<p>You can't get an exact composition just from density, but it is a clue.</p>
<p>In our solar system there are &quot;rocky&quot; planets with a density of about 5 to 6 g/cm³</p>
<p>And there are gas planets with a density of between 0.5 and 2 g/cm³</p>
<p>So if an exoplanet is found to have a density of 5.3 g/cm³  and a fairly small mass we can guess that the composition is rock (probably siliates with an iron core just like the rocky planets in our solar system)  And if the density is 1.1 g/cm³ we can guess that the planet is made of gasses like hydrogen and helium, perhaps with &quot;ices&quot; like methane or H₂O. If the mass is much greater than that  of Jupiter, then the planet is likely be be a (sub)-brown-dwarf.</p>
<p>You need both the mass and the density to make this judgement, as the density of gas giants depends on their mass, and larger gas giants have a greater density.</p>

CC BY-SA 4.0
2020-09-16T21:21:58.933Z
You need BOTH the mass and density to make these judgements. Gas giants can have densities in excess of 10 g/cm$^3$.
ProfRob
CC BY-SA 4.0
2020-09-16T20:41:52.687Z
2020-09-17T04:54:28.420Z
2020-09-17T04:54:28.420Z
James K
<p>I have seen some articles and lectures that state but i cant seem to put my finger on on how does density affect composition and how are you able to determine composition from exoplanet density.</p>

CC BY-SA 4.0
2020-09-17T02:27:35.910Z
Density does not determine composition uniquely, particularly not with the massive error bars that current mass determinations still have. But it can constrain them, setting gaseous from non-gaseous planets broadly apart.
AtmosphericPrisonEscape
<exoplanet>
I have seen so many articles but how can finding exoplanet density determine composition
Physics Geek
# I have seen so many articles but how can finding exoplanet density determine composition

(Asked by: username_0 on 2020-09-16)

I have seen some articles and lectures that state but i cant seem to put my finger on on how does density affect composition and how are you able to determine composition from exoplanet density.

**Tags:** exoplanet

> **Comments:**
> **username_1**: Density does not determine composition uniquely, particularly not with the massive error bars that current mass determinations still have. But it can constrain them, setting gaseous from non-gaseous planets broadly apart.  

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## 1 answers

### Answer by username_2 on 2020-09-16. Score: 1

You can't get an exact composition just from density, but it is a clue.

In our solar system there are "rocky" planets with a density of about 5 to 6 g/cm³

And there are gas planets with a density of between 0.5 and 2 g/cm³

So if an exoplanet is found to have a density of 5.3 g/cm³ and a fairly small mass we can guess that the composition is rock (probably siliates with an iron core just like the rocky planets in our solar system) And if the density is 1.1 g/cm³ we can guess that the planet is made of gasses like hydrogen and helium, perhaps with "ices" like methane or H₂O. If the mass is much greater than that of Jupiter, then the planet is likely be be a (sub)-brown-dwarf.

You need both the mass and the density to make this judgement, as the density of gas giants depends on their mass, and larger gas giants have a greater density.

> **Comments:**
> **username_3**: You need BOTH the mass and density to make these judgements. Gas giants can have densities in excess of 10 g/cm$^3$.  

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