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An exoplanet with density 0.31 grams per cubic centimeter has been found. Is this the least dense exoplanet we know of?

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The article you link to refers to Borsato et al. 2019, 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

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, Roberts et al.) have found densities of around $\rho\sim0.03\text{ - }0.06\text{ g cm}^{-3}$ for both planets.

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  • $\begingroup$ 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) better illustrate the point ? $\endgroup$ – astrosnapper Nov 19 at 18:57
  • $\begingroup$ Or a plot of density Vs mass for transiting exoplanets. Such as the one I constructed to answer astronomy.stackexchange.com/questions/13382/… $\endgroup$ – Rob Jeffries Nov 19 at 22:14

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