What value of density would a planet need to be classified as a mini-Neptune?

Question

EDIT: well, now I have more

For reference:

• A terrestrial planet is a planet that is made out of silicates and metals.

• A Mini-Neptune is, as far as I know, the smallest type of giant planet (arguably, not even giant), and a miniature version of an ice giant.

Mini-Neptunes differ from terrestrial planets in that their atmospheres are significantly more voluminous, often being made out of hydrogen/helium or volatiles. This means that, while they may be roughly as massive as some of the larger terrestrial planets, they are significantly less dense and therefore significantly greater in volume.

However, what's the dividing line? Where, density-wise, does a planet stop being a terrestrial planet and start being a Mini-Neptune?

For the lower limit of that, we have the density of water, at about 1 gram per cubic centimeter; from what I've read, water is one of the densest things to make up a gas planet's atmosphere, so this seemed like a good lower bracket - but Mini-Neptunes will still be denser than this, since they're not all liquid water, hence the ambiguity.

For the upper limit, we have Mars, which is approximately 3.93 grams per cubic centimeter and yet is quite obviously not a Mini-Neptune.

And, for a confounding quasi-middle ground, we have the density of silicate rock, which, depending on which one of the million online sources you believe, is around 2.5 to 3 grams per cubic centimeter, which makes me think that we could conceivably have a planet that's around those densities without having it be a mini-Neptune - but I, of course, am not well versed in such things, which is why I'm here.

I'm of the impression that the density "cutoff" isn't a clear line, but instead a sort of hazy it-might-be-a-steam-atmosphere/it-might-just-have-a-massive-surface-ocean type of thing. However, give it your best shot.

So, the question: what is the smallest value of density a planet have before it's no a longer terrestrial planet, and are there any boundary cases?

Answer 1

There is no hard rule for these classification and every astrophysicist may draw their own boundaries. Usually the distinctions made are like earth-like - superearth - neptunes - jupiters and you can add mini-neptunes between earths and superearths to that. These are not fixed categories, they are chosen to give the audience an idea of what type of planet we talk about in reference to the important properties of the solar-system planets they reference in the word:

• mars-like or mini-earths: small terrestrial planet with minimal atmosphere
• earth-like and super-earths: terrestrial, assumed to be composed from silicates and a metal core, with comparatively small atmosphere. The mass is comparable to that of Earth or up to ~10 earth masses for super earths. Though usually a Venus-like planet would also fall in this category: the atmosphere there is also only a small part of its mass. Densities for the terrestrial planets in the solar system ranges from 4g/cm^3 to about 5.5g/cm^3.
• neptunes and also mini neptunes: similar to super earths, but at colder temperatures, so that one can assume rather an ice giant than a terrestrial planet and possibly with an extended atmosphere with respect to total mass (for comparison Neptune has about 15 earth masses). The ice giants (Uranus and Neptune) have densities between 1.2 and 1.7g/cm^3.
• jupiters: large gasous planets with an extended atmosphere and a comparatively small solid core with masses starting at around saturn size. It's hard to distinguish in density from the ice giants: Jupiter has a density of 1.3g/cm^3 and Saturn of 0.7g/cm^3. Difference is rather in composition and mass and derived models for the interior.

There ARE planets between super earths and mini-neptunes which are hard to classify with this scheme and which seem to fall right between these categories. You will then have to use your own judgement or use further criteria or measurements to possibly improve a distinction (if there is not even an actual smooth transition so that these categories might be arbitrary anyway: maybe one can one day land on Neptune's solid core below the dense atmosphere. Maybe one will want to draw the boundary not on the density of the overall planet but by the pressure of the atmosphere on the (average) solid surface or that for neptunes the gas is beyond the critical point so that there is no distinction anymore between gas and liquid at the solid surface and for super-earths this is not yet the case.