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ABSTRACT Growing numbers of exoplanet detections continue to reveal the diverse nature of planetary systems. Planet formation around late-type M dwarfs is of particular interest. These systems provide practical laboratories to measure exoplanet occurrence rates for M dwarfs, thus testing how the outcomes of planet formation scale with host mass, and how they compare to Sun-like stars. Here, we report the discovery of TOI-6478 b, a cold ($$T_{\text{eq}}=204\,$$ K) Neptune-like planet orbiting an M5 star ($$R_\star =0.234\pm 0.012\, \text{R}_\odot$$, $$M_\star =0.230\pm 0.007\, \text{M}_\odot$$, $$T_{\text{eff}}=3230\pm 75\,$$ K) that is a member of the Milky Way’s thick disc. We measure a planet radius of $$R_b=4.6\pm 0.24\, \text{R}_{\oplus }$$ on a $$P_b=34.005019\pm 0.000025\,$$ d orbit. Using radial velocities, we calculate an upper mass limit of $$M_b\le 9.9\, \text{M}_{\oplus }$$ ($$M_b\le 0.6\, \text{M}_{\text{Nep}})$$, with $$3\, \sigma$$ confidence. TOI-6478 b is a milestone planet in the study of cold Neptune-like worlds. Due to its large atmospheric scale height, it is amenable to atmospheric characterization with facilities such as JWST, and will provide an excellent probe of atmospheric chemistry in this cold regime. It is one of very few transiting exoplanets that orbit beyond their system’s ice-line whose atmospheric chemical composition can be measured. Based on our current understanding of this planet, we estimate TOI-6478 b’s spectroscopic features (in transmission) can be $$\sim 2.5\times$$ as high as the widely studied planet K2-18 b. 

ABSTRACT A new generation of observatories is enabling detailed study of exoplanetary atmospheres and the diversity of alien climates, allowing us to seek evidence for extraterrestrial biological and geological processes. Now is therefore the time to identify the most unique planets to be characterized with these instruments. In this context, we report on the discovery and validation of TOI-715 b, a $$R_{\rm b}=1.55\pm 0.06\rm R_{\oplus }$$ planet orbiting its nearby (42 pc) M4 host (TOI-715/TIC 271971130) with a period $$P_{\rm b} = 19.288004_{-0.000024}^{+0.000027}$$ d. TOI-715 b was first identified by TESS and validated using ground-based photometry, high-resolution imaging and statistical validation. The planet’s orbital period combined with the stellar effective temperature $$T_{\rm eff}=3075\pm 75~\rm K$$ give this planet an installation $$S_{\rm b} = 0.67_{-0.20}^{+0.15}~\rm S_\oplus$$, placing it within the most conservative definitions of the habitable zone for rocky planets. TOI-715 b’s radius falls exactly between two measured locations of the M-dwarf radius valley; characterizing its mass and composition will help understand the true nature of the radius valley for low-mass stars. We demonstrate TOI-715 b is amenable for characterization using precise radial velocities and transmission spectroscopy. Additionally, we reveal a second candidate planet in the system, TIC 271971130.02, with a potential orbital period of $$P_{02} = 25.60712_{-0.00036}^{+0.00031}$$ d and a radius of $$R_{02} = 1.066\pm 0.092\, \rm R_{\oplus }$$, just inside the outer boundary of the habitable zone, and near a 4:3 orbital period commensurability. Should this second planet be confirmed, it would represent the smallest habitable zone planet discovered by TESS to date.