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GJ 9827 is a bright, nearby K7V star orbited by two super-Earths and onemini-Neptune on close-in orbits. The system was first discovered using K2 dataand then further characterized by other spectroscopic and photometricinstruments. Previous literature studies provide several mass measurements forthe three planets, however, with large variations and uncertainties. To betterconstrain the planetary masses, we added high-precision radial velocitymeasurements from ESPRESSO to published datasets from HARPS, HARPS-N, and HIRESand we performed a Gaussian process analysis combining radial velocity andphotometric datasets from K2 and TESS. This method allowed us to model thestellar activity signal and derive precise planetary parameters. We determinedplanetary masses of $M_b = 4.28_{-0.33}^{+0.35}$ M${_\oplus}$, $M_c =1.86_{-0.39}^{+0.37}$ M${_\oplus}$, and $M_d = 3.02_{-0.57}^{+0.58}$M${_\oplus}$, and orbital periods of $1.208974 \pm 0.1$ days for planet b,$3.648103_{-0.000010}^{+0.000013}$ days for planet c, and $6.201812 \pm0.9$ days for planet d. We compared our results to literature values andfound that our derived uncertainties for the planetary mass, period, and radialvelocity amplitude are smaller than the previously determined uncertainties. Wemodeled the interior composition of the three planets using themachine-learning-based tool ExoMDN and conclude that GJ 9827 b and c have anEarth-like composition, whereas GJ 9827 d has an hydrogen envelope, which,together with its density, places it in the mini-Neptune regime.

The compact multi-planet system GJ 9827 revisited with ESPRESSO