<p>The article you link to refers to <a href="https://academic.oup.com/mnras/article/484/3/3233/5290320#130413347" rel="nofollow noreferrer">Borsato et al. 2019</a>, which attempted to rectify discrepancies in the measured properties of planets in the Kepler-9 system between transit timing variation measurements and radial velocity measurements. They arrived at <span class="math-container">$\rho\sim0.31^{+0.05}_{-0.06}\text{ g cm}^{-3}$</span> for Kepler-9c. However, Borsato et al.'s Figure 10 shows that there are other exoplanets in this mass regime with substantially lower densities, e.g. WASP-107b, which comes in at about <span class="math-container">$\rho\sim0.19\text{ g cm}^{-3}$</span>:</p>

<p><a href="https://i.sstatic.net/KwiuC.png" rel="nofollow noreferrer"><img src="https://i.sstatic.net/KwiuC.png" alt="Plot of exoplanet densities"></a> </p>

<p>Even WASP-107b, however, doesn't hold the record for the least dense exoplanet. The three planets in the Kepler-51 system, Kepler-51b, Kepler-51c, and Kepler-51d, may hold that record. Multiple groups (<a href="https://arxiv.org/abs/1401.2885" rel="nofollow noreferrer">Masuda 2014</a>, <a href="https://www.exoplanetscience2.org/sites/default/files/submission-attachments/exoplanets2_roberts_kepler51b_51d_spectra.pdf" rel="nofollow noreferrer">Roberts et al.</a>) have found densities of around <span class="math-container">$\rho\sim0.03\text{ - }0.06\text{ g cm}^{-3}$</span> for both planets.</p>

CC BY-SA 4.0
2019-11-19T18:57:56.570Z
Just a thought: Would an alternative version of the diagram that is often shown with the lines of constant density corresponding to particular bulk densities (e.g. [this one](https://www.researchgate.net/figure/Mass-radius-diagram-for-exoplanets-for-which-the-mass-and-radius-have-been-both-measured_fig9_323392649)) better illustrate the point ?
astrosnapper
CC BY-SA 4.0
2019-11-19T22:14:59.947Z
Or a plot of density Vs mass for transiting exoplanets. Such as the one I constructed to answer https://astronomy.stackexchange.com/questions/13382/planets-classification-by-density?r=SearchResults
ProfRob
CC BY-SA 4.0
2019-11-19T16:42:27.523Z
2019-11-19T16:42:27.523Z
HDE 226868
<p><a href="https://phys.org/news/2019-06-density-exoplanets.html" rel="nofollow noreferrer">An exoplanet with density 0.31 grams per cubic centimeter has been found.</a> Is this the least dense exoplanet we know of? </p>

CC BY-SA 4.0
2019-11-19T22:17:17.140Z
See https://astronomy.stackexchange.com/questions/13382/planets-classification-by-density?r=SearchResults
ProfRob
CC BY-SA 4.0
2019-11-19T22:21:16.233Z
And pretty much a duplicate of https://astronomy.stackexchange.com/questions/8440/maximum-and-minimum-gas-giant-ice-giant-densities/8444?r=SearchResults#8444
ProfRob
<exoplanet><density>
What is the least dense exoplanet?
usernumber
# What is the least dense exoplanet?

(Asked by: username_0 on 2019-11-19)

[An exoplanet with density 0.31 grams per cubic centimeter has been found.](https://phys.org/news/2019-06-density-exoplanets.html) Is this the least dense exoplanet we know of?

**Tags:** exoplanet, density
> **Comments:**
> **username_1**: See https://astronomy.stackexchange.com/questions/13382/planets-classification-by-density?r=SearchResults  
> **username_1**: And pretty much a duplicate of https://astronomy.stackexchange.com/questions/8440/maximum-and-minimum-gas-giant-ice-giant-densities/8444?r=SearchResults#8444  

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## 1 answers

### Answer by username_2 on 2019-11-19. Score: 2 | Accepted answer

The article you link to refers to [Borsato et al. 2019](https://academic.oup.com/mnras/article/484/3/3233/5290320#130413347), which attempted to rectify discrepancies in the measured properties of planets in the Kepler-9 system between transit timing variation measurements and radial velocity measurements. They arrived at $\rho\sim0.31^{+0.05}\_{-0.06}\text{ g cm}^{-3}$ for Kepler-9c. However, Borsato et al.'s Figure 10 shows that there are other exoplanets in this mass regime with substantially lower densities, e.g. WASP-107b, which comes in at about $\rho\sim0.19\text{ g cm}^{-3}$:

[![Plot of exoplanet densities](https://i.sstatic.net/KwiuC.png)](https://i.sstatic.net/KwiuC.png)

Even WASP-107b, however, doesn't hold the record for the least dense exoplanet. The three planets in the Kepler-51 system, Kepler-51b, Kepler-51c, and Kepler-51d, may hold that record. Multiple groups ([Masuda 2014](https://arxiv.org/abs/1401.2885), [Roberts et al.](https://www.exoplanetscience2.org/sites/default/files/submission-attachments/exoplanets2_roberts_kepler51b_51d_spectra.pdf)) have found densities of around $\rho\sim0.03\text{ - }0.06\text{ g cm}^{-3}$ for both planets.

> **Comments:**
> **username_3**: Just a thought: Would an alternative version of the diagram that is often shown with the lines of constant density corresponding to particular bulk densities (e.g. [this one](https://www.researchgate.net/figure/Mass-radius-diagram-for-exoplanets-for-which-the-mass-and-radius-have-been-both-measured_fig9_323392649)) better illustrate the point ?  
> **username_1**: Or a plot of density Vs mass for transiting exoplanets. Such as the one I constructed to answer https://astronomy.stackexchange.com/questions/13382/planets-classification-by-density?r=SearchResults  

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