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Super-Earths belong to a class of planet not found in the Solar System, butwhich appear common in the Galaxy. Given that some super-Earths are rocky,while others retain substantial atmospheres, their study can provide clues asto the formation of both rocky planets and gaseous planets, and - in particular- they can help to constrain the role of photo-evaporation in sculpting theexoplanet population. GJ 9827 is a system already known to host 3 super-Earthswith orbital periods of 1.2, 3.6 and 6.2 days. Here we use new HARPS-N radialvelocity measurements, together with previously published radial velocities, tobetter constrain the properties of the GJ 9827 planets. Our analysis can'tplace a strong constraint on the mass of GJ 9827 c, but does indicate that GJ9827 b is rocky with a composition that is probably similar to that of theEarth, while GJ 9827 d almost certainly retains a volatile envelope. Therefore,GJ 9827 hosts planets on either side of the radius gap that appears to dividesuper-Earths into pre-dominantly rocky ones that have radii below $\sim 1.5R_\oplus$, and ones that still retain a substantial atmosphere and/or volatilecomponents, and have radii above $\sim 2 R_\oplus$. That the less heavilyirradiated of the 3 planets still retains an atmosphere, may indicate thatphotoevaporation has played a key role in the evolution of the planets in thissystem.

Masses and radii for the three super-Earths orbiting GJ 9827, and implications for the composition of small exoplanets