<p>Whether of not you include the <em>mass</em> of the atmosphere doesn't matter much. For instance, the mass of Earth's atmosphere is roughly <span class="math-container">$10^{-6}$</span> of Earth's mass, corresponding to the uncertainty in the gravitational constant <span class="math-container">$G$</span> used to calculate the mass in the first place.</p>

<p>But the <em>volume</em> that you use for calculating the average density could potentially matter, depending on your definition of the height of the atmosphere. For Earth, the lowest-orbiting satellites are at a height of <span class="math-container">$\sim100\,\mathrm{km}$</span>, so if you include that, your volume will be 5% larger. This number can be much lower if you define the height of the atmosphere to be the scale height (0.3%), or much higher if you include the thermosphere (1600%).</p>

<p>Because the height of the atmosphere is quite arbitrary, this is rarely if ever included in the average density of a (rocky) planet.</p>

CC BY-SA 4.0
2020-02-05T23:48:42.050Z
For gas giants the gaseous component obviously has to be included both for the mass and the volume. So I believe that for reasons of consistency, this will be done too, for the rocky planets. A common convention is to take as radius the radius of the 1 bar level of atmospheric pressure, which will yield comparative radii for most planets and moons with atmospheres, and those without.
AtmosphericPrisonEscape
CC BY-SA 4.0
2020-02-07T15:03:32.963Z
@AtmosphericPrisonEscape You're probably right. I know about the "1 bar" rule, but I don't know whether or not the part above this arbitrary height is included in the mass. But it would make sense if it were.
pela
CC BY-SA 4.0
2020-02-04T12:23:31.563Z
2020-02-04T12:23:31.563Z
pela
<p>When the average density for a planet is stated does it incorporate the mass of the atmosphere, when a planet has an atmosphere, or is the mass of the atmosphere ignored?</p>

<planet><density>
Factors included in the average density of planets
Fred
# Factors included in the average density of planets

(Asked by: username_0 on 2020-02-04)

When the average density for a planet is stated does it incorporate the mass of the atmosphere, when a planet has an atmosphere, or is the mass of the atmosphere ignored?

**Tags:** planet, density


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## 1 answers

### Answer by username_1 on 2020-02-04. Score: 5 | Accepted answer

Whether of not you include the *mass* of the atmosphere doesn't matter much. For instance, the mass of Earth's atmosphere is roughly $10^{-6}$ of Earth's mass, corresponding to the uncertainty in the gravitational constant $G$ used to calculate the mass in the first place.

But the *volume* that you use for calculating the average density could potentially matter, depending on your definition of the height of the atmosphere. For Earth, the lowest-orbiting satellites are at a height of $\sim100\,\mathrm{km}$, so if you include that, your volume will be 5% larger. This number can be much lower if you define the height of the atmosphere to be the scale height (0.3%), or much higher if you include the thermosphere (1600%).

Because the height of the atmosphere is quite arbitrary, this is rarely if ever included in the average density of a (rocky) planet.

> **Comments:**
> **username_2**: For gas giants the gaseous component obviously has to be included both for the mass and the volume. So I believe that for reasons of consistency, this will be done too, for the rocky planets. A common convention is to take as radius the radius of the 1 bar level of atmospheric pressure, which will yield comparative radii for most planets and moons with atmospheres, and those without.  
> **username_1**: @username_2 You're probably right. I know about the "1 bar" rule, but I don't know whether or not the part above this arbitrary height is included in the mass. But it would make sense if it were.  

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