TOI-2345: Thick Disk Exoplanet System
- TOI-2345 is a kinematic thick disk mid-K dwarf with precise Gaia astrometry and high-resolution spectroscopy confirming its old, α-enhanced, and metal-poor nature.
- The system hosts two transiting planets that span the small-planet radius valley, with the ultra-short-period planet b being rocky and c retaining a volatile envelope.
- Comprehensive TESS and CHEOPS photometry, HARPS spectroscopy, and Bayesian modeling establish TOI-2345 as a benchmark for understanding exoplanet formation and evolution in chemically distinct Galactic environments.
TOI-2345 is a kinematic thick disk K-dwarf (mid–K spectral type) located at a distance of 81.27 ± 0.30 pc (Gaia DR3: π=12.297 ± 0.015 mas) and notable for hosting two transiting sub-Neptune-sized exoplanets that precisely span the small-planet radius valley. The host star is metal-poor ([Fe/H]=–0.10 ± 0.07 dex), α-enhanced ([Mg/H]=+0.17 ± 0.05 dex, [Si/H]=+0.12 ± 0.07 dex), with (U, V, W)ₗₛᵣ=(–29.97 ± 0.06, –100.23 ± 0.12, –13.10 ± 0.19) km s⁻¹ and thick-disk membership probability P(thick disk)=85%. The system's planets, TOI-2345 b and TOI-2345 c, have orbital periods of 1.05 and 21 days, respectively. Comprehensive photometric and spectroscopic monitoring, coupled with Bayesian hierarchical modeling and interior-structure inference using devolatilized stellar abundances, allows TOI-2345 to serve as a benchmark for exoplanet formation, radius valley sculpting, and population processes in chemically distinct Galactic environments (Eschen et al., 14 Oct 2025).
1. Host Star Characteristics
TOI-2345's stellar properties are established through combined Gaia DR3 astrometry, photometric analyses, and high-resolution spectroscopy. The infrared-flux method yields a radius R⋆=0.729 ± 0.007 R☉ and mass M⋆=0.727 ± 0.033 M☉, with a mean density ρ⋆=1.88 ± 0.10 ρ☉. The effective temperature is Tₑ𝚏𝚏=4687 ± 60 K and surface gravity log g=4.57 ± 0.06 dex. Low projected rotational velocity (v sin i=1.41 ± 0.32 km s⁻¹) and an isochrone-inferred age of 6.3 ± 4.7 Gyr indicate an old, slowly rotating star. Enrichment in [Mg/H] and [Si/H] relative to Fe, along with large space velocity and kinematics, confirm thick-disk, α-enhanced, and metal-poor character.
2. Planetary System Architecture and Orbital Properties
The system comprises two confirmed transiting planets:
| Parameter | TOI-2345 b | TOI-2345 c |
|---|---|---|
| Orbital period (P, d) | 1.0528573(–2.6/+2.5)×10⁻⁶ | 21.064302(–4.1/+4.1)×10⁻⁵ |
| Semi-major axis (a, AU) | 0.01705(–0.0023/+0.0022) | 0.1257(–0.0017/+0.0016) |
| Inclination (°, i) | 86.9(–1.8/+1.3) | 89.914(–0.057/+0.083) |
| Eccentricity (e) | 0 (fixed) | 0 (fixed) |
No significant transit-timing variations are detected. TOI-2345 b orbits at ultra-short period (<1.1 d), at a=0.01705 AU, while TOI-2345 c is separated by over an order of magnitude in period and distance, orbiting at a=0.1257 AU. Both orbits are consistent with zero eccentricity.
3. Observational Data and Modeling Framework
TOI-2345 is observed photometrically by TESS (sectors 3, 4: 30-min cadence; sectors 30, 31: 10-min cadence) with SPOC PDCSAP light curves (2-hr CDPP ≈ 180–200 ppm), and by CHEOPS with 5 visits at 60 s exposures (final PSF-photometry, RMS ≈ 630 ppm). Spectroscopic follow-up comprises 26 epochs from HARPS (S/N₅₀ ≈ 33, R ≈ 115,000; wavelength: 380–680 nm), processed using both standard DRS and the s-bart template-matching code (the latter reducing RMS RV scatter from 4.6 to 3.4 m s⁻¹).
Joint data modeling employs the juliet v2 package, combining batman (transit modeling) and radvel (radial velocities) under a dynesty nested sampling framework to obtain posterior distributions and ln Z model evidence. A Matern-3/2 Gaussian Process is applied to TESS photometry to model residual systematics. “Jitter” and “offset” terms are explicitly modeled for each dataset; limb-darkening is parametrized with q₁ and q₂ following Kipping (2013).
4. Physical Properties and Interior Structure of Planets
Derived planetary parameters from the joint fit are:
| Parameter | TOI-2345 b | TOI-2345 c |
|---|---|---|
| Radius (Rₚ) [R⊕] | 1.504(–0.044/+0.047) | 2.451(–0.046/+0.045) |
| Mass (Mₚ) [M⊕] | 3.49 ± 0.85 | 7.27(–2.45/+2.27) |
| Density (ρₚ) [g cm⁻³] | 5.64(–1.46/+1.48) | 2.71(–0.93/+0.86) |
| T_eq [K] | 1478 ± 20 | 544 ± 7 |
| Incident flux (Sₚ) [S⊕] | 791 ± 43 | 14.6 ± 0.8 |
| TSM | ≈43 | ≈33 |
Bulk densities are calculated as . The smaller, highly irradiated b is rock-dominated, while c is larger and less dense.
Interior structure modeling uses plaNETic (Egger et al.), with mass fractions of iron core (CMF), silicate mantle (MMF), water (WMF), and H/He (AMF), adopting compositional priors from devolatilized stellar abundances (ExoInt, Wang et al. 2019). Two prior sets are considered: B4 (water-poor) and A4 (water-rich). Inference results are summarized as follows:
| Planet | Prior | CMF | MMF | WMF | AMF |
|---|---|---|---|---|---|
| TOI-2345 b | B4 | 0.167⁺⁰.⁰⁹⁹₋⁰.¹¹¹ | 0.763⁺⁰.¹⁰⁷₋⁰.⁰⁹⁴ | ≈0.0003⁺⁰.⁰⁰³₋⁰.⁰⁰₃ | 0.070⁺⁰.039₋⁰.070 |
| TOI-2345 b | A4 | 0.16 ± 0.11 | 0.80 ± 0.11 | 0.026⁺⁰.⁰⁴³₋⁰.⁰¹₉ | ≈0 |
| TOI-2345 c | B4 | 0.16 ± 0.11 | 0.83 ± 0.11 | ≈0 | 0.0113⁺⁰.0034₋⁰.0036 |
| TOI-2345 c | A4 | 0.10⁺⁰.⁰⁷⁴₋⁰.⁰⁷¹ | 0.526⁺⁰.¹⁵⁸₋⁰.⁰⁹⁸ | 0.367⁺⁰.⁰⁹⁴₋⁰.²¹³ | 0.011⁺⁰.٠١٩₋⁰.٠١١ |
Water-poor priors indicate b is almost entirely rocky (bare super-Earth), while c requires a significant volatile/gas fraction if water-rich.
5. Radius Valley and Evolutionary Implications
The two planets straddle the empirical “radius valley” (Fulton 2017; Van Eylen 2018) at ≈1.7 R⊕: b (1.50 R⊕) is below (super-Earth), while c (2.45 R⊕) sits above (sub-Neptune). The high incident flux on b (S≈800 S⊕) is consistent with extreme atmospheric erosion via photo-evaporation or core-powered mass loss (Owen, Ginzburg 2018), yielding a bare, rocky composition. In contrast, c’s lower flux (S≈15 S⊕) allows the retention of a thin (~1%) H/He envelope or, alternatively, a significant water layer (A4 prior). The wide period separation exemplifies a “radius valley” system shaped by contrasting irradiation and evolutionary tracks.
TOI-2345’s association with the thick disk, its α-enhancement, and old age confirm that valley formation and ultra-short-period (USP) planet migration/evaporation operate robustly across Galactic populations. The migration scenario for b is supported by the lack of close-in planetary companions within 2 d, consistent with low-eccentricity migration mechanisms (Pu & Lai 2019).
6. Methodological Significance and Context in Exoplanet Demographics
The system is characterized using high-precision photometry (TESS, CHEOPS), precision RVs (HARPS + s-bart), and joint Bayesian inference (juliet + dynesty). The approach leverages modern Gaussian Process treatment of photometric noise, robust RV extraction via template matching, and interior modeling that links devolatilized stellar abundances to planetary composition. Masses and radii are determined to ≲10% precision, enabling rigorous interior constraints.
TOI-2345 expands the limited set of well-characterized planetary systems around thick disk stars and exemplifies the power of multi-instrument, multi-modal Bayesian characterization for exoplanet physics (Eschen et al., 14 Oct 2025). The system thereby provides a clear empirical testbed for theories of the origin and evolution of the small-planet radius valley in different Galactic contexts.
7. Implications for Planet Formation and Population Synthesis
The chemical, kinematic, and architectural properties of TOI-2345 indicate that evaporation-driven sculpting of the radius valley and the formation of ultra-short-period planets occur similarly in old, α-enhanced, metal-poor (thick disk) stellar populations as in solar-neighborhood thin disk counterparts. This supports universality in the underlying physical processes of atmospheric erosion, migration, and planet composition diversity across Galactic populations.
A plausible implication is that the observed radius valley is fundamentally governed by irradiation-driven mass-loss mechanisms, as the system conforms with expected mass, radius, and compositional transitions predicted by photo-evaporation and core-powered mass loss models across different stellar chemical environments. The system underscores the importance of interior-structure modeling anchored to precise, devolatilized stellar abundances in constraining exoplanet compositions.