TOI–757 b: an eccentric transiting mini–Neptune on a 17.5–d orbit
Authors: Alqasim A , Grieves N , Rosário N M , Gandolfi D , Livingston J H , Sousa S , Collins K A , Teske J K , Fridlund M , Egger J A , Cabrera J , Hellier C , Lanza A F , Van Eylen V , Bouchy F , Oelkers R J , Srdoc G , Shectman S , Günther M , Goffo E , Wilson T , Serrano L M , Brandeker A , Wang S X , Heitzmann A , Bonfanti A , Fossati L , Alibert Y , Delrez L , Sefako R , Barros S , Collins K I , Demangeon O D S , Albrecht S H , Alonso R , Asquier J , Barczy T , Barrado D , Baumjohann W , Beck T , Benz W , Billot N , Borsato L , Broeg C , Bryant E M , Butler R P , Cochran W D , Cameron A Collier , Correia A C M , Crane J D , Csizmadia Sz , Cubillos P E , Davies M B , Daylan T , Deleuil M , Deline A , Demory B -O , Derekas A , Edwards B , Ehrenreich D , Erikson A , Essack Z , Fortier A , Gazeas K , Gillon M , Gudel M , Hasiba J , Hatzes A P , Helling Ch , Hirano T , Howell S B , Hoyer S , Isaak K G , Jenkins J M , Kanodia S , Kiss L L , Korth J , Lam K W F , Laskar J , Etangs A Lecavelier des , Lendl M , Lund M B , Luque R , Mann A W , Magrin D , Maxted P F L , Mordasini C , Narita N , Nascimbeni V , Nowak G , Olofsson G , Osborn H P , Osborne H L M , Osip D , Ottensamer R , Pagano I , Palle E , Peter G , Piotto G , Pollacco D , Queloz D , Ragazzoni R , Rando N , Rauer H , Redfield S , Ribas I , Rice M , Ricker G R , Rieder M , Salmon S , Santos N C , Scandariato G , Seager S , Segransan D , Shporer A , Simon A E , Smith A M S , Stalport M , Szabo Gy M , Thompson I , Twicken J D , Udry S , Vanderspek R , Van Grootel V , Venturini J , Villaver E , Villaseñor J , Viotto V , Walter I , Walton N A , Winn J N , Yee S W 
Published: August 2024

We report the spectroscopic confirmation and fundamental properties of TOI$-$757 b, a mini$-$Neptune on a 17.5$-$d orbit transiting a bright star ($V\, =\, 9.7$ mag) discovered by the TESS mission. We acquired high$-$precision radial velocity measurements with the HARPS, ESPRESSO, and PFS spectrographs to confirm the planet detection and determine its mass. We also acquired space$-$borne transit photometry with the CHEOPS space telescope to place stronger constraints on the planet radius, supported with ground$-$based LCOGT photometry. WASP and KELT photometry were used to help constrain the stellar rotation period. We also determined the fundamental parameters of the host star. We find that TOI$-$757 b has a radius of $R_{\mathrm{p}} = 2.5 \pm 0.1 R_{\oplus }$ and a mass of $M_{\mathrm{p}} = 10.5^{+2.2}_{-2.1} M_{\oplus }$, implying a bulk density of $\rho _{\text{p}} = 3.6 \pm 0.8$ g cm$^{-3}$. Our internal composition modelling was unable to constrain the composition of TOI$-$757 b, highlighting the importance of atmospheric observations for the system. We also find the planet to be highly eccentric with e = 0.39$^{+0.08}_{-0.07}$, making it one of the very few highly eccentric planets among precisely characterized mini$-$Neptunes. Based on comparisons to other similar eccentric systems, we find a likely scenario for TOI$-$757 b’s formation to be high eccentricity migration due to a distant outer companion. We additionally propose the possibility of a more intrinsic explanation for the high eccentricity due to star$-$star interactions during the earlier epoch of the Galactic disc formation, given the low metallicity and older age of TOI$-$757.