TOI-5388 b: Earth-Sized Transiting Exoplanet

• TOI-5388 b is a transiting exoplanet orbiting the nearby M3 dwarf Wolf 346, characterized by an Earth-like radius (~0.99 R⊕) and a short 2.59-day orbital period.
• It was detected using TESS photometry and confirmed with ground-based transit and radial-velocity follow-up from CARMENES, revealing a shallow ~0.1% transit depth.
• The planet’s rocky composition is suggested by its radius while its mass remains constrained as an upper limit (<2.2 M⊕), prompting future high-precision RV studies.

Searching arXiv for the cited source paper to ground the article in current literature. TOI-5388 b is a transiting exoplanet orbiting the nearby early-M dwarf TOI-5388, also known as Wolf 346, Karmn J10087+355, and TIC 407591297. It was confirmed through TESS photometry together with ground-based photometry and radial-velocity follow-up measurements with CARMENES, and it is the terrestrial planet in a three-planet sample of short-period planets around M-dwarf hosts presented in "Characterization of two new transiting sub-Neptunes and a terrestrial planet around M-dwarf hosts" (Poultourtzidis et al., 12 Jan 2026). The planet is essentially Earth-sized, with a radius of $0.99^{+0.07}_{-0.06}\,R_\oplus$, a short orbital period of $2.5946748^{+0.0000042}_{-0.0000030}\,\mathrm{d}$, and only a $3\sigma$ upper limit on its mass, $M_{\rm p}<2.2\,M_\oplus$ (Poultourtzidis et al., 12 Jan 2026).

1. Discovery, confirmation, and observational status

TOI-5388 b was initially detected by TESS through the SPOC pipeline using 2-min cadence data, with additional detection by the Quick-Look Pipeline for full-frame images. The TESS observation summary reports sectors 21 and 48, an observation range from 2020-01-21 to 2022-02-26, a TESS magnitude of $T = 10.5737 \pm 0.0073$ mag, and 18 observed transits across the two sectors (Poultourtzidis et al., 12 Jan 2026).

The TESS signal was independently recovered with the SHERLOCK pipeline, which searched the combined 2-min cadence data over periods of 0.4–30 d and multiple detrending window lengths. The known 2.59 d signal was robustly recovered, and no additional candidate signals were found for TOI-5388. Ground-based follow-up transit observations were also obtained with the LCOGT 1 m network in $z_s$ and $i'$ bands and with MuSCAT2 on the TCS in $g', r', i', z_s'$.

Photometric validation alone was not sufficient. The DAVE centroid module was unreliable because of a bright background eclipsing binary, BD+36 2033, with $T \approx 9.73$ mag located about $1.7'$ northeast of the target and outside the aperture but still affecting centroid analysis. However, a pixel-level light-curve analysis confirmed that the transit depth variation is localized at the target position rather than at the bright neighbor. Gaia sources within the TESS aperture indicate only two much fainter stars, and the brightest blend, TIC 407591299, has $2.5946748^{+0.0000042}_{-0.0000030}\,\mathrm{d}$0 mag, about 5 magnitudes fainter, so flux contamination is negligible. TRICERATOPS yielded a final false-positive probability of about 0.20, so TOI-5388 b was not statistically validated by photometry alone. Its confirmation therefore relied on the radial-velocity detection of the host star and the absence of radial-velocity signatures of a binary (Poultourtzidis et al., 12 Jan 2026).

2. Host star: TOI-5388 (Wolf 346)

TOI-5388 b orbits Wolf 346, an M3.0 V star at a distance of $2.5946748^{+0.0000042}_{-0.0000030}\,\mathrm{d}$1. The stellar parameters reported for the host are $2.5946748^{+0.0000042}_{-0.0000030}\,\mathrm{d}$2, $2.5946748^{+0.0000042}_{-0.0000030}\,\mathrm{d}$3, $2.5946748^{+0.0000042}_{-0.0000030}\,\mathrm{d}$4, $2.5946748^{+0.0000042}_{-0.0000030}\,\mathrm{d}$5, $2.5946748^{+0.0000042}_{-0.0000030}\,\mathrm{d}$6, and $2.5946748^{+0.0000042}_{-0.0000030}\,\mathrm{d}$7 (Poultourtzidis et al., 12 Jan 2026). These quantities were derived using CARMENES spectra and the SteParSyn pipeline for $2.5946748^{+0.0000042}_{-0.0000030}\,\mathrm{d}$8, $2.5946748^{+0.0000042}_{-0.0000030}\,\mathrm{d}$9, and $3\sigma$0, together with the methods of Cifuentes et al. (2020) and Schweitzer et al. (2019) for luminosity, mass, and radius.

The star is relatively inactive in the diagnostics analyzed in the discovery paper. The projected rotation velocity is reported as $3\sigma$1, derived using the Reiners et al. (2018) method and used as input to SteParSyn. Generalized Lomb-Scargle periodograms of spectroscopic activity indicators, specifically H$3\sigma$2, Na D, and Ca II IRT, show no significant periodic peaks for TOI-5388. TESS SAP photometry exhibits some peaks near 15 d, but these were argued to be likely instrumental because they are tied to the TESS orbital period for Sector 48 and background variations; no corresponding period is seen in any other indicator. Long-term ground-based photometry shows a tentative $3\sigma$3 d peak in the TJO data, but this period is absent in the radial velocities and the dataset is not sufficient to confirm it (Poultourtzidis et al., 12 Jan 2026).

The absence of a secure rotation period and the lack of strong periodic activity signals have methodological consequences for the radial-velocity analysis. In contrast to the treatment of TOI-4529 in the same study, no Gaussian-process activity model was included for TOI-5388. This suggests that, within the model adopted in the paper, the radial-velocity noise budget is dominated by instrumental uncertainty together with short-term stellar jitter rather than by a strong rotationally modulated activity signal.

3. Orbital architecture and transit geometry

All final planetary parameters were derived from a joint fit of TESS photometry and CARMENES radial velocities. The orbital period is

$3\sigma$4

with transit mid-time

$3\sigma$5

The impact parameter is

$3\sigma$6

and the orbital inclination is

$3\sigma$7

Eccentricity was fixed to zero in all modeling, so the transit and radial-velocity fits assume a circular orbit (Poultourtzidis et al., 12 Jan 2026).

The joint fit yielded a fitted scaled semi-major axis

$3\sigma$8

and a more precise Keplerian value based on the stellar mass,

$3\sigma$9

Using $M_{\rm p}<2.2\,M_\oplus$0, the physical semi-major axis is approximately

$M_{\rm p}<2.2\,M_\oplus$1

consistent with a very close-in orbit (Poultourtzidis et al., 12 Jan 2026).

The transit is shallow, as expected for an Earth-sized planet transiting a small M dwarf. The planet-to-star radius ratio is

$M_{\rm p}<2.2\,M_\oplus$2

and the phase-folded TESS transit is described as a $M_{\rm p}<2.2\,M_\oplus$3 event. The light curves used were both SAP and PDCSAP products from SPOC. In the photometric analysis, each TESS light curve was detrended with a second-order polynomial in time; ExoTETHyS was used to compute quadratic limb-darkening coefficients in the TESS bandpass, which were then fixed in the transit model; Pylightcurve was used for the initial transit modeling; and the final joint fit was carried out with juliet and batman (Poultourtzidis et al., 12 Jan 2026).

Ground-based transit data were not incorporated into the final global fit for TOI-5388. The study states explicitly that all ground-based transits for this target were omitted because they failed the signal-to-noise and completeness criteria. They were nonetheless modeled individually to test for transit timing variations and for consistency of transit depth and shape. No significant transit timing variations or secular timing trends were detected (Poultourtzidis et al., 12 Jan 2026).

4. Radius, mass constraint, and radial-velocity modeling

The radius of TOI-5388 b is

$M_{\rm p}<2.2\,M_\oplus$4

which makes it essentially Earth-sized within uncertainties. The radial-velocity semi-amplitude from the joint analysis is

$M_{\rm p}<2.2\,M_\oplus$5

which is formally consistent with zero. Accordingly, the study reports no significant radial-velocity detection of the planetary signal and derives only a $M_{\rm p}<2.2\,M_\oplus$6 upper limit on the mass,

$M_{\rm p}<2.2\,M_\oplus$7

together with a $M_{\rm p}<2.2\,M_\oplus$8 upper density limit of

$M_{\rm p}<2.2\,M_\oplus$9

or

$T = 10.5737 \pm 0.0073$0

The density is computed in the usual way,

$T = 10.5737 \pm 0.0073$1

but because only an upper limit on $T = 10.5737 \pm 0.0073$2 is available, the quoted values are upper limits (Poultourtzidis et al., 12 Jan 2026).

The CARMENES VIS radial-velocity dataset for TOI-5388 comprises 42 measurements obtained between 2022-04-02 and 2024-06-23. The spectrograph covers 0.52–0.96 $T = 10.5737 \pm 0.0073$3m at $T = 10.5737 \pm 0.0073$4. The median internal precision is $T = 10.5737 \pm 0.0073$5, the observed RMS is $T = 10.5737 \pm 0.0073$6, and the median signal-to-noise ratio at 7370 Å is 333. The paper notes that the expected planetary semi-amplitude is about $T = 10.5737 \pm 0.0073$7 for an Earth-size planet at this period around a $T = 10.5737 \pm 0.0073$8 star, and that with 42 radial velocities the dataset would be able to detect a $T = 10.5737 \pm 0.0073$9 amplitude of $z_s$0 only in the best case. This instrumental and astrophysical noise environment prevents a precise mass determination but allows an upper limit (Poultourtzidis et al., 12 Jan 2026).

The radial-velocity model for TOI-5388 includes a single-planet Keplerian with period and phase constrained by TESS, a circular orbit, a systemic velocity offset $z_s$1, linear and quadratic trends, and a white-jitter term $z_s$2 added in quadrature to the internal radial-velocity errors. The reported values are

$z_s$3

$z_s$4

$z_s$5

all consistent with zero, and

$z_s$6

The joint fit was performed with juliet using RadVel for the radial-velocity model and dynesty nested sampling with 1000 live points and a convergence criterion $z_s$7 (Poultourtzidis et al., 12 Jan 2026).

5. Composition, thermal regime, and system-level interpretation

The central compositional result is that TOI-5388 b is most probably rocky, given its Earth-like radius. The paper states this explicitly while emphasizing that the mass remains unconstrained beyond an upper bound. In the mass-radius diagram presented in the study, the planet is plotted as a radius measurement with an upper limit in mass and lies close to the rocky-planet region. The comparison set consists of planets around M dwarfs with $z_s$8 and $z_s$9, together with compositional curves from Zeng et al. (2019): pure rock or Earth-like, 50% Earth-like rock plus 50% H$i'$0O water worlds, and 49.95% Earth-like plus 49.95% H$i'$1O plus 0.1% H$i'$2 at $i'$3 K (Poultourtzidis et al., 12 Jan 2026).

The allowed compositions are therefore consistent with Earth-like silicate-plus-iron interiors, rocky interiors with somewhat different core fraction, or, at the lower end of allowed densities, possibly a small water or volatile layer. Large, low-density water-rich or H/He-rich sub-Neptune configurations are excluded by the radius itself. The details text further states that a substantial, extended H/He envelope would generally inflate the radius above about $i'$4–$i'$5 at these equilibrium temperatures, which is not observed. This suggests that TOI-5388 b occupies the terrestrial rather than the sub-Neptune regime (Poultourtzidis et al., 12 Jan 2026).

The equilibrium temperature is reported as

$i'$6

computed assuming a Bond albedo of $i'$7 and full heat redistribution, using the standard zero-eccentricity relation

$i'$8

Given $i'$9 K and the small semi-major axis, the planet lies well interior to the traditional habitable zone. The study does not compute the incident flux explicitly, but the details text derives

$g', r', i', z_s'$0

and concludes that the planet is a hot terrestrial world rather than a temperate one (Poultourtzidis et al., 12 Jan 2026).

Within the observational scope of the paper, TOI-5388 is currently a single-planet system. Additional planets were searched for using SHERLOCK transit searches over 0.4–30 d, GLS periodograms of radial velocities and activity indicators, and transit timing variation analysis. No additional periodic signals attributable to further planets were found in either photometry or radial velocities. No explicit dynamical stability analysis was presented, but with only one known planet on a 2.6 d orbit, no obvious stability concern is identified in the source text (Poultourtzidis et al., 12 Jan 2026).

6. Position within the M-dwarf planet population and future prospects

Within the three-planet sample analyzed in the source paper, TOI-5388 b is the smallest planet. The other two planets are TOI-1243 b, with $g', r', i', z_s'$1, $g', r', i', z_s'$2, and $g', r', i', z_s'$3 d, and TOI-4529 b, with $g', r', i', z_s'$4, $g', r', i', z_s'$5 at $g', r', i', z_s'$6, and $g', r', i', z_s'$7 d. The study summarizes the compositional interpretation by classifying TOI-1243 b as likely a low-density small gaseous planet or water-world, TOI-4529 b as compatible with a water-world composition, and TOI-5388 b as a terrestrial planet whose radius suggests a rocky nature (Poultourtzidis et al., 12 Jan 2026).

The planet also sits below the radius valley discussed for small exoplanets. The details text states that the radius valley between about $g', r', i', z_s'$8 and $g', r', i', z_s'$9, identified around Sun-like stars, also appears around M dwarfs but with different underlying physics, including atmospheric loss and compositional diversity. At roughly $T \approx 9.73$0, TOI-5388 b lies well below that valley and is part of the rocky planet population around M dwarfs. The same population-level discussion notes that CARMENES contributes a large fraction of the precise mass measurements for small M-dwarf planets, with 24 of 66 planets in the mass-radius diagram using CARMENES radial velocities (Poultourtzidis et al., 12 Jan 2026).

Future work is limited primarily by the radial-velocity precision required for an Earth-sized planet in this regime. Synthetic radial-velocity time-series simulations in the paper estimate that more than 300 CARMENES radial velocities would be required to achieve a mass precision better than 15% for TOI-5388 b, the threshold adopted for meaningful interior modeling. Since only 42 radial velocities are currently available, the paper concludes that the present study can only place an upper limit on the mass (Poultourtzidis et al., 12 Jan 2026).

The atmospheric characterization outlook is mixed. The study computes the Transmission Spectroscopy Metric and Emission Spectroscopy Metric of Kempton et al. (2018) by Monte Carlo sampling 100,000 points from the mass-radius posteriors. For TOI-5388 b, it reports

$T \approx 9.73$1

and

$T \approx 9.73$2

The thresholds quoted in the paper are TSM $T \approx 9.73$3 for terrestrial planets with $T \approx 9.73$4 to place a target in the top quartile for transmission spectroscopy with JWST, and ESM $T \approx 9.73$5 for emission spectroscopy. The 1$T \approx 9.73$6 TSM range includes values above 10, so the planet may be promising for transmission spectroscopy if it has an atmosphere and if its mass is better constrained; the ESM is well below 7.5, so it is not a prime target for thermal emission spectroscopy with JWST. The paper does not present explicit atmospheric signal calculations or JWST spectral simulations for TOI-5388 b, unlike its treatment of TOI-1243 b, and emphasizes that more precise mass measurements are needed to constrain atmospheric scenarios meaningfully (Poultourtzidis et al., 12 Jan 2026).

The resulting picture is that of a nearby, Earth-sized, short-period planet around a small M3 dwarf, with an excellent radius measurement, a well-determined ephemeris, an equilibrium temperature of about 490 K, and a mass currently constrained only by an upper limit. The source paper treats these properties as strongly suggestive of a rocky composition and identifies TOI-5388 b as a valuable target for future ultra-precise radial-velocity campaigns and, contingent on improved mass constraints, transmission spectroscopy (Poultourtzidis et al., 12 Jan 2026).