TOI-6692 b: Eccentric Giant Exoplanet Characterization

• TOI-6692 b is a transiting giant exoplanet with a securely determined mass, radius, and moderate eccentricity, identified from a single TESS transit and extensive RV follow-up.
• Its 131-day orbital period and 11-hour transit duration were precisely refined using combined high-cadence ground-based photometry and EXOFASTv2 global fits.
• A persistent long-term RV drift indicates the presence of an outer companion, offering key insights into dynamical interactions and migration pathways in giant planet systems.

TOI-6692 b is a transiting giant exoplanet discovered via a single transit in TESS photometry and characterized through extensive radial velocity (RV) and ground-based photometric follow-up (Bieryla et al., 22 Jan 2026). It is notable for its moderate eccentricity, secure mass and radius determination at an orbital period exceeding 100 days, and evidence for an outer companion inferred from a long-term RV drift. The multi-modal discovery and subsequent characterization workflows exemplify best practices for confirming long-period single-transit TESS candidates.

1. Discovery Workflow and Observational Timeline

TOI-6692 b (TIC 324609409) was initially identified as a candidate by citizen-science teams—the Visual Survey Group and Planet Hunters TESS—as a community TESS Object of Interest (cTOI) in October 2021, later promoted to TOI-6692 in October 2023. TESS observed the target in Sectors 27 (600s cadence) and 39, 66, 67, 93, 94 (120s cadence), with only a single transit event (depth ≈ 6 ppt; duration ≈ 11 h) recorded in Sector 39. The single-transit nature precluded a SPOC pipeline TOI designation at the time.

The period was subsequently constrained using multi-instrument RV monitoring:

• Planet Finder Spectrograph (PFS, Magellan II 6.5m): 45 spectra (2022 May–2024 Jun; R ≈ 115,000)
• CHIRON (CTIO 1.5m): 31 spectra (2021 Sep–2022 Sep; R ≈ 80,000)
• FEROS (MPG 2.2m): 6 spectra (2023 Aug–2024 Jun; R ≈ 48,000)
• CORALIE (Euler 1.2m): 7 spectra (2024 Apr–Jul; R ≈ 60,000)

A total of 89 RVs revealed a 130-day Keplerian signal and a persistent long-term linear drift. Ground-based photometric follow-up was performed using the LCOGT network: twelve consecutive nights on six 1m telescopes at CTIO, SAAO, SSO with SINISTRO cameras in Sloan i′. While ingress and egress of the ∼11 hr transit were not captured, a 5–6 ppt in-transit signal was detected on 2025 May 24 UT, permitting a refined orbital period determination.

2. Fundamental Physical and Orbital Parameters

Fitting combined data with the EXOFASTv2 suite (incorporating TESS photometry, multi-instrument RVs, Gaia parallax, broadband SED, and spectroscopic priors), the following system parameters were derived:

Parameter	Value	Uncertainty/Notes
Orbital period	$P = 131.125 \pm 0.012$ days (updated global fit)	Previous: $130.57^{+0.42}_{-0.40}$ days
Transit epoch	$T_0 = 2459378.4174 \pm 0.0022$ BJD_TDB
Transit duration	$T_{14} = 11.06 \pm 0.24$ hours
Eccentricity	$e = 0.537 \pm 0.010$
Arg. of Periastron	$\omega_p = -16.6^{+7.9}_{-7.7}$ degrees
Planet mass	$M_p = 0.62^{+0.080}_{-0.070} M_J$
Planet radius	$R_p = 1.04 \pm 0.05 R_J$
Bulk density	$\rho_p \simeq 0.73$ g cm$^{-3}$	Computed: $0.62 M_J / 1.04^3 R_J$
Semi-major axis	$a = 0.512^{+0.014}_{-0.012}$ AU
RV semi-amplitude	$K = 28.5^{+3.2}_{-2.7}$ m s$^{-1}$
RV drift	$\dot{\gamma} = -0.0280^{+0.0046}_{-0.0045}$ m s$^{-1}$ day$^{-1}$

These metrics place TOI-6692 b within the regime of moderately inflated giant planets on eccentric orbits at intermediate periods.

3. Radial Velocity and Transit Characterization Equations

Standard equations quantifying the system's dynamics were employed:

• Radial velocity semi-amplitude:

$$K = \left(\frac{2\pi G}{P}\right)^{1/3} \frac{M_p \sin i}{(M_* + M_p)^{2/3}} \cdot \frac{1}{\sqrt{1-e^2}}$$

where $K$ is the RV semi-amplitude, $P$ the period, $M_p$ the planet’s mass, $M_*$ the stellar mass, $e$ the orbital eccentricity.

• Transit duration (assuming circular chord):

$$T_{14} = \frac{P}{\pi} \arcsin\left[\frac{\sqrt{(R_* + R_p)^2 - (a \cos i)^2}}{a}\right]$$

where $T_{14}$ is the total transit duration, $R_*$ the stellar radius, $R_p$ the planetary radius, $a$ the semi-major axis, and $i$ the inclination.

These relations were central to the EXOFASTv2 global fits used to infer system architecture.

4. Eccentricity and Migration Histories

The measured eccentricity ($e \approx 0.54$) is significant but falls distinctly below the $e \gtrsim 0.8$ regime characteristic of high-eccentricity tidal migrators (e.g., HD 80606 b). Instead, planet-planet scattering models such as those discussed by Petrovich (2014) predict $e \leq 0.8$ for runaway encounters, fully consistent with TOI-6692 b’s observed eccentricity. This suggests an evolutionary scenario involving dynamical interactions and subsequent weak tidal damping rather than recent high-e migration.

A plausible implication is that TOI-6692 b presents a valuable testbed for constraining intermediate-eccentricity migration mechanisms and the population outcomes of scattering events at moderate orbital periods.

5. Ephemeris Refinement and Prospects for Future Monitoring

The initial single TESS transit left an approximate $\pm$4.6 day uncertainty on the orbital period. The multi-night LCOGT in-transit detection in 2025 narrowed this to $\pm$0.012 day, yielding an updated ephemeris:

• Next transit windows can be scheduled with $<$20 min timing uncertainty.

Such precise period refinement enables targeted capture of full ingress/egress in future campaigns. Ongoing and proposed monitoring via ground- and space-based observatories (including LCOGT and CHEOPS) will permit detailed transit curve characterization and transmission spectroscopy (TSM $\simeq$ 23), supporting atmospheric and interior structure inference.

6. Long-Term Radial Velocity Trends and Outer Companion Constraints

A persistent linear RV slope ($\dot{\gamma} \sim -0.028$ m s$^{-1}$ day$^{-1}$ across $\sim$800 days) signals the gravitational influence of an outer companion. Speckle imaging with Zorro/Gemini-South excludes stellar companions brighter than $\Delta$mag $\leq$ 6 at separations 0.1–1.2″ (30–376 AU), imposing an upper mass limit $<$400 $M_J$ for resolved objects at $>$20 AU.

Interior to $\sim$20 AU, RV residuals constrain $M \sin i$ versus $a$. Continued RV monitoring is needed to establish the outer companion’s orbital period and mass, a plausible implication being the presence of low-mass substellar or planetary companions influencing TOI-6692 b’s long-term dynamical evolution.

7. Significance within TESS and Exoplanet Population Context

TOI-6692 b is one of the few single-transit TESS candidates at $P > 100$ days for which a secure mass, radius, and eccentricity have been measured. Its confirmation highlights the efficacy and necessity of coordinated citizen-science identification, high-cadence RV monitoring, and multi-night ground-based photometry for constraining ephemerides and system parameters in cases of limited transit coverage. The night-to-night stability demonstrated by LCOGT and similar facilities proves crucial for refining long-period single-transit planet discoveries.

For population studies, TOI-6692 b enters an exclusive regime, providing empirical leverage on late-stage migration channels and the architectures of giant planet systems beyond 1 AU (Bieryla et al., 22 Jan 2026).