# What is the least dense exoplanet?

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

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 $$\rho\sim0.19\text{ g cm}^{-3}$$:

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.

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