Is K2-56b a dense sub-Neptune?

K2-56 b is a long period (41.68d) sub-Neptune exoplanet that was previously reported as a high-density planet with interior composition lying in the transition region between pure rocky and a planet with a significant volatile envelope. 

In this work, we report the follow-up CHEOPS photometry and HARPS-N RV observations to provide in-depth characterisation of K2–56 b. We found that the exoplanet has a radius of 2.72 +/- 0.10 R_e and a mass of 14.11 +/- 3.06 M_e, corresponding to a bulk density of 3.85+/-0.94 g cm^{-3}. Unlike previous work, our results showed that K2-56 b is instead a low-density sub-Neptune with composition consistent with a planet with a significant gas envelope or a water world. With the precisely derived planet parameters, we modelled the interior structure of K2-56 b. If the planet were formed inside the iceline, it could host a H/He envelope of up to 2.5% with a very low envelope water mass fraction. Whereas if the planet were formed outside the ice line, it could hold an envelope mass fraction of up to nearly 40% and an envelope water mass fraction of almost 99%.

K2-56b orbits a bright host star and the sub-Neptune is likely to host a significant fraction of the gas envelope, making this exoplanet an attractive target for future atmospheric characterisation.