TOI-4529 b: M-dwarf Sub-Neptune

• TOI-4529 b is a sub-Neptune exoplanet orbiting an early-M dwarf with a measured radius of 1.77 R⊕ and a 3σ mass upper limit of 4.9 M⊕.
• It was discovered by TESS and confirmed through ground-based photometry and CARMENES radial-velocity follow-up that used transit modeling and GP analysis to constrain its orbit.
• Planned JWST observations, supported by a TSM near 102, aim to differentiate between a volatile-rich (water world) or rocky-plus-envelope composition.

Searching arXiv for the cited paper to ground the article in the primary source. arXiv search query: (Poultourtzidis et al., 12 Jan 2026) TOI-4529 b is a transiting sub-Neptune orbiting the early-M dwarf G 2–21, identified by TESS and confirmed through ground-based photometry and CARMENES radial-velocity follow-up. It was reported together with TOI-1243 b and TOI-5388 b in the study "Characterization of two new transiting sub-Neptunes and a terrestrial planet around M-dwarf hosts" (Poultourtzidis et al., 12 Jan 2026). The planet has a short orbital period of $5.879577^{+0.000011}_{-0.000010}\,\mathrm{d}$, a radius of $1.77^{+0.09}_{-0.08}\,R_\oplus$, and only an upper limit on its mass, $M_p \le 4.9\,M_\oplus$ at $3\sigma$, implying a $3\sigma$ density upper bound of $0.88\,\rho_\oplus$ (Poultourtzidis et al., 12 Jan 2026). Its position in mass-radius space falls within a highly degenerate regime in which water-rich, rocky-plus-envelope, and possibly bare rocky interpretations remain viable, although the current data favor a significant volatile component.

1. Discovery and system classification

TOI-4529 b was initially detected by TESS and subsequently confirmed through a combination of ground-based transit photometry and radial-velocity observations with CARMENES (Poultourtzidis et al., 12 Jan 2026). It belongs to a three-planet set reported in the same study, alongside TOI-1243 b and TOI-5388 b, and represents the system associated with TOI-4529, also cataloged as G 2–21.

The planet is classified as a sub-Neptune and orbits an M1.5 V host star. In the source study, the three reported planets are described as orbiting early-M dwarfs, and TOI-4529 b occupies the intermediate regime between Earth-sized rocky planets and larger volatile-rich sub-Neptunes. Its radius, $1.77^{+0.09}_{-0.08}\,R_\oplus$, places it near the small-planet regime where compositional inference from radius alone is intrinsically non-unique. This suggests that TOI-4529 b is of particular interest for comparative exoplanetology around M dwarfs, where mass-radius degeneracies and atmospheric retention are central problems.

2. Host star G 2–21

The host star G 2–21 is an early-M dwarf with spectral type M1.5 V (Poultourtzidis et al., 12 Jan 2026). Its atmospheric and fundamental parameters were derived from high-S/N VIS+NIR CARMENES spectra, which were co-added to form templates. The atmospheric parameters $T_{\mathrm{eff}}$, $\log g$, and $[\mathrm{Fe/H}]$ were fitted with the SteParSyn code using BT-Settl models, while bolometric luminosity was obtained by integrating the SED from Johnson B to WISE W4 with Gaia DR3 parallaxes. Radius and mass were then inferred through empirical mass-radius-luminosity relations.

Quantity	Value
Spectral type	M1.5 V
$1.77^{+0.09}_{-0.08}\,R_\oplus$0	$1.77^{+0.09}_{-0.08}\,R_\oplus$1
$1.77^{+0.09}_{-0.08}\,R_\oplus$2	$1.77^{+0.09}_{-0.08}\,R_\oplus$3 [cgs]
$1.77^{+0.09}_{-0.08}\,R_\oplus$4	$1.77^{+0.09}_{-0.08}\,R_\oplus$5
$1.77^{+0.09}_{-0.08}\,R_\oplus$6	$1.77^{+0.09}_{-0.08}\,R_\oplus$7
$1.77^{+0.09}_{-0.08}\,R_\oplus$8	$1.77^{+0.09}_{-0.08}\,R_\oplus$9
$M_p \le 4.9\,M_\oplus$0	$M_p \le 4.9\,M_\oplus$1

Additional stellar constraints include a projected rotational velocity of $M_p \le 4.9\,M_\oplus$2 and a rotation period of $M_p \le 4.9\,M_\oplus$3, derived from activity indicators and long-term photometry. These stellar properties are important because they directly enter the transit and radial-velocity inference chain: $M_p \le 4.9\,M_\oplus$4 sets the absolute planetary radius, $M_p \le 4.9\,M_\oplus$5 sets the conversion from Doppler semi-amplitude to planetary mass, and the stellar rotation period informs the activity model used in RV analysis.

3. Transit observations and orbital architecture

TESS observed G 2–21 in Sectors 42, 43, and 70 with 2-min cadence, yielding a total of nine transits (Poultourtzidis et al., 12 Jan 2026). The transit analysis used all available 2-min SAP/PDCSAP light curves, detrended with a 2nd-order polynomial in time. Quadratic limb-darkening coefficients were computed with ExoTETHyS for each band-pass. A global fit was performed jointly on TESS photometry and selected full ground-based transits from LCOGT and SAINT-EX using the Pylightcurve and juliet packages, with MCMC sampling via emcee and broad uniform or Gaussian priors for $M_p \le 4.9\,M_\oplus$6 and $M_p \le 4.9\,M_\oplus$7.

The resulting joint-fit transit solution is:

Parameter	Value
Orbital period $M_p \le 4.9\,M_\oplus$8	$M_p \le 4.9\,M_\oplus$9
Mid-transit epoch $3\sigma$0	$3\sigma$1
Scaled semimajor axis $3\sigma$2	$3\sigma$3
Inclination $3\sigma$4	$3\sigma$5
Impact parameter $3\sigma$6	$3\sigma$7
Radius ratio $3\sigma$8	$3\sigma$9
Transit depth $3\sigma$0	$3\sigma$1 ($3\sigma$2)

No significant transit-timing variations were detected; the O–C residuals are described as showing white noise. The geometry corresponds to a nearly edge-on orbit, as expected for a transiting configuration, and the measured transit depth is consistent with the radius ratio inferred from the joint fit. The radius was then obtained from the fitted $3\sigma$3 and the stellar radius, giving $3\sigma$4, with an uncertainty budget dominated by the photometric precision on $3\sigma$5 and the $3\sigma$6 uncertainty in $3\sigma$7.

4. Radial-velocity follow-up and mass constraint

Radial-velocity follow-up was carried out with CARMENES on the 3.5 m Calar Alto telescope. A total of 62 VIS spectra were acquired between 2022 Jul 10 and 2024 Jan 31, with median internal precision of approximately $3\sigma$8 and S/N at $3\sigma$9 of approximately 457 (Poultourtzidis et al., 12 Jan 2026). VIS radial velocities were extracted with the serval pipeline and nightly zero-point corrections.

Because the host star shows activity on a timescale comparable to the stellar rotation period, the RV analysis adopted a joint RV+GP model using a celerite quasiperiodic kernel, with a prior on $0.88\,\rho_\oplus$0 of $0.88\,\rho_\oplus$1. The inferred Keplerian semi-amplitude is not significantly different from zero:

$0.88\,\rho_\oplus$2

with a $0.88\,\rho_\oplus$3 upper limit of $0.88\,\rho_\oplus$4.

Using the standard circular-orbit RV mass relation with $0.88\,\rho_\oplus$5 and $0.88\,\rho_\oplus$6, the study derived

$0.88\,\rho_\oplus$7

The corresponding bulk-density upper limit is

$0.88\,\rho_\oplus$8

This non-detection in RV does not imply an absence of planetary mass; rather, it limits the mass to a regime in which multiple interior structures remain consistent with the data. The case is therefore one of constrained non-measurement: the radius is precise, but the mass remains bounded only from above.

5. Position in mass-radius space and interior interpretation

In the mass-radius diagram presented in the source study, TOI-4529 b lies in a highly degenerate region where pure rock (100% silicate), water-rich (50% rock–50% H$0.88\,\rho_\oplus$9O), and H–He-enveloped models intersect (Poultourtzidis et al., 12 Jan 2026). The study states that its location is consistent with a volatile-rich "water world," a rocky core plus a thin H–He envelope, and that a bare rocky composition cannot be excluded at the $1.77^{+0.09}_{-0.08}\,R_\oplus$0 mass limit.

The low density upper bound is the principal reason the system is compositionally ambiguous. A planet with radius near $1.77^{+0.09}_{-0.08}\,R_\oplus$1 can correspond to very different interiors depending on the actual mass and volatile inventory. In this case, the available data are insufficient to discriminate decisively among three scenarios:

• Volatile-rich interior: a significant H$1.77^{+0.09}_{-0.08}\,R_\oplus$2O layer, described in the source as a "water world."
• Rocky core plus thin envelope: a compact interior overlain by a modest H–He atmosphere.
• Possibly bare rocky: not excluded at the current $1.77^{+0.09}_{-0.08}\,R_\oplus$3 upper mass limit.

The source study states that TOI-4529 b "leans" toward a water-rich interior, similar to the population of sub-Neptunes orbiting M dwarfs suggested by Luque & Palle. This is an interpretive statement rather than a definitive classification. A plausible implication is that TOI-4529 b occupies a transition regime relevant to debates on whether small M-dwarf sub-Neptunes are predominantly water-rich, envelope-bearing, or compositionally heterogeneous. The study further notes that a tighter mass measurement would break the degeneracy.

6. Atmospheric characterization prospects with JWST

TOI-4529 b was identified in the source study as a favorable target for atmospheric transmission spectroscopy (Poultourtzidis et al., 12 Jan 2026). The reported transmission spectroscopy metric is

$1.77^{+0.09}_{-0.08}\,R_\oplus$4

and the emission spectroscopy metric is

$1.77^{+0.09}_{-0.08}\,R_\oplus$5

The $1.77^{+0.09}_{-0.08}\,R_\oplus$6 TSM interval straddles the threshold of $1.77^{+0.09}_{-0.08}\,R_\oplus$7 for small Neptunes, leading the study to classify the planet as a promising transmission target.

Simulated JWST spectra were generated with TauREx3+ExoTETHyS for several atmospheric scenarios. H/He atmospheres at $1.77^{+0.09}_{-0.08}\,R_\oplus$8 and $1.77^{+0.09}_{-0.08}\,R_\oplus$9 solar metallicity, under clear and hazy assumptions, produce H$T_{\mathrm{eff}}$0O and CH$T_{\mathrm{eff}}$1 features up to several $T_{\mathrm{eff}}$2. By contrast, a pure steam atmosphere, described as H$T_{\mathrm{eff}}$3O/N$T_{\mathrm{eff}}$4, yields signals below $T_{\mathrm{eff}}$5.

The predicted per-transit uncertainties are:

• NIRISS-SOSS: $T_{\mathrm{eff}}$6
• NIRSpec-G395H: $T_{\mathrm{eff}}$7
• MIRI-LRS: $T_{\mathrm{eff}}$8

Under those assumptions, a single transit would suffice to detect an H–He atmosphere, whereas $T_{\mathrm{eff}}$9 transits would be required to detect a steam-dominated envelope. The study further specifies the diagnostic spectral logic: detection of H$\log g$0O/CH$\log g$1 bands at approximately $\log g$2, $\log g$3, $\log g$4, and $\log g$5 would confirm H–He, while a muted spectrum with only weak H$\log g$6O would favor a steam world. This makes atmospheric spectroscopy not merely complementary but potentially decisive for inferring the planet’s interior and volatile inventory.

7. Scientific relevance within the M-dwarf small-planet sample

The source study situates TOI-4529 b within the broader sample of small planets around M dwarfs used to understand planet-formation and composition theories (Poultourtzidis et al., 12 Jan 2026). In that context, the system is notable for combining a well-measured radius with only an upper limit on mass, a configuration that is especially informative about current observational limits and model degeneracies.

Several aspects make TOI-4529 b scientifically consequential. First, the host is an early-M dwarf, a stellar class central to current transiting-planet demographics because short-period small planets are comparatively detectable around such stars. Second, the orbital period of about $\log g$7 places the planet in the compact, highly irradiated regime typical of many TESS M-dwarf discoveries. Third, the planet’s radius and density constraint place it directly in the region where compositional contours overlap, making it a test case for the extent to which radius alone can discriminate among rock, water-rich interiors, and tenuous H–He envelopes.

The current evidence does not establish a unique composition. A common misconception in such systems is that a sub-Neptune radius by itself implies a hydrogen-dominated atmosphere, or conversely that a low mass upper limit uniquely implies a water world. The source study supports neither simplification. Instead, it presents TOI-4529 b as a case in which pure rock, water-rich, and H–He-enveloped structures all remain admissible within present uncertainties, while the low-density bound shifts the balance of plausibility toward a volatile-rich solution. In this sense, TOI-4529 b functions as an observationally constrained but not yet compositionally resolved member of the M-dwarf sub-Neptune population.