TOI-5799 b: Radius-Valley Super-Earth

• TOI-5799 b is a validated super-Earth exoplanet in the radius valley, characterized by a 1.73 R⊕ size and a 4.16-day orbital period.
• Detected via TESS and confirmed with multi-band photometry and high-resolution imaging, its precise measurements inform atmospheric loss and interior composition theories.
• The system’s favorable host star and multi-planet architecture enable comparative studies on atmospheric composition and evolutionary pathways in small exoplanets.

TOI-5799 b is a validated transiting super-Earth exoplanet orbiting an early M2 dwarf. Detected by the TESS mission and characterized via an extensive multi-instrument observational campaign, TOI-5799 b possesses properties that place it directly within the "radius valley"—a critical regime in the exoplanet period–radius parameter space. Its combination of precise physical measurements, favorable host star characteristics, and its location within an architectural system containing an additional transiting planet establishes TOI-5799 b as a benchmark target for testing theories of planet formation, evolution, and atmospheric composition.

1. Discovery and Multi-Tiered Validation

TOI-5799 b was initially identified as a strong transiting planet candidate in TESS photometric data (sectors 54 and 81), exhibiting periodic flux dimmings consistent with a planetary transit signature. The validation protocol incorporated several stringent tests to rule out false positives:

• Statistical Validation: The TRICERATOPS package computed an exceptionally low false positive probability ($\mathrm{FPP} \simeq 5.3\times10^{-5}$), ruling out blends and eclipsing binaries as plausible explanations.
• Multi-Color Ground-Based Photometry: Transits were confirmed concurrently in multiple bandpasses via TUG-T100, TRAPPIST-North, LCO, and MuSCAT2 observations. The achromaticity of the signal—consistent transit depths across all filters—demonstrated the absence of confounding color effects.
• High-Resolution Imaging: Adaptive optics and speckle imaging from Palomar, SOAR, and other facilities conclusively excluded nearby unresolved sources that could produce a spurious transit signal.

This comprehensive approach confirmed the planetary nature of TOI-5799 b and provided robust constraints on possible astrophysical false positives.

2. Physical and Orbital Properties

Global modeling of the combined TESS and high-precision ground-based photometry yielded precise planetary and orbital parameters:

• Radius: $R_p = 1.733^{+0.096}_{-0.090}\,R_\oplus$
• Equilibrium Temperature: $T_{\rm eq} = 505 \pm 16$ K
• Orbital Period: $P \approx 4.1645$ days
• Host Star: Spectral type M2, $K_\mathrm{mag} < 11$
• Scaled Semi-Major Axis: $a/R_\star$ consistent with a short-period, close-in configuration

Other inferred orbital properties include a small impact parameter and high transit probability, both typical for close-in small planets transiting mid-M dwarfs.

Parameter	Value	Uncertainty
Radius ($R_p$)	$1.733\,R_\oplus$	$+0.096,\ -0.090$
Orbital Period ($P$)	$4.1645$ days	—
$T_\mathrm{eq}$	$505~\mathrm{K}$	$\pm16$

The physical characteristics are indicative of a planet with a substantial rocky component, pending confirmation via mass measurement.

3. The Radius Valley Context

TOI-5799 b is situated within the so-called "radius valley"—a region of the exoplanet population where planets between $\sim$1.5–2.0 $R_\oplus$ are underrepresented. This feature is interpreted as a result of atmospheric loss processes, such as photoevaporation or core-powered mass loss, that divide the planet population into predominantly rocky super-Earths and gas-rich sub-Neptunes.

• Significance: The measured radius ($\sim1.73\,R_\oplus$) places TOI-5799 b directly in this transition regime. Its existence enables critical tests of:
  • Atmosphere retention and loss models
  • Mechanisms responsible for the sculpting of the bimodal radius distribution

TOI-5799's planetary system structure, which includes TOI-5799 c (a second transiting planet with $R_p = 1.76^{+0.11}_{-0.10}~R_\oplus$ and $P = 14.01$ days), provides a comparative laboratory for evaluating composition and evolution near the inner edge of the habitable zone.

4. Radial Velocity and Atmospheric Characterization Prospects

The host star's brightness ($K_\mathrm{mag} < 11$) and the predicted radial velocity semi-amplitude ($K \sim 3.5$ m/s) of TOI-5799 b are favorable for precise mass measurement campaigns utilizing next-generation near-infrared spectrographs. A plausible implication is that, with sufficient RV precision over a modest number of orbits, both the mass and density regime of TOI-5799 b can be established, constraining its interior composition.

• JWST Transmission Spectroscopy: The planet's high Transmission Spectroscopy Metric (TSM) identifies it as an optimal target for atmospheric feature detection. Modeling with PandExo and PLATON indicates that strong broadband and molecular signatures—including H$_2$O, CH$_4$, and CO$_2$—should be accessible in the near- and mid-infrared.
• Comparative Analysis: The combination of radius, stellar brightness, and system architecture enables comparative atmospheric studies of both TOI-5799 b and TOI-5799 c.

The detectability of key atmospheric constituents will facilitate direct tests of competing scenarios for atmosphere retention and volatile content in the radius valley regime.

5. Implications for Planet Formation and Evolution

The evolutionary pathway of TOI-5799 b is tightly coupled to its location within the radius valley. Two dominant hypotheses are considered:

• Rocky Super-Earth Scenario: The planet may represent a core-dominated world that never accreted a significant volatile envelope, shaped by late-stage gas depletion in the primordial nebula.
• Atmospheric Loss Scenario: Alternatively, the planet could have originated as a sub-Neptune that experienced substantial photoevaporative mass loss or core-powered mass loss, resulting in the stripping of its H/He envelope.

The precise characterization of mass and atmospheric composition—via radial velocity follow-up and transmission spectroscopy—will directly inform which of these scenarios is preferred. A plausible implication is that the system may elucidate the processes governing the transition from rocky to gas-rich planets around low-mass stars, contributing to resolving the broader question of the origin of bimodality in the exoplanet radius distribution.

6. Comparative System Architecture and Broader Relevance

The TOI-5799 system, with both TOI-5799 b (short period: 4.1645 days, $T_{\rm eq} = 505$ K) and TOI-5799 c (longer period: 14.01 days, near the inner edge of the habitable zone), provides a multi-planet laboratory for examining planet formation and evolutionary pathways under nearly identical stellar irradiation and composition conditions. This facilitates:

• Direct comparative analysis of atmospheric evolution as a function of incident flux and orbital period
• Constraints on models of planetary migration and dynamical stability in compact systems around M dwarfs

A plausible implication is that such systems are invaluable for testing the universality of the radius valley and the timescales of atmospheric loss across varying stellar hosts.

7. Prospects for Ongoing and Future Study

The combination of confirmed planetary nature, amenability to mass measurement, and high transmission spectroscopy yield establishes TOI-5799 b as a priority target for follow-up campaigns. Key avenues for future research include:

• Precise Radial Velocity Monitoring: To constrain mass and, by extension, bulk density and interior structure.
• JWST and Ground-Based Atmospheric Characterization: To detect molecular features and infer atmospheric composition, metallicity, and cloud/haze properties.
• Formation and Evolutionary Modeling: Joint analysis of TOI-5799 b and c will inform the relative importance of formation time, migration history, and atmospheric loss processes in determining present-day physical properties of small planets orbiting mid-M dwarfs.

Continued paper of TOI-5799 b is likely to yield insights that are broadly applicable to the understanding of planetary system architectures, atmospheric evolution, and the origins of the small planet population around low-mass stars (Yalçınkaya et al., 5 Sep 2025).