TOI-654 b: M-dwarf Sub-Neptune

• TOI-654 b is a transiting sub-Neptune orbiting a mid-M dwarf with a 1.53-day period, a mass of 8.71 M⊕, and a radius of 2.38 R⊕.
• It is characterized using TESS photometry and Subaru/IRD radial velocities, yielding a bulk density of 3.59 g/cm³ that highlights mass–radius degeneracy.
• Its location near the M-dwarf radius valley and the Neptune desert edge makes it an excellent target for future atmospheric emission spectroscopy with JWST and Ariel.

TOI-654 b is a transiting short-period sub-Neptune orbiting the mid-M dwarf TOI-654, classified as M2V. Its orbital period is $P = 1.527561 \pm 0.000001$ day, its mass is $M_{\rm p} = 8.71 \pm 1.25\,M_\oplus$, and its radius is $R_{\rm p} = 2.378 \pm 0.089\,R_\oplus$. The resulting bulk density, $\rho_{\rm p} = 3.59 \pm 0.65$ g cm$^{-3}$, places it among small planets whose interior structure is not uniquely determined by mass and radius alone. In the 2025 analysis that reported its mass with Subaru/IRD and updated its radius from TESS and MuSCAT photometry, TOI-654 b was identified as lying near the radius valley for M-dwarf hosts and on the outer edge of the Neptune desert, while also being a suitable target for future atmospheric observations, especially emission spectroscopy with the James Webb Space Telescope and Ariel (Ikuta et al., 22 Jul 2025).

1. Stellar host and system definition

TOI-654 is a mid-M dwarf with spectral type M2V. Its effective temperature is $T_{\rm eff} = 3521^{+33}_{-48}$ K, its mass is $M_\star = 0.419 \pm 0.009\,M_\odot$, and its radius is $R_\star = 0.430 \pm 0.013\,R_\odot$. The metallicity derived from IRD spectroscopy is $[\mathrm{Fe/H}] = 0.12 \pm 0.18$ dex, the luminosity is $L_\star = 0.0246 \pm 0.0005\,L_\odot$, and the distance is $M_{\rm p} = 8.71 \pm 1.25\,M_\oplus$0 pc from Gaia DR3 parallax. Reported photometry includes Gaia $M_{\rm p} = 8.71 \pm 1.25\,M_\oplus$1 mag, TESS $M_{\rm p} = 8.71 \pm 1.25\,M_\oplus$2 mag, $M_{\rm p} = 8.71 \pm 1.25\,M_\oplus$3 mag, and 2MASS $M_{\rm p} = 8.71 \pm 1.25\,M_\oplus$4, $M_{\rm p} = 8.71 \pm 1.25\,M_\oplus$5, $M_{\rm p} = 8.71 \pm 1.25\,M_\oplus$6 mag (Ikuta et al., 22 Jul 2025).

The stellar rotation and activity diagnostics indicate a relatively quiet host. The projected rotation is constrained as $M_{\rm p} = 8.71 \pm 1.25\,M_\oplus$7 km s$M_{\rm p} = 8.71 \pm 1.25\,M_\oplus$8, no significant periodic photometric variability is reported in TESS, ZTF, or ASAS-SN, and the RV activity indicators FWHM, CRX, dLW, and dV show no significant periodic signals. The age is not constrained in the paper.

These host properties are important because the planet’s physical interpretation and atmospheric follow-up depend on the stellar context. A mid-M dwarf with low apparent activity is favorable for precise NIR radial velocities and for spectroscopic follow-up, while the small stellar radius amplifies both the transit depth and the planet-to-star flux ratio.

2. Orbital architecture and measured planetary parameters

The adopted solution for TOI-654 b is a circular orbit with $M_{\rm p} = 8.71 \pm 1.25\,M_\oplus$9 fixed. The orbital period is $R_{\rm p} = 2.378 \pm 0.089\,R_\oplus$0 day, the reference transit epoch is $R_{\rm p} = 2.378 \pm 0.089\,R_\oplus$1, the semi-major axis is $R_{\rm p} = 2.378 \pm 0.089\,R_\oplus$2 au, the inclination is $R_{\rm p} = 2.378 \pm 0.089\,R_\oplus$3 deg, and the impact parameter is $R_{\rm p} = 2.378 \pm 0.089\,R_\oplus$4. The transit depth is $R_{\rm p} = 2.378 \pm 0.089\,R_\oplus$5, corresponding to $R_{\rm p} = 2.378 \pm 0.089\,R_\oplus$6 ppm, and the total transit duration is $R_{\rm p} = 2.378 \pm 0.089\,R_\oplus$7 hr (Ikuta et al., 22 Jul 2025).

The eccentric fit yields $R_{\rm p} = 2.378 \pm 0.089\,R_\oplus$8 with $R_{\rm p} = 2.378 \pm 0.089\,R_\oplus$9 at 99.73% CL and $\rho_{\rm p} = 3.59 \pm 0.65$0 deg. The Bayesian evidence favors the circular model very slightly. The ephemeris is described as tightly constrained, and transit timing variations are not reported.

For a short-period sub-Neptune, the system is geometrically favorable for repeated transit and eclipse observations. A plausible implication is that observational cadence, rather than event rarity, is the limiting factor for future characterization.

3. Mass, radius, density, and interior interpretation

The radial-velocity semi-amplitude is $\rho_{\rm p} = 3.59 \pm 0.65$1 m s$\rho_{\rm p} = 3.59 \pm 0.65$2, yielding a planetary mass of $\rho_{\rm p} = 3.59 \pm 0.65$3. Combined with the updated radius $\rho_{\rm p} = 3.59 \pm 0.65$4, this gives a bulk density of $\rho_{\rm p} = 3.59 \pm 0.65$5 g cm$\rho_{\rm p} = 3.59 \pm 0.65$6. The equilibrium temperature is $\rho_{\rm p} = 3.59 \pm 0.65$7 K for $\rho_{\rm p} = 3.59 \pm 0.65$8 and $\rho_{\rm p} = 3.59 \pm 0.65$9 K for $^{-3}$0, and the incident flux is $^{-3}$1 (Ikuta et al., 22 Jul 2025).

Using the stated formulas, the semi-major axis follows from Kepler’s third law,

$^{-3}$2

which gives $^{-3}$3 au for $^{-3}$4 and $^{-3}$5 day, consistent with the tabulated value. The planetary surface gravity is

$^{-3}$6

or equivalently

$^{-3}$7

which yields $^{-3}$8 m s$^{-3}$9. The density expression,

$T_{\rm eff} = 3521^{+33}_{-48}$0

gives $T_{\rm eff} = 3521^{+33}_{-48}$1 g cm$T_{\rm eff} = 3521^{+33}_{-48}$2, consistent with the reported $T_{\rm eff} = 3521^{+33}_{-48}$3 g cm$T_{\rm eff} = 3521^{+33}_{-48}$4.

The mass–radius interpretation is explicitly degenerate. The measured density is consistent with either a rocky core plus a thin H/He envelope, for example $T_{\rm eff} = 3521^{+33}_{-48}$5 wt% H/He atop an Earth-like core, or a rocky core with a volatile-rich interior layer. The paper states that the planet is “equally close” to both composition families under sub-Neptune conditions, with surface $T_{\rm eff} = 3521^{+33}_{-48}$6 K at 1 mbar. This suggests that bulk properties alone do not uniquely resolve whether TOI-654 b is a lightly enveloped rocky planet or a volatile-rich sub-Neptune.

Parameter	Value	Note
$T_{\rm eff} = 3521^{+33}_{-48}$7	$T_{\rm eff} = 3521^{+33}_{-48}$8	Circular-orbit solution
$T_{\rm eff} = 3521^{+33}_{-48}$9	$M_\star = 0.419 \pm 0.009\,M_\odot$0	Updated radius
$M_\star = 0.419 \pm 0.009\,M_\odot$1	$M_\star = 0.419 \pm 0.009\,M_\odot$2 g cm$M_\star = 0.419 \pm 0.009\,M_\odot$3	Bulk density
$M_\star = 0.419 \pm 0.009\,M_\odot$4	$M_\star = 0.419 \pm 0.009\,M_\odot$5 m s$M_\star = 0.419 \pm 0.009\,M_\odot$6	Derived from $M_\star = 0.419 \pm 0.009\,M_\odot$7
$M_\star = 0.419 \pm 0.009\,M_\odot$8	$M_\star = 0.419 \pm 0.009\,M_\odot$9 K	For $R_\star = 0.430 \pm 0.013\,R_\odot$0

4. Observational basis and inference methodology

The mass determination is based on Subaru/IRD radial velocities covering 930–1740 nm at $R_\star = 0.430 \pm 0.013\,R_\odot$1. The data set consists of 85 spectra obtained over 2019-05-19 to 2023-05-11, with SNR 18–72 per pixel at 1000 nm, typical internal precision 6.32 m s$R_\star = 0.430 \pm 0.013\,R_\odot$2, and RV rms 9.43 m s$R_\star = 0.430 \pm 0.013\,R_\odot$3. Individual NIR spectra were combined into template spectra after telluric removal, and RVs were extracted via segment-wise forward-modeling. GLS periodograms revealed the planetary signal at $R_\star = 0.430 \pm 0.013\,R_\odot$4 d in RV with FAP $R_\star = 0.430 \pm 0.013\,R_\odot$5, while no significant power was seen in O–C residuals or in the activity indicators (Ikuta et al., 22 Jul 2025).

The photometric basis combines TESS PDC-SAP light curves from Sectors 9, 36, 45, 46, 62, and 72 at 2-min cadence with ground-based simultaneous multicolor photometry from MuSCAT2 on the TCS 1.52 m and MuSCAT3 on the FTN 2 m. Two high-quality MuSCAT events, on 2021-03-19 for MuSCAT2 and 2021-01-28 for MuSCAT3, were used in the joint fit. Ground-based systematics were modeled with a joint transit+baseline framework using jaxoplanet and a Gaussian Process baseline per band with a Matérn 3/2 kernel, common timescale, band-dependent amplitudes, and jitter terms per band. Limb darkening was modeled with the quadratic law in transformed coefficients $R_\star = 0.430 \pm 0.013\,R_\odot$6, with priors from LDtk based on PHOENIX models and uncertainties enlarged by $R_\star = 0.430 \pm 0.013\,R_\odot$7 to accommodate M-dwarf model systematics.

The paper also notes that TOI-654.01 had already been statistically validated by Hord et al. (2024) using TESS data from Sectors 9, 36, 45, and 46 together with Gaia DR3, while TFOP reconnaissance photometry ruled out nearby eclipsing binaries and centroid offsets. The final parameter estimation used a joint fit of TESS, MuSCAT2/3 photometry, and IRD RVs with jaxoplanet and NumPyro, employing the No-U-Turn Sampler and comparing circular and eccentric models via Bayesian evidence.

5. Radius valley, Neptune desert, and evolutionary significance

TOI-654 b is described as one of the unique planets located around the radius valley and also on the outer edge of the Neptune desert. The radius valley is the observed bimodality in small-planet radii near $R_\star = 0.430 \pm 0.013\,R_\odot$8, separating bare rocky super-Earths from sub-Neptunes with volatile envelopes. At $R_\star = 0.430 \pm 0.013\,R_\odot$9 and $[\mathrm{Fe/H}] = 0.12 \pm 0.18$0 day, TOI-654 b lies near this valley for M-dwarf hosts. The Neptune desert is a paucity of short-period ($[\mathrm{Fe/H}] = 0.12 \pm 0.18$1–4 days) Neptune/sub-Neptune-size planets, and TOI-654 b lies on the outer edge of the desert in period and near its lower-radius boundary (Ikuta et al., 22 Jul 2025).

This location makes the system informative for testing atmospheric loss mechanisms, including photoevaporation and core-powered mass loss, as well as the stellar-mass dependence of the radius valley. The paper further notes that the planet’s short period and moderate density suggest either a migrated, volatile-rich planet that retained some envelope, or a partially stripped sub-Neptune that has lost a significant fraction of its primordial H/He.

Because the host is a mid-M dwarf with no strong RV or photometric activity signals, TOI-654 b is a comparatively clean case for examining these processes around low-mass stars. A plausible implication is that it can serve as a discriminating benchmark for whether the M-dwarf radius valley and the Neptune desert are primarily sculpted by irradiation-driven erosion or by earlier formation and migration pathways.

6. Atmospheric characterization prospects

The atmospheric metrics reported in the paper are a Transmission Spectroscopy Metric of $[\mathrm{Fe/H}] = 0.12 \pm 0.18$2 and an Emission Spectroscopy Metric of $[\mathrm{Fe/H}] = 0.12 \pm 0.18$3. The authors identify TOI-654 b as suitable especially for emission observations with JWST and Ariel. For equilibrium temperature under isotropic reradiation,

$[\mathrm{Fe/H}] = 0.12 \pm 0.18$4

the tabulated values are $[\mathrm{Fe/H}] = 0.12 \pm 0.18$5 K for $[\mathrm{Fe/H}] = 0.12 \pm 0.18$6 and $[\mathrm{Fe/H}] = 0.12 \pm 0.18$7 K for $[\mathrm{Fe/H}] = 0.12 \pm 0.18$8 (Ikuta et al., 22 Jul 2025).

Assuming an isothermal atmosphere,

$[\mathrm{Fe/H}] = 0.12 \pm 0.18$9

the scale height is approximately $L_\star = 0.0246 \pm 0.0005\,L_\odot$0 km for H/He with $L_\star = 0.0246 \pm 0.0005\,L_\odot$1 and approximately $L_\star = 0.0246 \pm 0.0005\,L_\odot$2 km for a heavier water-rich atmosphere with $L_\star = 0.0246 \pm 0.0005\,L_\odot$3. The order-of-magnitude transmission signal per scale height,

$L_\star = 0.0246 \pm 0.0005\,L_\odot$4

is approximately $L_\star = 0.0246 \pm 0.0005\,L_\odot$5 ppm for H/He and approximately $L_\star = 0.0246 \pm 0.0005\,L_\odot$6 ppm for a water-rich atmosphere. A clear atmosphere with $L_\star = 0.0246 \pm 0.0005\,L_\odot$7–7 scale heights would therefore yield $L_\star = 0.0246 \pm 0.0005\,L_\odot$8–450 ppm in the H/He case, whereas a heavier atmosphere would remain at the level of tens of ppm. The secondary-eclipse contrast in a blackbody approximation,

$L_\star = 0.0246 \pm 0.0005\,L_\odot$9

is of order a few $M_{\rm p} = 8.71 \pm 1.25\,M_\oplus$00 ppm in the mid-IR, consistent with $M_{\rm p} = 8.71 \pm 1.25\,M_\oplus$01.

The main science objective of such observations is to distinguish between a thin H/He envelope and a water-rich envelope or interior, thereby addressing the mass–radius degeneracy. The paper also identifies atmospheric metallicity and aerosols or haze as key targets. Transmission spectroscopy is described as moderate rather than optimal, whereas thermal emission at 8–12 $M_{\rm p} = 8.71 \pm 1.25\,M_\oplus$02m is presented as promising. The low apparent stellar activity and the use of NIR instrumentation and multicolor photometry are favorable for this program.