TOI-2449 b / NGTS-36 b: Warm Jupiter Exoplanet

• TOI-2449 b/NGTS-36 b is a warm Jupiter exoplanet with a 106-day orbit around a bright G-type star.
• Combined TESS, NGTS, and high-resolution spectroscopic data yielded precise measurements of its mass, radius, and orbital eccentricity.
• Its uninflated density and measurable heavy element content offer key insights into gas giant formation, migration, and atmospheric evolution.

TOI-2449 b, also known as NGTS-36 b, is a “warm Jupiter” exoplanet characterized by a 106-day orbital period around a bright G-type star. Its discovery and detailed characterization expand the modest sample of known long-period transiting giant planets, whose properties provide crucial insights into gas giant formation, migration, and atmospheric evolution. TOI-2449 b’s measured physical parameters—including its mass, radius, bulk density, interior composition, and orbital eccentricity—exemplify the relatively unaltered state of warm exoplanets, especially with respect to the effects of lower stellar irradiation and reduced tidal forces compared to hot Jupiters.

1. Detection and Observational Workflow

TOI-2449 b was initially flagged in the Transiting Exoplanet Survey Satellite (TESS) data as a single transit event, consistent with the challenge that most long-period transiting giants present only mono-transit signals in TESS. To accurately determine its orbital period, ground-based photometry was undertaken using the Next Generation Transit Survey (NGTS). NGTS captured additional partial transits, confirming a consistent transit depth with TESS and narrowing the possible orbital period aliases. A subsequent NGTS detection definitively established the 106.14468-day period.

High-resolution spectroscopic observations were performed using four instruments—CHIRON, CORALIE, FEROS, and HARPS—to obtain radial velocity (RV) measurements. These RV data are essential for confirming the planetary nature of the candidate and deriving its mass and orbital parameters. The analysis implemented a joint fit of the photometric data and radial velocity signals using the modelling tool “juliet,” applying a Keplerian model alongside sinusoidal components to capture long-term trends, notably a ∼3-year signal attributed to the host star’s magnetic activity cycle.

Instrument	Data Type	Role in Workflow
TESS	Photometry	Initial transit detection
NGTS	Photometry	Period confirmation, transit depth
CHIRON/CORALIE/FEROS/HARPS	Spectroscopy	Radial velocity, mass and eccentricity determination

2. Physical and Orbital Parameters

The combined photometric and RV modelling yielded highly precise system properties:

• Orbital period: $P = 106.14468^{+0.00022}_{-0.00021}$ days.
• Planetary mass: $M_p = 0.70^{+0.05}_{-0.04} \, M_J$.
• Planetary radius: $R_p = 1.001 \pm 0.009 \, R_J$.
• Semi-major axis: $a = 0.449^{+0.011}_{-0.008}$ au.
• Eccentricity: $e = 0.098^{+0.028}_{-0.030}$.
• Transit duration: ~8.26 hours.
• Additional RV periodicity: ≈3 years, attributed to stellar magnetic activity (not planetary companion).

These parameters position TOI-2449 b within the warm Jupiter regime and indicate a gas giant with properties minimally affected by extreme stellar irradiation. The orbital eccentricity is modestly nonzero, which may reflect a non-trivial migration or dynamical history.

3. Implications of Bulk Properties and Orbital Dynamics

A 106-day orbital period subjects TOI-2449 b to substantially lower stellar irradiation compared to hot Jupiters, suppressing radius inflation and preserving the planet’s intrinsic interior structure and composition. The mass-radius combination yields a density consistent with a non-inflated gas giant. Its semi-major axis provides a context for formation scenarios involving migration from several astronomical units.

The measured orbital eccentricity ($e \approx 0.10$) is notable as the expectation from disc-driven migration or multiple-body interactions is for partial orbital circularization. A plausible implication is that TOI-2449 b has experienced processes that did not fully dampen the eccentricity, such as incomplete tidal circularization or interactions with another massive body.

The ∼3-year RV signal is best explained by the host star’s magnetic activity cycle, as indicated by chromospheric indices (e.g., $H_\alpha$) and RV diagnostics, rather than by another planetary body.

4. Interior Composition and Evolution Models

Interior characterization was achieved using a planetary evolution code (“completo”) to construct interior models assuming predominance of hydrogen and helium with a fraction of heavy elements. The derived heavy element content is $M_Z = 11^{+6}_{-5} \, M_\oplus$. No significant radius inflation mechanisms are needed, congruent with the modest equilibrium temperature.

Metal enrichment is quantified as $\frac{Z_p}{Z_\star} = 3.3^{+2.5}_{-1.8}$, where $Z_p$ and $Z_\star$ represent the planet’s and star’s heavy element mass fractions, respectively. This suggests that TOI-2449 b likely formed at greater orbital separations, accreting heavy elements and gas, and subsequently migrated inward.

Parameter	Value	Interpretation
Heavy element mass ($M_Z$)	$11^{+6}_{-5} \, M_\oplus$	Core + envelope metals
Metal enrichment ($Z_p/Z_\star$)	$3.3^{+2.5}_{-1.8}$	Compared to host star

5. Prospects for Atmospheric Characterization

With an equilibrium temperature of approximately 400 K (assuming a Jupiter-like Bond albedo), TOI-2449 b is optimally situated for detailed atmospheric studies of temperate giant planets. At these temperatures, nitrogen chemistry transitions become accessible, specifically the transition from $\mathrm{N}_2$ dominance (in hot atmospheres) to $\mathrm{NH}_3$ (in cooler conditions). This presents opportunities for transmission spectroscopy to probe molecular features indicative of atmospheric composition and elemental ratios.

Synthetic transmission spectra, generated using equilibrium chemistry models (with petitRADTRANS), demonstrate prominent absorption features for $\mathrm{CH}_4$ and $\mathrm{NH}_3$ at wavelengths near 1.5, 2.2, 3.3, and 3 μm. This facilitates studies of the C–N–O ratios and constrains formation and migration models.

6. Broader Context and Future Directions

TOI-2449 b/NGTS-36 b augments the sample of long-period transiting warm Jupiters, providing a benchmark for testing theories of giant planet formation, accretion histories, and post-formation migration. Its well-constrained mass, radius, orbital elements, and interior metallicity make it a critical target for advancing models of planet-star metallicity correlation and migration pathways.

The detailed characterization of such warm giants—especially those whose bulk properties and atmospheric states are relatively unperturbed—deepens understanding of the physical processes governing giant planet evolution. The equilibrium temperature and observable atmospheric features position TOI-2449 b as a valuable object for future atmospheric studies, specifically on nitrogen-bearing molecules, enabling direct comparison with both hot Jupiters and sub-Jovian companions.

A plausible implication is that continued detection and multi-technique characterization of similar systems will further illuminate the diversity of warm giant exoplanets, refine models of heavy element accretion, and clarify the timescales and mechanisms underpinning orbital migration.