Characterization of two new transiting sub-Neptunes and a terrestrial planet around M-dwarf hosts

TL;DR

This study reports the confirmation and characterization of three transiting planets around nearby M-dwarfs (TOI-1243, TOI-4529, TOI-5388) using TESS data complemented by ground-based photometry and CARMENES RVs. Precise radii are obtained for all three, with robust mass for TOI-1243 b and upper mass limits for TOI-4529 b and TOI-5388 b, revealing a range of possible interior compositions from rocky to water-rich or H/He-enveloped structures. The planets span periods of 2.59–5.88 days and exhibit radii from ~1.0 to ~2.3 R_earth, highlighting degeneracies in mass–radius space and underscoring the value of RVs for interior characterization. The authors also assess JWST transmission spectroscopy prospects, showing TOI-4529 b as a particularly favorable target for atmospheric studies, while contributing valuable data toward the population of small planets around M dwarfs and informing planet formation theories.

Abstract

We report the confirmation of three transiting exoplanets orbiting TOI-1243 (LSPM~J0902+7138), TOI-4529 (G~2--21), and TOI-5388 (Wolf~346) that were initially detected by TESS through ground-based photometry and radial velocity follow-up measurements with CARMENES. The planets present short orbital periods of $4.65$, $5.88$, and $2.59$ days, and they orbit early-M dwarfs (M2.0V, M1.5V, and M3.0V, respectively). We were able to precisely determine the radius of all three planets with a precision of $< 7\, \%$, the mass of TOI-1243 b with a precision of $19\, \%$, and upper mass limits for TOI-4529 b and TOI-5388 b. The radius of TOI-1243 b is $2.33\pm0.12\, {R_{\oplus}}$, its mass is $7.7 \pm 1.5\,{M_{\oplus}}$, and the mean density is $0.61 \pm 0.15 \, {ρ_\oplus}$. The radius of TOI-4529 b is $1.77 ^{+0.09}_{-0.08} \, {R_{\oplus}}$, the $3 σ$ upper mass limit is $4.9 \, {M_{\oplus}}$, and the $3 σ$ upper density limit is $0.88\, {ρ_\oplus}$. The third planet, TOI-5388 b, is Earth-sized with a radius of $0.99 ^{+0.07}_{-0.06} \, {R_{\oplus}}$, a $3 σ$ upper mass limit of $2.2 \, {M_{\oplus}}$, and a $3 σ$ upper density limit of $2.2\, {ρ_\oplus}$. While TOI-5388 b is most probably rocky, given its Earth-like radius, TOI-1243 b and TOI-4529 b are located in a highly degenerate region in the mass-radius space. TOI-4529 b appears to lean toward a water-world composition. TOI-1243 b has enough mass to host a significant H-He envelope, although a water-world and pure rocky compositions are also consistent with the data. Our analysis indicates that future atmospheric observations using JWST can aid in determining their real composition. The sample of small planets around M dwarfs is widely used to understand planet formation and composition theories, and our study adds three planets to this sample.

Figures (15)

• Figure 1: Br$\gamma$ Gemini North NIRI adaptive-optics image and $5 \sigma$ contrast limits for TOI-1243.
• Figure 2: TESS phase-folded and detrended relative flux of TOI-1243 b (top), TOI-4529 b (middle), and TOI-5388 b (bottom). The black line shows the best-fit model, and the purple points show the binned flux.
• Figure 3: RV phase-folded and detrended plot for TOI-1243 b (top), TOI-4529 b (middle), and TOI-5388 b (bottom). The purple points show the binned RVs, the black line shows the best-fit model, and gray shaded area shows the 1 $\sigma$ uncertainty of the RV model.
• Figure 4: Mass-radius diagram for the three discussed planets in comparison with validated exoplanets around M dwarfs with masses of $M_{\rm p} < 30$ M$_{\oplus}$ and $R_{\rm p} < 4$ R$_{\oplus}$. The exoplanet data were downloaded from NASA Exoplanet Archive on 2025 August 2. The black dots highlight planets characterized with CARMENES. The diagram features several compositional lines as computed by planet_growth_models2019. The $50\%$ Earth-like + $50\%$ H$_2$O and $49.95\%$ Earth-like + $49.95\%$ H$_2$O + 0.1 H$_2$ compositional lines are plotted for an equilibrium temperature of 500 K, that is, the closest to the derived temperature of the exoplanets under analysis.
• Figure 5: Output from the centroids module for TOI-1243 b in TESS Sector 60 (top), for TOI-4529 b in TESS Sector 42 (middle), and for TOI-5388 b in TESS Sector 21 (bottom). The dashed white lines outline the aperture mask for LC extraction. A star symbol indicates the cataloged position of the target, while a purple triangle shows the average out-of-transit photocenter. Individual photocenters are marked by small red dots, and the large red circle denotes the overall difference image photocenter. The panels show: upper left - difference image; upper right - average out-of-transit image; lower left - average in-transit image; lower right - S/N of the mean difference image. A color bar indicates the number of electrons/sec for each case mentioned. The difference image reveals a centroid offset without artifacts slightly contaminated by the presence of a nearby source.