Abstract Many important advances in planet formation theory have come from the discovery of unexpected planets. The thousands of discovered exoplanets have unveiled demographic trends, such as the bimodality of planetary radius distribution known as the radius valley. Modelling these trends can probe underlying processes, e.g. the formation environment and atmospheric evolution. Here, we report the discovery and characterisation of a four-planet system around the kinematically thick-disk M-dwarf LHS 1903 with orbital periods of 2.16, 6.23, 12.57, and 29.32 days that becomes the only known M-dwarf hosting four small, well-characterised planets spanning the radius valley. We utilise high-precision transit photometry from the Transiting Exoplanet Survey Satellite (TESS) and the CHaracterising ExOPlanets Satellite (CHEOPS) to measure the radii of LHS 1903 b, c, d, and e (1.382+/-0.046, 2.046^+0.078_-0.074, 2.500^+0.078_-0.077, and 1.732^+0.059_-0.058 R_oplus). Combined with HARPS-N radial velocity data, we determine the planetary bulk densities (1.24^+0.21_-0.19, 0.53^+0.11_-0.09, 0.38^+0.09_-0.08, and 1.11^+0.33_-0.31 rho_oplus). Our compositional analysis finds that planet b is rocky, planets c and d have extended atmospheres, and LHS 1903 e does not have a gaseous envelope. Our discovery that planet e, the longest-period well-characterised terrestrial M-dwarf planet, lacks an extended atmosphere causes tension with thermally-driven mass loss radius valley predictions, but supports a gas-depleted formation explanation. The observed broken atmospheric-mass fraction trend is at odds with current formation theory, but provides further evidence for a gas-depleted formation environment for terrestrial M-dwarf planets.

ABSTRACT We report the discovery and characterization of a pair of sub-Neptunes transiting the bright K-dwarf TOI-1064 (TIC 79748331), initially detected in the Transiting Exoplanet Survey Satellite (TESS) photometry. To characterize the system, we performed and retrieved the CHaracterising ExOPlanets Satellite (CHEOPS), TESS, and ground-based photometry, the High Accuracy Radial velocity Planet Searcher (HARPS) high-resolution spectroscopy, and Gemini speckle imaging. We characterize the host star and determine $T_{\rm eff, \star }=4734\pm 67\,\mathrm{ K}$, $R_{\star }=0.726\pm 0.007\, \mathrm{ R}_{\odot }$, and $M_{\star }=0.748\pm 0.032\, \mathrm{ M}_{\odot }$. We present a novel detrending method based on point spread function shape-change modelling and demonstrate its suitability to correct flux variations in CHEOPS data. We confirm the planetary nature of both bodies and find that TOI-1064 b has an orbital period of Pb = 6.44387 ± 0.00003 d, a radius of Rb = 2.59 ± 0.04 R⊕, and a mass of $M_{\rm b} = 13.5_{-1.8}^{+1.7}$ M⊕, whilst TOI-1064 c has an orbital period of $P_{\rm c} = 12.22657^{+0.00005}_{-0.00004}$ d, a radius of Rc = 2.65 ± 0.04 R⊕, and a 3σ upper mass limit of 8.5 M⊕. From the high-precision photometry we obtain radius uncertainties of ∼1.6 per cent, allowing us to conduct internal structure and atmospheric escape modelling. TOI-1064 b is one of the densest, well-characterized sub-Neptunes, with a tenuous atmosphere that can be explained by the loss of a primordial envelope following migration through the protoplanetary disc. It is likely that TOI-1064 c has an extended atmosphere due to the tentative low density, however further radial velocities are needed to confirm this scenario and the similar radii, different masses nature of this system. The high-precision data and modelling of TOI-1064 b are important for planets in this region of mass–radius space, and it allow us to identify a trend in bulk density–stellar metallicity for massive sub-Neptunes that may hint at the formation of this population of planets.

Exoplanets transiting bright nearby stars are key objects for advancing our knowledge of planetary formation and evolution. The wealth of photons from the host star gives detailed access to the atmospheric, interior and orbital properties of the planetary companions. ν2 Lupi (HD 136352) is a naked-eye (V = 5.78) Sun-like star that was discovered to host three low-mass planets with orbital periods of 11.6, 27.6 and 107.6 d via radial-velocity monitoring1. The two inner planets (b and c) were recently found to transit2, prompting a photometric follow-up by the brand new Characterising Exoplanets Satellite (CHEOPS). Here, we report that the outer planet d is also transiting, and measure its radius and mass to be 2.56 ± 0.09 R⊕ and 8.82 ± 0.94 M⊕, respectively. With its bright Sun-like star, long period and mild irradiation (~5.7 times the irradiation of Earth), ν2 Lupi d unlocks a completely new region in the parameter space of exoplanets amenable to detailed characterization. We refine the properties of all three planets: planet b probably has a rocky mostly dry composition, while planets c and d seem to have retained small hydrogen–helium envelopes and a possibly large water fraction. This diversity of planetary compositions makes the ν2 Lupi system an excellent laboratory for testing formation and evolution models of low-mass planets. Three planets orbit the Sun-like star ν2 Lupi. CHEOPS data show that all of them are transiting and show remarkable diversity. In particular, dry and gas-poor inner planet b has experienced extensive atmospheric loss, while planets c and d are water rich and have a small gaseous envelope of primordial origin.