TOI-6223 b: A Neptune-Sized Transiting Exoplanet

• TOI-6223 b is a Neptune-sized exoplanet identified by TESS and confirmed with multifaceted ground-based observations, including high-resolution imaging and statistical validation.
• Its physical parameters—5.12 R⊕ radius, 714 K equilibrium temperature, and a 3.86-day orbit—place it in the Neptunian ridge, bridging the gap between super-Earths and hot Neptunes.
• The planet is a prime target for precision radial velocity measurements and atmospheric characterization, which are key for studying planet migration and envelope retention mechanisms.

TOI-6223 b is a transiting Neptune-sized exoplanet orbiting an early M dwarf star with a short period of approximately 3.86 days. Discovered and validated using data from the Transiting Exoplanet Survey Satellite (TESS), as well as extensive ground-based and ancillary observations, TOI-6223 b exhibits physical and contextual properties that make it a salient target for precision radial velocity (RV) measurements and atmospheric characterization, contributing to the empirical mapping of exoplanet population features in the period–radius parameter space (Yalçınkaya et al., 5 Sep 2025).

1. Detection Methods and Validation Strategy

TOI-6223 b was identified in TESS sectors 57 and 84 via both the Science Processing Operations Center (SPOC) and the Quick Look Pipeline (QLP). The TESS data yielded a signal-to-noise ratio of 19.2 with a transit depth of approximately 7782 ppm. A multifaceted validation protocol combined photometric confirmation and assessment of false positive scenarios:

• Ground-based light curves (TUG-T100, Artemis, MuSCAT2) confirmed recurring transit signatures with achromatic depths, ruling out color-dependent contamination.
• High-resolution imaging using speckle and adaptive optics (AO) excluded significant nearby stellar companions, mitigating background eclipsing binary scenarios.
• Statistical validation with TRICERATOPS returned a false positive probability (FPP) of

$\mathrm{FPP} = (1.309 \pm 2.215)\times10^{-5},$

providing robust confirmation of the planet’s existence.

2. Physical and Orbital Properties

Joint modeling of TESS and ground-based light curves characterizes TOI-6223 b as a warm, Neptune-sized body:

Parameter	Value
Planetary radius	$R_p=5.12^{+0.24}_{-0.25}\;R_\oplus$
Equilibrium temp.	$T_{\rm eq}=714\pm14\;\mathrm{K}$
Orbital period	$P=3.85556\pm0.00040$ days
Host brightness	$K_{mag} < 11$

These parameters place TOI-6223 b as slightly larger than Neptune and orbiting with a semimajor axis corresponding to a moderately irradiated regime for its stellar type.

3. Contextual Placement in the Period–Radius Diagram

In the period–radius plane, TOI-6223 b occupies what is termed the “Neptunian ridge.” This region demarcates the transition between the sparsely populated Neptunian desert (short-period Neptune analogs are rare, likely due to photoevaporation or dynamical instabilities) and the more moderately populated “savanna” zone.

• Comparison to the radius valley: Several planets in the same study (TOI-1743 b, TOI-5799 b/c) are super-Earths ($R_p \sim 1.7$–$1.8\,R_\oplus$) within the so-called “radius valley,” thought to separate predominantly rocky from volatile-rich planets.
• TOI-6223 b's markedly larger size and location above the valley provide an empirical anchor for the Neptunian ridge population.
• A plausible implication is that TOI-6223 b’s present orbit and bulk properties may result from high-eccentricity migration rather than in situ formation, bolstered by the planet’s short period and the architecture of its system.

4. Observational Prospects and Follow-up Potential

Several aspects of TOI-6223 b render it a model target for future characterization campaigns:

• Radial velocity signals: The expected RV semi-amplitude, $K_\star=14.9^{+5.3}_{-3.4}\;\mathrm{m\,s}^{-1}$, is well within the sensitivity range of current spectrometers, enabling mass, density, and eccentricity measurements.
• Atmospheric transmission: Transmission Spectroscopy Metric (TSM) is reported at $62\pm18$, reflecting strong atmospheric signal prospects. Simulations with PandExo and PLATON tools predict prominent near-infrared features (e.g., H$_2$O, CH$_4$, CO$_2$).
• Host star brightness: With $K_{mag}<11$, TOI-6223 b's host is suitable for precision RV and JWST/NIRSpec atmospheric observations.

5. Comparative Analysis with Super-Earths in the Population

Contrasting TOI-6223 b with the super-Earths in the same study:

Planet	$R_p\;(R_\oplus)$	Period (days)	Regime
TOI-6223 b	$5.12^{+0.24}_{-0.25}$	$3.86$	Neptunian ridge
TOI-1743 b	$1.83^{+0.11}_{-0.10}$	$4.27$	Radius valley
TOI-5799 b/c	$1.733^{+0.096}_{-0.090}$, $1.76^{+0.11}_{-0.10}$	$4.17,\;14.01$	Radius valley

• While the super-Earths illuminate properties and formation mechanisms at the rocky-to-gaseous planetary transition (the radius valley), TOI-6223 b provides insight into the dynamics and survivability of Neptune-sized planets at short orbital periods.
• This suggests that comparative studies between these populations can constrain theories of photoevaporation, core-powered mass-loss, and dynamical migration in close-in exoplanet systems.

6. Implications for Exoplanet Formation and Evolution

TOI-6223 b's position, physical parameters, and robust validation support several avenues of theoretical and empirical research:

• The planet offers an anchor point for mapping the otherwise under-populated Neptunian ridge and places constraints on envelope retention and migration efficiency at short orbital periods.
• Planned RV and JWST atmospheric campaigns will yield mass, density, and compositional constraints, discriminating among core-heavy and envelope-rich formation scenarios.
• A plausible implication is that planets like TOI-6223 b, with high FPP validation and atmospheric accessibility, will catalyze further revision of exoplanet demographic models, especially with respect to close-in Neptune analogs and their evolutionary trajectories.

TOI-6223 b thus serves as a critical datapoint for understanding the diversity and evolutionary history of short-period Neptune-class exoplanets, complementing existing surveys of the super-Earth and mini-Neptune populations (Yalçınkaya et al., 5 Sep 2025).