Characterising TOI-732 b and c: New insights into the M-dwarf radius and density valley

Bonfanti, A.; Brady, M.; Wilson, T. G.; Venturini, J.; Egger, J. A.; Brandeker, A.; Sousa, S. G.; Lendl, M.; Simon, A. E.; Queloz, D.; Olofsson, G.; Adibekyan, V.; Alibert, Y.; Fossati, L.; Hooton, M. J.; Kubyshkina, D.; Luque, R.; Murgas, F.; Mustill, A. J.; Santos, N. C.; Van Grootel, V.; Alonso, R.; Asquier, J.; Bandy, T.; Bárczy, T.; Barrado Navascues, D.; Barros, S. C. C.; Baumjohann, W.; Bean, J.; Beck, M.; Beck, T.; Benz, W.; Bergomi, M.; Billot, N.; Borsato, L.; Broeg, C.; Collier Cameron, A.; Csizmadia, Sz.; Cubillos, P. E.; Davies, M. B.; Deleuil, M.; Deline, A.; Delrez, L.; Demangeon, O. D. S.; Demory, B. -O.; Ehrenreich, D.; Erikson, A.; Fortier, A.; Fridlund, M.; Gandolfi, D.; Gillon, M.; Güdel, M.; Günther, M. N.; Heitzmann, A.; Helling, Ch.; Hoyer, S.; Isaak, K. G.; Kasper, D.; Kiss, L. L.; Lam, K. W. F.; Laskar, J.; Lecavelier des Etangs, A.; Magrin, D.; Maxted, P. F. L.; Mordasini, C.; Nascimbeni, V.; Ottensamer, R.; Pagano, I.; Pallé, E.; Peter, G.; Piotto, G.; Pollacco, D.; Ragazzoni, R.; Rando, N.; Rauer, H.; Ribas, I.; Scandariato, G.; Ségransan, D.; Seifahrt, A.; Smith, A. M. S.; Stalport, M.; Stefánsson, G.; Steinberger, M.; Stürmer, J.; Szabó, Gy. M.; Thomas, N.; Udry, S.; Villaver, E.; Walton, N. A.; Westerdorff, K.; Zingales, T.
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Astronomy and Astrophysics

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Context. TOI-732 is an M dwarf hosting two transiting planets that are located on the two opposite sides of the radius valley. Inferring a reliable demographics for this type of systems is key to understanding their formation and evolution mechanisms.
Aims: By doubling the number of available space-based observations and increasing the number of radial velocity (RV) measurements, we aim at refining the parameters of TOI-732 b and c. We also use the results to study the slope of the radius valley and the density valley for a well-characterised sample of M-dwarf exoplanets.
Methods: We performed a global Markov chain Monte Carlo analysis by jointly modelling ground-based light curves and CHEOPS and TESS observations, along with RV time series both taken from the literature and obtained with the MAROON-X spectrograph. The slopes of the M-dwarf valleys were quantified via a support vector machine (SVM) procedure.
Results: TOI-732b is an ultrashort-period planet (P = 0.76837931-0.00000042+0.0000039 days) with a radius Rb = 1.325-0.058+0.057R⊕, a mass Mb = 2.46 ± 0.19 M⊕, and thus a mean density ρb = 5.8-0.8+1.0 g cm-3, while the outer planet at P = 12.252284 ± 0.000013 days has Rc = 2.39-0.11+0.10R⊕, Mc = 8.04-0.48+0.50M⊕, and thus ρc = 3.24-0.43+0.55 g cm-3. Even with respect to the most recently reported values, this work yields uncertainties on the transit depths and on the RV semi-amplitudes that are smaller up to a factor of ~1.6 and ~2.4 for TOI-732 b and c, respectively. Our calculations for the interior structure and the location of the planets in the mass-radius diagram lead us to classify TOI-732 b as a super-Earth and TOI-732 c as a mini-Neptune. Following the SVM approach, we quantified d log Rp,valley / d logP = -0.065-0.013+0.024, which is flatter than for Sun-like stars. In line with former analyses, we note that the radius valley for M-dwarf planets is more densely populated, and we further quantify the slope of the density valley as d log ρ^valley / d log P = -0.02-0.04+0.12.
Conclusions: Compared to FGK stars, the weaker dependence of the position of the radius valley on the orbital period might indicate that the formation shapes the radius valley around M dwarfs more strongly than the evolution mechanisms.

TESS and CHEOPS detrended light curves are available at the CDS via anonymous ftp to ( or via

This article uses data from CHEOPS programme CH_PR100031.