TIC118798035: A Benchmark Exoplanetary System

• TIC118798035 System is a compact exoplanetary system with three transiting warm giant planets orbiting a G-dwarf star, confirmed through TESS, photometry, and RV spectroscopy.
• The integration of space-based photometry with ground-based RV data and N-body modeling reveals significant transit timing variations that elucidate gravitational interactions and orbital dynamics.
• Accurate mass, radius, and bulk metallicity measurements make TIC118798035 a critical benchmark for comparative planetology and future atmospheric studies via transmission spectroscopy.

The TIC118798035 system is a compact exoplanetary system hosting three transiting giant planets orbiting a G-dwarf star. Identified in TESS full-frame images and confirmed with ground-based photometry and high-precision radial velocity spectroscopy, this system possesses uniquely well-characterized planetary masses, radii, and orbital dynamics. The planets exhibit distinct transit timing variations (TTVs) due to mutual gravitational interactions, enabling simultaneous modeling of transit and RV data via N-body integrations. TIC118798035 is notable for being the only known system with more than two transiting planets larger than 0.5 $R_J$ with detailed physical and orbital characterization, making it a critical benchmark for comparative planetary sciences and atmospheric studies.

1. Detection and Observational Confirmation

The initial identification of the three planetary candidates in TIC118798035 arose from periodic transit-like signals in TESS mission full-frame images. Automated machine-learning algorithms flagged these candidates, necessitating extensive follow-up to discriminate genuine planetary transits from false positives due to blending or background eclipsing binaries. Ground-based photometric time series confirmed the on-target origin of the transits. Radial velocity (RV) follow-up observations employed FEROS (MPG 2.2 m), HARPS (ESO 3.6 m), and ESPRESSO (VLT) spectrographs, yielding RV curves consistent with multiple giant planet companions.

The combination of high-cadence space-based photometry and phase-resolved RVs allowed a robust statistical confirmation, excluding scenarios such as hierarchical stellar systems or diluted eclipsing binaries. The methodology employed joint fitting of transit and RV data, providing self-consistent orbital and dynamical parameters for all three planets.

2. Planetary Properties and Orbital Dynamics

Planet masses, radii, and orbital periods were derived simultaneously using advanced N-body modeling frameworks—specifically, packages such as “juliet” and Exo-Striker—which incorporate both RV semi-amplitudes and TTV signals in their likelihood construction. Noteworthy is the adoption of non-parametric transit timing fits, avoiding strict periodicity assumptions and instead accommodating correlated deviations induced by planet–planet interactions.

The planets’ measured properties are:

Planet	Mass ($M_J$)	Radius ($R_J$)	Period (days)
TIC118798035 b	$0.0250_{-0.0022}^{+0.0024}$	$0.655\pm0.018$	$11.5075_{-0.0015}^{+0.0017}$
TIC118798035 c	$0.403_{-0.025}^{+0.024}$	$0.973\pm0.023$	$22.5644_{-0.0024}^{+0.0027}$
TIC118798035 d	$0.773_{-0.052}^{+0.051}$	$0.923\pm0.044$	$48.9244_{-0.0054}^{+0.0048}$

All three planets are classified as "warm giants"—their orbital periods are sufficiently long to avoid the severe radius inflation processes observed in ultra-short-period hot Jupiters. Significant TTVs were detected for each planet, providing the leverage to tightly constrain planetary masses, relative inclinations, and eccentricities in a manner superior to RV-only or transit-only methodologies.

3. Bulk Metallicities and Formation Constraints

Determination of the planets’ bulk metallicity (mass fraction of heavy elements) utilized planetary interior modeling anchored by the empirically derived masses and radii. The derived metallicities do not uniformly adhere to the mass–metallicity trend observed among solar system giants, wherein lower-mass gas giants are typically more metal-rich.

For instance, while the Saturn-mass (c) and Jupiter-mass (d) planets marginally align with existing empirical mass–metallicity relations, the sub-Neptune (b) exhibits a lower limit for metallicity that may eclipse theoretical predictions, especially in plausible water-rich structural models. This result suggests divergent formation pathways or post-formation processing, such as differential accretion histories or dynamical migration and resonance escape.

The apparent deviations from solar system benchmarks invoke scenarios with locally enhanced solid accretion, rapid core formation, or alternatively, gas-poor envelope accumulation beyond the snow line with subsequent inward migration. The observed heavy element mass budgets help constrain proto-disk chemistry, migration histories, and dynamical sculpting in the system.

4. Orbital Interactions and Transit Timing Variations

Mutual gravitational interactions among the planets produce detectable non-Keplerian perturbations, manifesting as transit timing variations (TTVs). The modeling adopted does not enforce strictly periodic transit ephemerides; instead, each transit is individually scheduled, with N-body orbital fits explaining correlated offset patterns.

The parameter estimation treats TTVs and RVs as a coupled inference problem, with the N-body model providing dynamical consistency over the full dataset. This permits the isolation of subtle mass ratios and possible orbital resonances, and rules out oblique orbital configurations or additional massive companions within the detection completeness.

5. Comparative Planetology and Future Atmospheric Studies

TIC118798035 is, as documented, the only system with more than two transiting planets exceeding 0.5 $R_J$ for which all masses and radii are accurately measured. This unique configuration allows direct comparative planetology—probing differences in formation, migration, and atmospheric composition across a continuous mass range from sub-Neptune to Jupiter.

The system serves as an ideal target for future transmission spectroscopy campaigns, especially with observatories like JWST. Studying atmospheric metallicity gradients and chemical tracers (e.g., CO$_2$, H$_2$O, CH$_4$) across its planets will corroborate or challenge current models of planet formation and migration. Direct observation of features in such a well-controlled laboratory setting, where planetary masses and radii are precisely known, will provide stringent tests of atmospheric enrichment processes and the universality of mass–metallicity relations.

6. Broader Relevance and Outlook

The compact arrangement and diversity of giant planets in TIC118798035, together with the well-resolved TTVs and RVs, contribute critical empirical evidence to models of exoplanet system architecture. The system straddles the boundary between dynamically active multi-planet architectures and the classical isolated giant planet regime. Its detailed paper informs both the statistical frequency and dynamical outcomes of multi-giant planet formation, particularly in the warm orbital regime outside strong stellar irradiation.

Continued high-precision transit monitoring and RV tracking, as well as dedicated atmospheric characterization, will resolve whether the compositional anomalies reflect formative chemical gradients in the natal disk, post-formation dynamical instability, or stochasticity in late-stage planetesimal accretion. This suggests an ongoing role for TIC118798035 as a reference case in exoplanet systematics, atmospheric physics, and the theory of planet formation and evolution.