The discovery of a planetary candidate orbiting the habitable zone of our closest neighbor, Proxima Centauri (Gl 551), shook the planetary community as few other discoveries have done in recent years. It not only showed that the nearest star to the Sun could host a planetary system but also that, given the right conditions, it could host a habitable rocky planet. 2020 brought back the attention to Proxima with the announcement of a second planetary candidate in the system, the confirmation of Proxima b, using ESPRESSO, and the suggestion of a third, very low mass. Now, after an intense observational campaign, we can confidently say there's a third companion with just one quarter of the mass of the Earth.

Proxima d is one of the lightest exoplanets known to date. It orbits its parent star at 0.029 AU, with a period of 5.12 days. It's induced RV semi-amplitude, of just 39 cm/s on a V-mag 11 star, highlights the capabilities of the ESPRESSO spectrograph, installed at the VLT telescope array of the Paranal observatory. The signal has an amplitude that is just 1/5 of the amplitude of the activity-induced radial velocity signal, which shows that, under the right conditions, current techniques can detect planetary signals much smaller than activity signals. The discovery of Proxima d opens the door to the characterization of the population of very low mass planets of the solar neighborhood. It shows that radial velocity studies are now capable of detecting exoplanets with masses similar to the Earth and much smaller.