---
title: 'TOI-4529 b: M-dwarf Sub-Neptune'
url: https://www.emergentmind.com/topics/toi-4529-b
type: topic
---

# TOI-4529 b: M-dwarf Sub-Neptune

Searching arXiv for the cited paper to ground the article in the primary source.
arXiv search query: 2601.07414
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" [2601.07414]. 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 $0.88\,\rho_\oplus$ [2601.07414]. 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 [2601.07414]. 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 [2601.07414]. 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 |
| $T_{\mathrm{eff}}$ | $3697 \pm 71\,\mathrm{K}$ |
| $\log g$ | $4.61 \pm 0.07$ [cgs] |
| $[\mathrm{Fe/H}]$ | $-0.24 \pm 0.07$ |
| $R_\star$ | $0.480 \pm 0.019\,R_\odot$ |
| $M_\star$ | $0.482 \pm 0.023\,M_\odot$ |
| $\rho_\star$ | $6.1 \pm 0.8\,\mathrm{g\,cm^{-3}}$ |

Additional stellar constraints include a projected rotational velocity of $v\sin i < 2\,\mathrm{km\,s^{-1}}$ and a rotation period of $P_{\mathrm{rot}} \approx 20.7 \pm 1.4\,\mathrm{d}$, derived from activity indicators and long-term photometry. These stellar properties are important because they directly enter the transit and radial-velocity inference chain: $R_\star$ sets the absolute planetary radius, $M_\star$ 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 [2601.07414]. 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 $P$ and $T_0$.

The resulting joint-fit transit solution is:

| Parameter | Value |
|---|---|
| Orbital period $P$ | $5.879577^{+0.000011}_{-0.000010}\,\mathrm{d}$ |
| Mid-transit epoch $T_0$ | $2459701.3329 \pm 0.0006\,\mathrm{BJD}$ |
| Scaled semimajor axis $a/R_\star$ | $21.5^{+1.2}_{-2.9}$ |
| Inclination $i$ | $88.7^{+0.9}_{-0.8}\,^\circ$ |
| Impact parameter $b$ | $0.32 \pm 0.20$ |
| Radius ratio $R_p/R_\star$ | $0.0338^{+0.0009}_{-0.0008}$ |
| Transit depth $\delta$ | $\approx 0.00114$ ($\approx 1140\,\mathrm{ppm}$) |

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 $R_p/R_\star$ and the stellar radius, giving $R_p = 1.77^{+0.09}_{-0.08}\,R_\oplus$, with an uncertainty budget dominated by the photometric precision on $R_p/R_\star$ and the $4\%$ uncertainty in $R_\star$.

## 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 $1.9\,\mathrm{m\,s^{-1}}$ and S/N at $7370\,\text{\AA}$ of approximately 457 [2601.07414]. 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 $P_{\mathrm{rot}}$ of $20 \pm 3\,\mathrm{d}$. The inferred Keplerian semi-amplitude is not significantly different from zero:
$$
K = 0.95^{+0.58}_{-0.79}\,\mathrm{m\,s^{-1}},
$$
with a $3\sigma$ upper limit of $K_{3\sigma} \simeq 2.84\,\mathrm{m\,s^{-1}}$.

Using the standard circular-orbit RV mass relation with $e=0$ and $\sin i \approx 1$, the study derived
$$
M_p \le 4.9\,M_\oplus \quad (3\sigma).
$$
The corresponding bulk-density upper limit is
$$
\rho_p \le 4.8\,\mathrm{g\,cm^{-3}} \quad (\approx 0.88\,\rho_\oplus,\ 3\sigma).
$$

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$_2$O), and H–He-enveloped models intersect [2601.07414]. 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 $3\sigma$ mass limit.

The low density upper bound is the principal reason the system is compositionally ambiguous. A planet with radius near $1.8\,R_\oplus$ 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$_2$O 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 $3\sigma$ 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 [2601.07414]. The reported transmission spectroscopy metric is
$$
\mathrm{TSM} = 102^{+60}_{-31},
$$
and the emission spectroscopy metric is
$$
\mathrm{ESM} = 4.3^{+1.1}_{-1.0}.
$$
The $1\sigma$ TSM interval straddles the threshold of $\mathrm{TSM} > 80$ 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 $1\times$ and $100\times$ solar metallicity, under clear and hazy assumptions, produce H$_2$O and CH$_4$ features up to several $10^2\,\mathrm{ppm}$. By contrast, a pure steam atmosphere, described as H$_2$O/N$_2$, yields signals below $40\,\mathrm{ppm}$.

The predicted per-transit uncertainties are:
- NIRISS-SOSS: $\sim 69\,\mathrm{ppm}$
- NIRSpec-G395H: $\sim 74\,\mathrm{ppm}$
- MIRI-LRS: $\sim 78\,\mathrm{ppm}$

Under those assumptions, a single transit would suffice to detect an H–He atmosphere, whereas $\gtrsim 5$ transits would be required to detect a steam-dominated envelope. The study further specifies the diagnostic spectral logic: detection of H$_2$O/CH$_4$ bands at approximately $1.4$, $2.3$, $3.3$, and $4.5\,\mu\mathrm{m}$ would confirm H–He, while a muted spectrum with only weak H$_2$O 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 [2601.07414]. 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 $5.88\,\mathrm{d}$ 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.

Source: https://www.emergentmind.com/topics/toi-4529-b