A transiting hot Jupiter with two outer siblings orbiting an intermediate-mass post main-sequence star

Exoplanetary systems with multiple giant planets present an opportunity to understand planet formation, migration processes, and long-term system-wide dynamical interactions. In particular, they provide constraints to distinguish between smooth disk-driven migration or more dynamically excited system evolution pathways. We report the discovery and characterization of a unique multi-planet system hosting three gas giant planets orbiting the post-main sequence star TOI-375. The innermost planet, TOI-375 b, was initially detected by the TESS mission and then confirmed with photometric follow-up observations conducted using MEarth and LCOGT, and radial velocity measurements obtained with FEROS and CHIRON. The radial velocity data revealed the presence of two additional planetary candidates, TOI-375 c and TOI-375 d. We find that TOI-375 b is a hot Jupiter with an orbital period of $9.45469 \pm 0.00002$ days, mass $0.745 \pm 0.053,M_\mathrm{J}$, radius $0.961 \pm 0.043, R_\mathrm{J}$, and eccentricity $0.087 \pm 0.042$. The outer two planets, TOI-375 c and TOI-375 d, are warm-cold and cold Jupiters with orbital periods of $115.5^{+2.0}{-1.6}$ days and $297.9^{+28.9}{-18.6}$ days, and minimum masses of $2.11 \pm 0.22, M_\mathrm{J}$ and $1.40 \pm 0.28, M_\mathrm{J}$, respectively. In terms of formation and overall system architecture, the physical properties of TOI-375 b are consistent with the core accretion scenario, while the current configuration of the system could be explained by both disk-driven and high-eccentricity migration scenarios. The discovery of TOI-375 as the first known system hosting three or more fully evolved gas giants, with at least one transiting planet, makes it an excellent candidate for testing formation and migration theories.

💡 Research Summary

The paper reports the discovery and comprehensive characterization of a remarkable three‑planet system orbiting the evolved, intermediate‑mass star TOI‑375 (M★ ≈ 1.44 M⊙, R★ ≈ 2.90 R⊙, age ≈ 2.9 Gyr). The innermost planet, TOI‑375 b, was initially identified as a transiting candidate by the Transiting Exoplanet Survey Satellite (TESS) with a period of 9.45469 days and a shallow ~1000 ppm depth. Follow‑up ground‑based photometry from the MEarth‑South array and the Las Cumbres Observatory Global Telescope (LCOGT) network captured two partial transits (one ingress on 09 Sep 2019 and another ingress on 21 Aug 2019), confirming the transit signal and refining the ephemeris.

High‑resolution spectroscopy was obtained with the FEROS spectrograph on the MPG 2.2 m telescope (49 spectra) and the CHIRON spectrograph on the SMARTS 1.5 m telescope (16 spectra), yielding a total of 66 radial‑velocity (RV) measurements spanning 2018–2019. The RV time series shows a dominant 9.45‑day sinusoid consistent with the transiting hot Jupiter, plus two longer‑period signals at ~115 days and ~298 days. A joint photometric‑RV model (implemented with the EXOFASTv2 framework) simultaneously fits the transit light curves, the RV data, and the stellar spectral energy distribution, incorporating Gaussian priors on stellar parameters derived from a high‑S/N FEROS template analyzed with the SPECIES code.

The derived planetary parameters are:

• TOI‑375 b: mass 0.745 ± 0.053 M_J, radius 0.961 ± 0.043 R_J, mean density ≈ 0.84 g cm⁻³, orbital eccentricity e = 0.087 ± 0.042, and a modest equilibrium temperature (~1500 K) given the star’s sub‑giant luminosity.

• TOI‑375 c: minimum mass (M sin i) 2.11 ± 0.22 M_J, period 115.5 + 2.0/‑1.6 days, eccentricity poorly constrained but consistent with modest values, placing it in the “warm‑cold Jupiter” regime.

• TOI‑375 d: minimum mass (M sin i) 1.40 ± 0.28 M_J, period 297.9 + 28.9/‑18.6 days, also a cold Jupiter analogue.

No nearby stellar companions were detected in high‑resolution speckle imaging (SOAR/HRCam) down to Δmag ≈ 4.9 at 1″, nor in Gaia DR3 source catalogs within 3″, allowing the authors to adopt a dilution factor of unity for the transit modeling.

Stellar characterization employed a combination of spectroscopic analysis (effective temperature T_eff = 5259 ± 136 K, surface gravity log g = 3.672 ± 0.011, metallicity