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

TL;DR

This paper reports the discovery and characterization of TOI-375, a post-main-sequence star hosting three gas giants, including a transiting hot Jupiter. By combining TESS transit data, ground-based photometry, and radial-velocity measurements from FEROS and CHIRON, the authors derive the architecture: a transiting TOI-375 b with P ≈ 9.45 days and M ≈ 0.745 MJ, R ≈ 0.961 RJ, plus two outer giants TOI-375 c and d with P ≈ 115.5 and ≈ 297.9 days and minimum masses ≈ 2.11 MJ and ≈ 1.40 MJ, respectively. The system’s low eccentricity and lack of strong trends favor disk-driven migration for the inner planet, though high-eccentricity pathways cannot be ruled out. Interior modelling of TOI-375 b supports core-accretion with substantial heavy-element enrichment (Zp/Z★ ≈ 10) and no observed inflation despite high irradiation, validating current planet-formation theories. Overall, TOI-375 provides a rare, valuable case for testing giant-planet formation and migration theories in multi-planet systems around evolved stars.

Abstract

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.

Figures (10)

• Figure 1: Position of TOI-375 in the HR diagram (black star), using the stellar parameters derived here (see Table 3). For comparison, the position of known giant stars hosting transiting giant planets are overplotted. Two different PARSEC models parsec for 1.0 and 1.45 ${\rm M}_{\odot}$, are overplotted. The solid and dotted lines correspond to Z$_\star$=0.017 and 0.020, respectively.
• Figure 1: Target pixel file (TPF) for TOI-375 sectors; the target star is marked with a white cross on top of a red point, and are numbered as 1. Smaller numbered red points are the closest stars to the target (drawn from Gaia) with Gaia magnitude differences with the target of $|\Delta G| < 6$. There are no nearby sources of contamination for TOI-375. Plot made using tpfplotter.
• Figure 2: Speckle autocorrelation function for TOI-375, obtained with the SOAR telescope. The black dots represent the $5\sigma$ contrast curve, and the solid line shows a linear fit to the data at separations smaller and larger than $\sim 0.2$ arcseconds.
• Figure 3: Periodogram of the radial velocities (top) and activity indices (middle), along with the window function of the observations (bottom) for the FEROS data of TOI-375. False-alarm probabilities are indicated by horizontal lines: dashed red for 1% FAP and dotted blue for 0.1% FAP on each panel. Green vertical lines mark peaks in the radial-velocity periodogram with FAP below 1% or corresponding to a confirmed a priori planetary signal, such as the 9.4-day signal. In contrast, magenta vertical lines indicate signals that are unlikely to be of planetary origin.
• Figure 4: In green, we show the Phase-folded light curves from each instrument (or sector in the case of TESS) with the best-fit transit model for TOI-375 b derived using juliet. In blue, we show the potential single transit event of one of the outer planets on TESS sector 2. Data points are shown in their original time sampling (small green or blue points) and binned in phase with 6-minute intervals (white markers). The solid black line represents the best-fit transit model. We do not include the transit of TOI-375 c in the final model as there is not enough evidence to support it is from true planetary origin.