LHS 1903 provides evidence for gas-depleted formation of planets around M-dwarfs
Authors: Wilson Thomas , Simpson Anna , Cameron Andrew Collier , Cloutier Ryan , Adibekyan Vardan , John Ancy Anna , Alibert Y , Stalport Manu , Egger J , Bonfanti Andrea , Billot Nicolas , Guterman Pascal , Maxted Pierre , Simon Attila , Sousa Sergio , Fridlund Malcolm , Beck M , Bekkelien Anja , Salmon S , Grootel Valerie Van , Fossati Luca , Mustill Alexander , Osborn Hugh , Zingales Tiziano , Hooton M , Affer Laura , Aigrain Suzanne , Alonso Roi , Anglada-Escude G , Antoniadis-Karnavas Alexandros , Barczy T , Barrado David , Barros Susana , Baumjohann Wolfgang , Beck T , Benz W , Biondi Federico , Bonfils Xavier , Borsato Luca , Brandeker Alexis , Broeg Christopher , Buchhave Lars , Buder Maximilian , Cabrera Juan , Gaxiola S Carrazco , Charnoz Sébastien , Ciardi David , Collins Karen , Collins Kevin , Cosentino Rosario , Csizmadia Szilard , Cubillos P , Dalal Shweta , Damasso Mario , Davenport James , Davies Melvyn , Deleuil Magali , Delrez L , Demangeon Olivier , Demory Brice-Olivier , DiTomasso Victoria , Dragomir Diana , Dressing Courtney , Dumusque Xavier , Ehrenreich David , Erikson Anders , Esparza-Borges Emma , Fortier Andrea , Fukuda Izuru , Fukui Akihiko , Gandolfi Davide , Ghedina Adriano , Giacalone Steven , Gill Holden , Gillon Michaël , Chew Y Gomez Maqueo , Güdel Manuel , Guerra Pere , Günther Maximilian , Hara Nathan , Harutyunyan Avet , Hayashi Yuya , Haywood Raphaëlle , Holcomb Rae , Horne Keith , Hoyer S , Huang Chelsea , Ikoma Masahiro , Isaak K , Jackman James , Jenkins Jon , Jensen Eric , Jontof-Hutter Daniel , Kawai Yugo , Kiss Laszlo , Lakeland Ben , Laskar J , Latham David , des Etangs Alain Lecavelier , Leleu Adrien , Lendl Monika , de Leon J , Lienhard Florian , López-Morales Mercedes , Lovis Christophe , Lund Michael , Luque Rafael , Magrin D , Malavolta Luca , Fiorenzano Aldo , Mayo Andrew , Mayor Michel , Mordasini C , Mortier Annelies , Murgas Felipe , Narita Norio , Nascimbeni Valerio , Nicholson Belinda , Olofsson Göran , Ottensamer Roland , Pagano Isabella , Palethorpe Larissa , Palle Enric , Parviainen Hannu , Pedani Marco , Pepe Francesco , Peter Gisbert , Pinamonti Matteo , Piotto Giampaolo , Pollacco Don , Poretti Ennio , Queloz Didier , Quinn Sam , Ragazzoni R , Rando N , Rapetti David , Ratti F , Rauer Heike , Rescigno Federica , Ribas Ignasi , Rice William , Ricker George , Robertson Paul , de Roche Thierry , Sabin L , Santos Nuno , Sasselov Dimitar , Savel Arjun , Scandariato Gaetano , Schanche Nicole , Schroffenegger Urs , Schwarz Richard , Seager Sara , Sefako Ramotholo , Ségransan Damien , Shporer Avi , Silva André , Smith Alexis , Sozzetti Alessandro , Steller Manfred , Szabo Gyula , Tamura Motohide , Thomas Nicolas , Tuson Amy , Udry Stéphane , Vanderburg Andrew , Vanderspek Roland , Venturini J , Verrecchia Francesco , Walton Nicholas , Watson Christopher , Wells Robert , Winn Joshua , Zambelli Roberto , Ziegler Carl 
Published: September 2023

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.