Densities of small planets around the M dwarfs TOI-4336 A and TOI-4342 with ESPRESSO: Three sub-Neptunes, one super-Earth, and a Neptune-mass candidate

TL;DR

The paper presents precise masses and radii for four planets orbiting two M-dwarfs, TOI-4336 A and TOI-4342, revealing a temperate sub-Neptune and a super-Earth around TOI-4336 A and two similar-sized sub-Neptunes around TOI-4342, plus a Neptune-mass candidate. Using ESPRESSO and NIRPS spectroscopy alongside TESS and ground-based photometry, the authors model stellar activity with Gaussian Processes to robustly extract planetary signals, deriving densities that suggest H$_2$/He/H$_2$O envelopes for the sub-Neptunes and a rocky or water-rich interior for the super-Earth. Detailed interior modeling indicates envelope mass fractions and core–mantle compositions, with TOI-4336 A b emerging as a premier JWST target due to its high TSM at a modest equilibrium temperature. The TOI-4342 system adds a dynamical dimension with a near 2:1 resonance between the transiting planets and a non-transiting Neptune-mass candidate, offering a valuable laboratory for atmospheric studies and formation/evolution scenarios around low-mass stars.

Abstract

We present the characterization of two planetary systems orbiting the M dwarfs TOI-4336 A (M3.5V) and TOI-4342 (M0V), each hosting two transiting planets previously validated with TESS and ground-based observations. We refined the photometry of the TOI-4342 system using TESS and LCOGT data, and characterized the host stars with NIRPS and ESPRESSO spectroscopy. High-precision ESPRESSO radial velocities allowed us to constrain the planetary masses and investigate their potential compositions. The TOI-4336 A system is composed of a sub-Neptune with a period of 16.34 days, a radius of $2.14 \pm 0.08$ Re, and a mass of $3.33 \pm 0.36$ Me, along with an inner super-Earth on a 7.59-day orbit with a radius of $1.25 \pm 0.07$ Re and a mass of $1.55 \pm 0.13$ Me. The TOI-4342 system hosts two sub-Neptunes of similar sizes ($2.33 \pm 0.09$ Re and $2.35 \pm 0.09$ Re), with periods of 5.54 and 10.69 days. Their masses are measured to be $7.3 \pm 1.3$ Me and $4.8 \pm 1.4$ Me, respectively. The RVs also reveal a planet candidate around TOI-4342, likely non-transiting, with a period of 47.5 days and a minimum mass of $17.8 \pm 3.0$ Me. With precise radii and masses, we derived bulk densities and explored possible compositions. The TOI-4336 A sub-Neptune and super-Earth have densities of $1.87 \pm 0.30$ and $4.35 \pm 0.79$ g cm$^{-3}$, while the two similar-sized sub-Neptunes in TOI-4342 show distinct densities of $3.18 \pm 0.67$ and $2.01 \pm 0.63$ g cm$^{-3}$. All four planets are excellent targets for future atmospheric characterization with JWST, and their multi-planet nature makes them especially interesting for comparative planetology. Notably, TOI-4336 A b stands out as one of the best-known targets in its size and temperature regime, with a TSM of 138, comparable to benchmark planets such as K2-18 b and LHS 1140 b.

Figures (15)

• Figure 1: Radial velocity measurements of TOI-4336 A (left) and TOI-4342 (right) obtained with ESPRESSO, using the CCF method (blue points) and the LBL method (red points).
• Figure 2: Time series of the RVs, FWHM, and $\Delta T_{3500\,\mathrm{K}}$ of TOI-4336 A after correction for the inferred offsets. The measurements are shown as white dots with error bars that include the fitted jitter. In the top panel, the full RV + activity model with its 1$\sigma$ uncertainty is shown in blue, while the GP-only component is shown in brown. In the middle and bottom panels, the activity models are displayed in yellow and pink, respectively, with their associated shaded uncertainties.
• Figure 3: Phase-folded ESPRESSO RVs with the best-fit model from our analysis (Sect. \ref{['subsect:RVs_TOI4336']}) and the corresponding residuals for TOI-4336 A b (top) and TOI-4336 A c (bottom). The error-bars include the fitted jitter added in quadrature.
• Figure 4: Phase-folded TESS and LCO light curves of TOI-4342 b (left) and TOI-4342 c (right), along with their residuals to the best-fit model from our analysis (Sect. \ref{['subsubsect:photometry_TOI4342']}), shown as black lines. Colored circles represent the data binned in 10-minute intervals.
• Figure 5: Time series of the RVs, FWHM, $\Delta T_{4000\,\mathrm{K}}$, and TESS binned photometry (sector 66 and 67) of TOI-4342 after correction for the inferred offsets. The measurements are shown as white dots with error bars that include the fitted jitter. In the top panel, the full RV + activity model with its 1$\sigma$ uncertainty is shown in blue, while the GP-only component is shown in brown. In the three other panels, the activity models are displayed in green, pink, and purple, with their associated shaded uncertainties.