Detection and characterisation of a 106-day transiting Jupiter : TOI-2449 b / NGTS-36 b

TL;DR

TOI-2449 b / NGTS-36 b is a transiting warm Jupiter with a 106-day period around a bright G0-type star. The team combines TESS and NGTS photometry with RVs from CORALIE, CHIRON, FEROS, and HARPS in a joint Juliet analysis to derive $M_p = 0.70^{+0.05}_{-0.04}$ M$_J$, $R_p = 1.001 \\pm 0.009$ R$_J$, and a modest eccentricity $e = 0.098^{+0.028}_{-0.030}$, placing it at $a = 0.450^{+0.010}_{-0.010}$ au with $T_{eq} \approx 400^{+40}_{-70}$ K. Interior structure modelling yields $M_z = 11^{+6}_{-5}$ M$_\oplus$ and $Z_p/Z_* \approx 3.3^{+2.5}_{-1.8}$, indicating a modest heavy-element enrichment for a planet of this mass. A separate 3-year RV signal is attributed to the star’s magnetic activity cycle rather than a companion. This system provides a valuable benchmark for studying nitrogen chemistry in temperate giant atmospheres and for testing formation and migration theories of warm Jupiters, with RM and transmission spectroscopy offering promising avenues for deeper characterization.

Abstract

Only a handful of transiting giant exoplanets with orbital periods longer than 100 days are known. These warm exoplanets are valuable objects as their radius and mass can be measured leading to an in-depth characterisation of the planet's properties. Thanks to low levels of stellar irradiation and large orbital distances, the atmospheric properties and orbital parameters of warm exoplanets remain relatively unaltered by their host star, giving new insights into planetary formation and evolution. We aim at extending the sample of warm giant exoplanets with precise radii and masses. Our goal is to identify suitable candidates in the Transiting Exoplanet Survey Satellite (TESS) data and perform follow-up observations with ground-based instruments. We use the Next Generation Transit Survey (NGTS) to detect additional transits of planetary candidates in order to pinpoint their orbital period. We also monitored the target with several high-resolution spectrographs to measure the planetary mass and eccentricity. We report the discovery of a 106-day period Jupiter-sized planet around the G-type star TOI-2449 / NGTS-36. We jointly modelled the photometric and radial velocity data and find that the planet has a mass of 0.70 Mj and a radius of 1.002 Rj. The planetary orbit has a semi-major axis of 0.449 au and is slightly eccentric. We detect an additional 3-year signal in the radial velocity data likely due to the stellar magnetic cycle. Based on the planetary evolution models considered here, we find that TOI-2449 b / NGTS-36 b contains 11 Me of heavy elements and has a marginal planet-to-star metal enrichment of 3.3. Assuming a Jupiter-like Bond albedo, TOI-2449 b / NGTS-36 b has an equilibrium temperature of 400 K and is a good target for understanding nitrogen chemistry in cooler atmospheres.

Figures (16)

• Figure 1: Radial velocities of TOI-2449. Top: Radial velocity data spans 1136 days (CHIRON : blue, CORALIE: green, FEROS : grey, HARPS: orange). Bottom: Generalised Lomb-Scargle periodogram of the radial velocities. The black line marks the 1% false alarm probability.
• Figure 2: Generalised Lomb-Scargle periodogram of the radial velocities, the radial velocity residuals after joint modelling (see Section \ref{['sec:joint_fit']}), FWHM of the CCF, and $\rm H_{\alpha}$ indicator. Vertical lines highlight the relevant periods (red : stellar rotation, green : significant signals). The grey line marks the 1% false alarm probability.
• Figure 3: Posterior distribution of the orbital period for a model with up to three planets (blue histogram). The prior on the orbital period is identical for the three planets and shown in grey. The effective sample size (ESS) is equal to 22790 samples.
• Figure 4: Top: TESS and NGTS detrended photometric observations of TOI-2449 (TESS in green, NGTS in orange and blue) with the common transit model (solid lines). NGTS data is binned at 10 minute cadence to match the TESS cadence. Bottom: each panel shows the residuals between the transit model and the respective light curve.
• Figure 5: Radial velocity timeseries with median model (top panel) and residuals (bottom panel).