<p>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:</p>
<ul>
<li>mars-like or mini-earths: small terrestrial planet with minimal atmosphere</li>
<li>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.</li>
<li>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.</li>
<li>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.</li>
</ul>
<p>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 <em>actual</em> 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.</p>

CC BY-SA 4.0
2023-01-11T10:03:08.917Z
2023-01-11T10:03:08.917Z
planetmaker
<p>EDIT: well, <em>now</em> I have more</p>
<p>For reference:</p>
<ul>
<li><p>A <a href="https://en.wikipedia.org/wiki/Terrestrial_planet" rel="nofollow noreferrer">terrestrial planet</a> is a planet that is made out of <a href="https://en.wikipedia.org/wiki/Silicate" rel="nofollow noreferrer">silicates</a> and <a href="https://en.wikipedia.org/wiki/Metal" rel="nofollow noreferrer">metals</a>.</p>
</li>
<li><p>A <a href="https://en.wikipedia.org/wiki/Mini-Neptune" rel="nofollow noreferrer">Mini-Neptune</a> is, as far as I know, the smallest type of <a href="https://en.wikipedia.org/wiki/Gas_giant" rel="nofollow noreferrer">giant planet</a> (arguably, not even giant), and a miniature version of an <a href="https://en.wikipedia.org/wiki/Ice_giant" rel="nofollow noreferrer">ice giant</a>.</p>
</li>
</ul>
<p>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 <a href="https://en.wikipedia.org/wiki/Super-Earth" rel="nofollow noreferrer">some</a> of the <a href="https://en.wikipedia.org/wiki/Mega-Earth" rel="nofollow noreferrer">larger</a> terrestrial planets, they are significantly less dense and therefore significantly greater in volume.</p>
<p>However, what's the dividing line? Where, density-wise, does a planet stop being a terrestrial planet and start being a Mini-Neptune?</p>
<p>For the lower limit of that, we have the density of water, <a href="https://www.usgs.gov/special-topics/water-science-school/science/water-density" rel="nofollow noreferrer">at about 1 gram per cubic centimeter</a>; 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.</p>
<p>For the upper limit, we have Mars, <a href="https://nssdc.gsfc.nasa.gov/planetary/factsheet/marsfact.html" rel="nofollow noreferrer">which is approximately 3.93 grams per cubic centimeter</a> and yet is quite obviously not a Mini-Neptune.</p>
<p>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.</p>
<p>I'm of the impression that the density &quot;cutoff&quot; 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.</p>
<p>So, the question: <strong>what is the smallest value of density a planet have before it's no a longer terrestrial planet, and are there any boundary cases?</strong></p>

CC BY-SA 4.0
2022-01-29T08:40:55.670Z
porosity is IMHO the wrong word to use here: you are not asking about capilary space between solids. 'Less dense' is correct. The density differences are not due to differences in porosity but in elemental composition and chemistry.
planetmaker
CC BY-SA 4.0
2022-01-29T08:44:01.083Z
@planetmaker Changed.
KEY_ABRADE
CC BY-SA 4.0
2022-01-29T09:06:35.667Z
I don't think you can say that below a certain density planets are mini-Neptunes. There are superpuff planets with very low density that are a few earth masses but larger than Neptune and are thus not terrestrial planets or mini-Neptunes. So there is no simple dividing line based on density.
sno
CC BY-SA 4.0
2022-01-29T11:01:17.520Z
I know gas-giants and I also do know ice giants but it's my first time hearing mini-Neptune.
Nilay Ghosh
CC BY-SA 4.0
2022-01-29T17:44:01.563Z
The exoplanet K2-110 b is described as a mini-Neptune but has an earth-like density. https://www.aanda.org/articles/aa/pdf/2017/08/aa28932-16.pdf Perhaps the term mini-Neptune is not used consistently and so is not well defined.
sno
<planet><astrophysics><planetary-atmosphere><planetary-science><geology>
What value of density would a planet need to be classified as a mini-Neptune?
KEY_ABRADE
# What value of density would a planet need to be classified as a mini-Neptune?

(Asked by: username_0 on 2022-01-22)

EDIT: well, *now* I have more

For reference:

* A [terrestrial planet](https://en.wikipedia.org/wiki/Terrestrial_planet) is a planet that is made out of [silicates](https://en.wikipedia.org/wiki/Silicate) and [metals](https://en.wikipedia.org/wiki/Metal).
* A [Mini-Neptune](https://en.wikipedia.org/wiki/Mini-Neptune) is, as far as I know, the smallest type of [giant planet](https://en.wikipedia.org/wiki/Gas_giant) (arguably, not even giant), and a miniature version of an [ice giant](https://en.wikipedia.org/wiki/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](https://en.wikipedia.org/wiki/Super-Earth) of the [larger](https://en.wikipedia.org/wiki/Mega-Earth) 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](https://www.usgs.gov/special-topics/water-science-school/science/water-density); 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](https://nssdc.gsfc.nasa.gov/planetary/factsheet/marsfact.html) 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?**

**Tags:** planet, astrophysics, planetary-atmosphere, planetary-science, geology

> **Comments:**
> **username_1**: porosity is IMHO the wrong word to use here: you are not asking about capilary space between solids. 'Less dense' is correct. The density differences are not due to differences in porosity but in elemental composition and chemistry.  
> **username_0**: @username_1 Changed.  
> **username_2**: I don't think you can say that below a certain density planets are mini-Neptunes. There are superpuff planets with very low density that are a few earth masses but larger than Neptune and are thus not terrestrial planets or mini-Neptunes. So there is no simple dividing line based on density.  
> **username_3**: I know gas-giants and I also do know ice giants but it's my first time hearing mini-Neptune.  
> **username_2**: The exoplanet K2-110 b is described as a mini-Neptune but has an earth-like density. https://www.aanda.org/articles/aa/pdf/2017/08/aa28932-16.pdf Perhaps the term mini-Neptune is not used consistently and so is not well defined.  

---

## 1 answers

### Answer by username_1 on 2023-01-11. Score: 2

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.


---

