G.M. has received fundings from the Ariel Postdoctoral Fellowship program of the Swedish National Space Agency (SNSA). S.V.J. acknowledges the support of the DFG priority program SPP 1992 “Exploring the Diversity of Extrasolar Planets” (JE 701/5-1). M.P.T. and L.P.M. acknowledge financial support through grants CEX2021-001131-S and PID2020-117404GB-C21, funded by MCIU/AEI/10.13039/501100011033. L.P.M. also acknowledges funding through the grant PRE2020-095421, funded by MCIU/AEI/10.13039/501100011033 and by FSE Investing in your future. This work is partly supported by JSPS KAK-ENHI Grant Number JP18H05439, JST CREST Grant Number JPMJCR1761. This article is based on observations made with the MuSCAT2 instrument, developed by ABC, at Telescopio Carlos Sánchez operated on the island of Tenerife by the IAC in the Spanish Observatorio del Teide. We acknowledge financial support from the Agencia Estatal de Investigación (AEI/10.13039/501100011033) of the Ministerio de Ciencia e Innovación and the ERDF “A way of making Europe” through projects PID2022-137241NB-C4[1:4], PID2021-125627OB-C31, PID2019-109522GB-C5[1:4], and the Centre of Excellence “Severo Ochoa" and “María de Maeztu” awards to the Instituto de Astrofísica de Canarias (CEX2019-000920-S), Instituto de Astrofísica de Andalucía (CEX2021-001131-S) and Institut de Ciències de l'Espai (CEX2020-001058-M). This work was also funded by the Generalitat de Catalunya/CERCA programme, the DFG through grant HA3279/14-1 and the priority program SPP 1992 “Exploring the Diversity of Extrasolar Planets” (JE 701/5-1), the JSPS KAK-ENHI through grant JP18H05439, the JST CREST through grant JPMJCR1761, and the Israel Science Foundation through grant 1404/22.

We report the confirmation and mass determination of a mini-Neptune transiting the M3.5 V star TOI-4438 (G 182-34) every 7.44 days. A transit signal was detected with NASA’s TESS space mission in the sectors 40, 52, and 53. In order to validate the planet TOI-4438 b and to determine the system properties, we combined TESS data with high-precision radial velocity measurements from the CARMENES spectrograph, spanning almost one year, and ground-based transit photometry. We found that TOI-4438 b has a radius of Rb = 2.52 ± 0.13 R⊕ (5% precision), which together with a mass of Mb = 5.4 ± 1.1 M⊕ (20% precision), results in a bulk density of ρb = 1.85−0.44+0.51 g cm−3 (~28% precision), aligning the discovery with a volatile-rich planet. Our interior structure retrieval with a pure water envelope yields aminimum water mass fraction of 46% (1σ). TOI-4438 b is a volatile-rich mini-Neptune with likely H/He mixed with molecules, such as water, CO2, and CH4. The primary star has a J-band magnitude of 9.7, and the planet has a high transmission spectroscopy metric (TSM) of 136 ± 13. Taking into account the relatively warm equilibrium temperature of Teq = 435 ± 15 K, and the low activity level of its host star, TOI-4438 b is one of the most promising mini-Neptunes around an M dwarf for transmission spectroscopy studies.

With funding from the Spanish government through the "Severo Ochoa Centre of Excellence" accreditation (CEX2021-001131-S).

With funding from the Spanish government through the "Severo Ochoa Centre of Excellence" accreditation (CEX2020-001058-M).

With funding from the Spanish government through the "Severo Ochoa Centre of Excellence" accreditation (CEX2019-000920-S).

Goffo, E., et al.

Peer reviewed