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Alexander Mustill. Profile picture.

Alexander Mustill


Alexander Mustill. Profile picture.

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


  • A. Bonfanti
  • A.J. Mustill
  • M.B. Davies
  • K. Westerdorff
  • T. Zingales

Summary, in English

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-732 b is an ultrashort-period planet (P = 0.76837931−+000000004200000039 days) with a radius Rb = 1.325+−00057058 R☉, a mass Mb = 2.46 ± 0.19 M☉, and thus a mean density ρb = 5.8+−1008 g cm−3, while the outer planet at P = 12.252284 ± 0.000013 days has Rc = 2.39+−001011 R☉, Mc = 8.04+−005048 M☉, and thus ρc = 3.24+−005543 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 log P = −0.065+−00024013, 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 ρ̂valley/d log P = −0.02+−001204. 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.


  • Astrophysics
  • eSSENCE: The e-Science Collaboration
  • Mathematics (Faculty of Sciences)
  • LUNARC, Centre for Scientific and Technical Computing at Lund University
  • LU Profile Area: Natural and Artificial Cognition

Publishing year





Astronomy and Astrophysics



Document type

Journal article


EDP Sciences


  • Astronomy, Astrophysics and Cosmology


  • planets and satellites: fundamental parameters
  • stars: fundamental parameters
  • techniques: photometric
  • techniques: radial velocities
  • Landforms
  • Markov processes
  • Planets
  • Satellites
  • Time series analysis
  • Doublings
  • Evolution mechanism
  • Formation and evolutions
  • Formation mechanism
  • Planets and satellites: fundamental parameters
  • Radial velocity
  • Stars:fundamental parameters
  • Support vectors machine
  • Techniques: photometric
  • Techniques: radial velocities
  • Support vector machines




  • ISSN: 0004-6361