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Description
In recent years, the advent of a new generation of radial velocity instruments has allowed us to detect lower and lower mass planets, breaking the one Earth-mass barrier. Here we report a new milestone in this context, by announcing the detection of the lightest planet measured so far using radial velocities: L 98-59 b, a rocky planet with half the mass of Venus which is part of a system composed of three known transiting terrestrial planets (planets b to d). We announce the discovery of a fourth non-transiting planet with a minimum mass of 3.06_-0.37_^+0.33^M_{Earth)_ and an orbital period of 12.796_-0.019_^+0.020^ days and report hints for the presence of a fifth non-transiting terrestrial planet. If confirmed, with a minimum mass of 2.46_-0.82_^+0.66^M_{Earth}_ and an orbital period 23.15_-0.17_^+0.60^ days, this planet would sit in the middle of the habitable zone of the L 98-59 system. L 98-59 is a bright M-dwarf located 10.6pc away. Positioned at the border of the continuous viewing zone of the James Webb space telescope, this system is destined to become a corner stone for comparative exoplanetology of terrestrial planets. The three transiting planets have transmission spectrum metrics ranging from 49 to 255 which undoubtedly make them prime targets for atmospheric characterization with the James Webb space telescope, the Hubble space telescope, Ariel or ground-based facilities like NIRPS or ESPRESSO. With equilibrium temperature ranging from 416 to 627K, they offer a unique opportunity to study the diversity of warm terrestrial planets without the unknowns associated with different host stars. L 98-59 b and c have densities of 3.6_-1.5_^+1.4^ and 4.57_-0.85_^+0.77^g/cm^3^ respectively and have very similar bulk compositions with a small iron core, representing only 12 to 14% of the total mass, and a small amount of water. However, with a density of 2.95_-0.51_^+0.79^g/cm^3^ and despite a similar core mass fraction, up to 30% of L 98-59 d's mass could be made of water.
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