TOI-1243 b: Sub-Neptune Orbiting M-Dwarf

• TOI-1243 b is a transiting sub-Neptune exoplanet orbiting an early M-dwarf, defined by its measured radius, mass, and density.
• Its 4.66-day orbital period and high transit impact parameter were determined through combined TESS photometry and ground-based follow-up.
• The planet’s ambiguous interior—potentially a water-world or H-He envelope—makes it a prime candidate for atmospheric characterization with JWST.

TOI-1243 b is a confirmed transiting sub-Neptune orbiting the nearby early M dwarf TOI-1243, also identified as LSPM J0902+7138, TIC 219698776, and Karmn J09029+716. It was reported as one of three confirmed transiting exoplanets around M-dwarf hosts in a study that combined TESS photometry, ground-based transit follow-up, and radial-velocity measurements from CARMENES. Among the three planets in that study, TOI-1243 b is the only one with a clear radial-velocity mass detection, and it is therefore the system’s best-measured and most compositionally informative case. Its reported properties are an orbital period of $4.6594779^{+0.0000042}_{-0.0000045}\ \mathrm{d}$, a radius of $2.33 \pm 0.12\,R_\oplus$, a mass of $7.7 \pm 1.5\,M_\oplus$, and a mean density of $0.61 \pm 0.15\,\rho_\oplus$, placing it in a regime where several interior-composition models remain viable (Poultourtzidis et al., 12 Jan 2026).

1. Discovery and confirmation

TOI-1243 b was originally identified as a TESS object of interest and then confirmed through the standard follow-up chain used throughout the study: TESS 2-minute cadence photometry, multiple ground-based transit light curves, and radial-velocity measurements from CARMENES. In the confirmation flow, the TESS signal was first vetted with SPOC data validation, then re-checked with independent photometric vetting tools including DAVE and TRICERATOPS, and finally established as planetary with CARMENES radial velocities. For TOI-1243 specifically, the TESS transit signal passed the data validation tests, and the centroid analysis localized the source close to the target, with only a small contamination-related offset from nearby stars. The authors also emphasize that TOI-1243 b was the only one of the three planets for which they obtained a clear RV mass detection (Poultourtzidis et al., 12 Jan 2026).

Within the broader sample presented in the same work, TOI-1243 b belongs to a set of short-period planets orbiting early-M dwarfs. Its role in the paper is distinctive because it anchors the mass-radius discussion more strongly than the two companions discovered around the other host stars, whose masses are constrained only by upper limits. This gives TOI-1243 b disproportionate weight in the paper’s discussion of small-planet composition around M dwarfs (Poultourtzidis et al., 12 Jan 2026).

2. Host star

The host star is classified as an M2.0: V star. The stellar parameters derived in the study are $T_{\rm eff} = 3515 \pm 79\ \mathrm{K}$, $\log g = 4.73 \pm 0.12$ (cgs), $[{\rm Fe/H}] = -0.20 \pm 0.16$ dex, $$L_{\rm bol} = 358.5 \pm 7.2 \times 10^{-4}\,L_\odot$$, $R_\star = 0.511 \pm 0.024\,R_\odot$, $M_\star = 0.515 \pm 0.027\,M_\odot$, and $2.33 \pm 0.12\,R_\oplus$0. The star is at a distance of $2.33 \pm 0.12\,R_\oplus$1. The study also reports $2.33 \pm 0.12\,R_\oplus$2, and no robust rotation period was found for this star (Poultourtzidis et al., 12 Jan 2026).

These stellar parameters are central to the derivation of the planetary radius, mass, and density. In the study’s inference chain, the transit constrains $2.33 \pm 0.12\,R_\oplus$3, the radial velocities constrain $2.33 \pm 0.12\,R_\oplus$4, and the stellar properties set the scale for converting those observables into absolute planetary parameters. The relatively nearby distance and early-M-dwarf classification also frame TOI-1243 b as part of the observationally important population of small planets around M dwarfs used to test planet-formation and composition theories (Poultourtzidis et al., 12 Jan 2026).

3. Orbital and transit properties

The joint photometric and radial-velocity analysis yields an orbital period of

$2.33 \pm 0.12\,R_\oplus$5

with

$2.33 \pm 0.12\,R_\oplus$6

The transit geometry is characterized by a high impact parameter,

$2.33 \pm 0.12\,R_\oplus$7

and the scaled semimajor axis from the joint fit is reported as

$2.33 \pm 0.12\,R_\oplus$8

The equilibrium temperature, assuming $2.33 \pm 0.12\,R_\oplus$9, is

$7.7 \pm 1.5\,M_\oplus$0

The paper also lists a Keplerian semi-major axis ratio,

$7.7 \pm 1.5\,M_\oplus$1

in the derived-parameter row labeled “Keplerian,” which comes from the stellar mass and the orbital period through Kepler’s law (Poultourtzidis et al., 12 Jan 2026).

The radius ratio is reported as

$7.7 \pm 1.5\,M_\oplus$2

This directly underlies the planet-radius estimate and, combined with the high impact parameter, defines the transit geometry used in the joint fit. The short period and moderate equilibrium temperature place TOI-1243 b among close-in small planets around M dwarfs that are accessible to both transit and radial-velocity characterization (Poultourtzidis et al., 12 Jan 2026).

4. Observational material and inference workflow

TOI-1243 b was observed in TESS sectors 14, 20, 40, 47, 53, 54, and 74, with 33 transits in total. Ground-based transit follow-up for this target included an LCOGT 1 m $7.7 \pm 1.5\,M_\oplus$3 observation on 2021 Feb 3; MuSCAT3 multicolor observations on 2021 Feb 12; SAINT-EX on 2021 Nov 12; MuSCAT2 on 2022 Jan 21 and 2022 Oct 30; LCOGT CTIO and SSO $7.7 \pm 1.5\,M_\oplus$4 observations in 2022 Dec and 2023 Sep; and additional ground-based transit and long-term photometric monitoring from TJO, e-EYE, and LCOGT. The paper explicitly states that the TESS and ground-based data were cleaned of systematics with polynomial detrending, and that the final planet selection used the best-quality transits only (Poultourtzidis et al., 12 Jan 2026).

On the spectroscopic side, the CARMENES VIS channel radial velocities were used for the mass determination, while the NIR data were used mainly for activity indicators and stellar parameters. For TOI-1243 b, the CARMENES data set comprises 39 RVs, with a median internal precision of $7.7 \pm 1.5\,M_\oplus$5, RMS $7.7 \pm 1.5\,M_\oplus$6, and median S/N 245 at 7370 Å. The paper’s stated modeling assumptions are that the orbit is circular and the argument of periastron is fixed to $7.7 \pm 1.5\,M_\oplus$7; for the RV modeling the authors used RadVel within juliet, and for the transit modeling they used Pylightcurve and later batman in the joint fit. A dedicated simulation in the paper suggests that about 80–100 CARMENES observations would be needed to measure the planet mass to better than 15% (Poultourtzidis et al., 12 Jan 2026).

5. Measured planetary properties

The reported planet radius is

$7.7 \pm 1.5\,M_\oplus$8

and the study explicitly states that the planetary radii are all measured with precision better than 7%. For TOI-1243 b, the radial-velocity semi-amplitude is

$7.7 \pm 1.5\,M_\oplus$9

which is translated to a planetary mass of

$0.61 \pm 0.15\,\rho_\oplus$0

The discussion states that this is a “$0.61 \pm 0.15\,\rho_\oplus$1 detection.” The corresponding bulk density is

$0.61 \pm 0.15\,\rho_\oplus$2

or, in Earth-density units,

$0.61 \pm 0.15\,\rho_\oplus$3

The abstract summarizes the mass precision as 19% (Poultourtzidis et al., 12 Jan 2026).

These values define TOI-1243 b as the best-characterized planet in the three-planet sample presented in the paper. The measured radius places it in the sub-Neptune regime, while the density is low enough that bulk composition cannot be read off uniquely from mass and radius alone. This is the central empirical result that drives the paper’s later discussion of interior degeneracy and atmospheric follow-up (Poultourtzidis et al., 12 Jan 2026).

6. Interior interpretation and atmospheric characterization

The paper places TOI-1243 b in a highly degenerate region in mass-radius space. Its main conclusion is that the observed radius, mass, and density can be explained by more than one internal structure. Specifically, the study states that the mass is high enough for the planet to host a significant H-He envelope, but that a water-world composition is also equally likely, and a purely rocky interior cannot be ruled out either. The discussion summarized for the system also states that the planet “seems to lean toward a water-world composition,” and, if that interpretation is correct, it would be “one of the lowest-density planets of its size discovered around an M-dwarf star.” At the same time, the current measurement set does not uniquely distinguish between a dense rocky super-Earth, a water-rich world, or a planet with a modest H-He envelope (Poultourtzidis et al., 12 Jan 2026).

This compositional ambiguity is the basis for the paper’s emphasis on atmospheric follow-up. The study reports a transmission spectroscopy metric of

$0.61 \pm 0.15\,\rho_\oplus$4

and an emission spectroscopy metric of

$0.61 \pm 0.15\,\rho_\oplus$5

concluding that the planet is potentially appealing for atmospheric characterization, though its ESM is below the recommended threshold for emission work. The authors simulate JWST observations for several atmospheric cases, including clear and hazy H/He atmospheres and a steam H$0.61 \pm 0.15\,\rho_\oplus$6O atmosphere, using NIRISS-SOSS, NIRSpec-G395H, and MIRI-LRS. In those synthetic spectra, H/He atmospheres could produce H$0.61 \pm 0.15\,\rho_\oplus$7O and CH$0.61 \pm 0.15\,\rho_\oplus$8 features of a few hundred ppm, whereas a steam atmosphere would have much weaker features, $0.61 \pm 0.15\,\rho_\oplus$9 ppm. Their simulated single-transit uncertainties are large enough that one or a few JWST transits should detect an H/He atmosphere if present, while a water-vapor atmosphere would require more than five transits. TOI-1243 b is therefore presented as a particularly promising JWST target precisely because atmospheric spectroscopy could break the compositional degeneracy implied by the bulk measurements (Poultourtzidis et al., 12 Jan 2026).