Earth but 60% larger radius, and the most likely cause of something like that would be a rocky planet with a very thick gas envelope, e.g. a "gas dwarf" as you call it. (For Kepler-138d to be entirely rocky with that mass and radius it would have to be even less dense than the Moon).

In a sense, all planets start with a fairly thick atmosphere before the star in the center starts its work. The initial jumpstarting fusion impulse strips the atmospheres of the closest planets unless they are really heavy. Whatever atmosphere we have now, was aquired later.

[snowblizz]

... Isn't "planet" a good term for it? The bigger ones are gas giants, the smaller ones are dwarf planets, so the bog standard planet seems fine.

Right.

Because isn't this sort of a question where we are trying to define what "fish" means by only having looked at 9 aquatic species we have encountered so far.

Or "tree" or "fruit" or "berry" and any number of words/things everyone knows what it is but trying to nail it down categorically is almost impossible to suit actual practice.

[halfeye]

Depends what you mean by "bigger". Kepler-138d is estimated to have the same mass as Earth but 60% larger radius, and the most likely cause of something like that would be a rocky planet with a very thick gas envelope, e.g. a "gas dwarf" as you call it. (For Kepler-138d to be entirely rocky with that mass and radius it would have to be even less dense than the Moon).

I don't think that works. If there's a lot of gas, it would tend to blow away if the gravity was low. With that density, even if it's light rock all the way to the edge, the surface gravity would be less than the Earth's.

[factotum]

I don't think that works. If there's a lot of gas, it would tend to blow away if the gravity was low. With that density, even if it's light rock all the way to the edge, the surface gravity would be less than the Earth's.

It could consist of a very dense rocky core covered in a very thick, large atmosphere, is the point--it would only have to be less dense than the Moon if the entire thing was rock. I'm not sure exactly how close to its primary Kepler-138d is, either, which would affect how well it could keep its atmosphere.

[Lvl 2 Expert]

Right.

Because isn't this sort of a question where we are trying to define what "fish" means by only having looked at 9 aquatic species we have encountered so far.

Or "tree" or "fruit" or "berry" and any number of words/things everyone knows what it is but trying to nail it down categorically is almost impossible to suit actual practice.

Actually, that's kind of an argument for having a size based classification, rather than trying to work with terms like terrestrial planets. "A mesoplanet is any planet between one lunar mass (or: large enough to not be classified a dwarf planet) and one quarter of a Neptune mass in size" is a classification that just works.


It may not be very useful, but it does work.

[Rogar Demonblud]

The measurements used are Me (Earth Mass), Mj (Jovian Mass) and Ms (Sol Mass). As far as I know, there is no measure for moons.

[Lvl 2 Expert]

The measurements used are Me (Earth Mass), Mj (Jovian Mass) and Ms (Sol Mass). As far as I know, there is no measure for moons.

But if you want to want to make a point without looking up reasonable values and forcing your audience to look up values to make sure they're reasonable Lunar masses and Neptune masses work just fine.

[Rogar Demonblud]

You want to argue that we should be using a different yardstick for measurements, take it up with the International Astronomical Union. Rocky planets are defined by a ratio of Me (Mercury is 0.055Me, Mars is 0.107Me), gas giants by Mj (Saturn is 0.30Mj). Stars are calculated using Ms (referred to as Sols, which generally you only hear about in trying to describe singularities (which are on the megasol track, FYI)).

I can link to a primer on basic astronomy, if anybody wants.

[Bohandas]

You know, it strikes me that if there's not a name already then maybe the name ought to be of a fantasy/mythological bent, to go along with "dwarf" and "giant"

[Rogar Demonblud]

Well, there is, sort of. Hadean, Chthonian, Aquean, etc. Earth is an Aquean, BTW.

[Lvl 2 Expert]

I'm not trying to define yardsticks of measurement Rogar, I'm trying to get an idea across to other forummers.

Imagine I was saying "we could call any animal between the mass of the smallest mouse species and the size of a medium bear macroanimals". That doesn't mean I think biologists should stop using kilograms.

If it was previously unclear what I meant I hope this fixes that and you can stop putting words in my mouth and acting like I know too little to be considered a reasonable participant in this discussion.

[Lvl 2 Expert]

You know, it strikes me that if there's not a name already then maybe the name ought to be of a fantasy/mythological bent, to go along with "dwarf" and "giant"

I'm in. Earth is now an orc planet.

[Rogar Demonblud]

I'm not trying to define yardsticks of measurement Rogar, I'm trying to get an idea across to other forummers.

Imagine I was saying "we could call any animal between the mass of the smallest mouse species and the size of a medium bear macroanimals". That doesn't mean I think biologists should stop using kilograms.

If it was previously unclear what I meant I hope this fixes that and you can stop putting words in my mouth and acting like I know too little to be considered a reasonable participant in this discussion.

Unfortunately, it doesn't help, because they're still vague terms. Luna is actually larger than Pluto or Ceres, so it should fall under dwarf planets, as should Io. Phrasing it as 10% of Earth (Mars) is a little easier for people to grasp, also, since this is the only planet most people are familiar with. Referring to Neptune isn't going to help, as anybody trying to figure out what that means is going to be given measurements related to Earth (17.147Me, 1.14g, 57.74 Ve, etc).

There really isn't an easy solution, I'm afraid. I wish there was.

Also, I didn't mean to offend you. I'm just bad at trying to explain this without a chalkboard full of pictures and maths.

[Rogar Demonblud]

I'm in. Earth is now an orc planet.

As long as you don't say Orcus, because that's taken.

[Peelee]

I'm in. Earth is now an orc planet.

This, this is what I want in life.

Luna

You mean the Moon?

Oir sun is named the Sun, ourcmoon is named the Moon, and our solar system is named the Solar System. We are a simple people.

[Rogar Demonblud]

Our moon is named Luna, after an ancient goddess of the moon.

Our sun is named Sol, after an ancient god of the sun.

Our planet is named Gaia, after an ancient goddess not only of the earth (the original Mother Earth) but of life itself.

Most people don't bother to use them because "Our X" works fine...for now. Won't once we actually get serious about getting off planet.

[Peelee]

Our moon is named Luna, after an ancient goddess of the moon.

Our sun is named Sol, after an ancient god of the sun.

Our planet is named Gaia, after an ancient goddess not only of the earth (the original Mother Earth) but of life itself.

Most people don't bother to use them because "Our X" works fine...for now. Won't once we actually get serious about getting off planet.

Nope. Moon is named the Moon, sun is named the Sun, our planet is named Earth. You'll never see NASA, for example, use Luna Gaia or Sol because those aren't the names.

People like to think our moon is named Luna (or Selene) because it sounds so much nicer than the truth, but it ain't the truth.

The fun thing is solar and lunar are based off Sol and Luna... which aren't their names. Guess it just sounded better than Sunnar and Moonar.

ETA: It you want, we can both try to provide sources, but I suspect after a bit of looking around you won't want to.

[Bohandas]

Nope. Moon is named the Moon, sun is named the Sun, our planet is named Earth. You'll never see NASA, for example, use Luna Gaia or Sol because those aren't the names.

People like to think our moon is named Luna (or Selene) because it sounds so much nicer than the truth, but it ain't the truth.

The fun thing is solar and lunar are based off Sol and Luna... which aren't their names. Guess it just sounded better than Sunnar and Moonar.

ETA: It you want, we can both try to provide sources, but I suspect after a bit of looking around you won't want to.

On a related note, those things where you can supposedly pay money to name a star are scams. They're not recognized by any astronomical or scientific body (nor even by other name-a-star scams)

[Peelee]

On a related note, those things where you can supposedly pay money to name a star are scams. They're not recognized by any astronomical or scientific body (nor even by other name-a-star scams)

Aye. IAU is the only entity that can do naming.

[snowblizz]

Nope. Moon is named the Moon, sun is named the Sun, our planet is named Earth. You'll never see NASA, for example, use Luna Gaia or Sol because those aren't the names.

In English. You don't have a monopoly on deciding what something we all share is named. I believe an ancinet Egyptian would protest he named it "Ra" way before anyone ever managed to spit out a syllable of English.


Also, hilariously that ties into what I was on about earlier. The sun, the moon and earth are effectively names that became categories. There's a hilarious exchange in the Schlock Mercenary comic about that. Where an alien describes a lifeform as "Othlike", to which a human goes "yoo mean earth-like?" Or a planet the inhabitants named "Wet" and one of the human chacters note "that's a bit unoriginal" only to be told it compares favourably to the inhabitants of "Earth".

At some point in the future, the definitions of planets and moon might (probably) shift to something more along like "in which way are they useful to us".