🔭 astrophysics

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

This paper reports the discovery and characterization of TOI-375, a unique multi-planet system orbiting an intermediate-mass post-main-sequence star that hosts three gas giants, including a transiting hot Jupiter and two outer warm/cold Jupiters, providing a valuable laboratory for testing planet formation and migration theories.

Original authors: Y. Reinarz, M. I. Jones, R. Brahm, N. Espinoza, M. Tala Pinto, T. Trifonov, A. Jordán, L. Acuña-Aguirre, T. Henning, F. Rojas, C. Ziegler, D. M. Conti, C. Briceño, N. Law, A. W. Mann, K. A. Collins, J

Published 2026-02-04

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CC BY 4.0

Original authors: Y. Reinarz, M. I. Jones, R. Brahm, N. Espinoza, M. Tala Pinto, T. Trifonov, A. Jordán, L. Acuña-Aguirre, T. Henning, F. Rojas, C. Ziegler, D. M. Conti, C. Briceño, N. Law, A. W. Mann, K. A. Collins, J. M. Irwin, D. Charbonneau

Original paper licensed under CC BY 4.0 (http://creativecommons.org/licenses/by/4.0/). ✨ This is an AI-generated explanation of the paper below. It is not written or endorsed by the authors. For technical accuracy, refer to the original paper. Read full disclaimer

Imagine the solar system as a busy neighborhood. Usually, we think of our Sun as the quiet, stable landlord, and the planets as its tenants. But in this new discovery, the landlord (a star named TOI-375) is getting old and changing shape, and it has three massive "tenants" (gas giant planets) living with it.

Here is the story of this unique family, explained simply:

The Landlord: An Aging Star

The star TOI-375 is not a young, energetic star like our Sun. It is an "intermediate-mass" star that has finished its main life stage. Think of it like a person who has retired from their main job and is now entering a new, slightly unstable phase of life. It is swelling up and cooling down, preparing to become a giant red star. Despite this change, it still hosts a very interesting family.

The Tenants: Three Giant Planets

The astronomers found three huge planets orbiting this aging star. They are all "gas giants," meaning they are made mostly of gas (like Jupiter and Saturn in our own solar system), but they are much bigger and heavier.

The "Hot" Tenant (TOI-375 b): This is the star's favorite child, living very close to the landlord. It's a "Hot Jupiter."
- The Analogy: Imagine a dog that lives right next to the fireplace. It's very hot there! This planet orbits its star in just 9.5 days. Because it's so close, it's scorching hot (about 1,373°C).
- What we know: We can actually see this planet pass in front of the star (a "transit"), like a bug flying across a porch light. This lets us measure its size and weight accurately. It's about the size of Jupiter but slightly lighter.
The "Warm" Tenant (TOI-375 c): This planet lives a bit further out.
- The Analogy: Think of this one as living in the living room, a comfortable distance from the fireplace. It takes about 115 days to go around the star. It's a "Warm Jupiter."
The "Cold" Tenant (TOI-375 d): This one lives on the outskirts of the neighborhood.
- The Analogy: This planet is like a tenant living in the backyard shed, far away from the heat. It takes nearly 300 days to complete one orbit. It's a "Cold Jupiter."

The Big Mystery: How Did They Get There?

For a long time, scientists have argued about how giant planets like these end up so close to their stars. There are two main theories, like two different ways a family might move into a house:

Theory A: The Smooth Slide (Disk Migration). Imagine the planets were born far away and slowly slid inward on a smooth, icy slide (the gas disk around the star) while they were growing. This would keep them in a neat, flat line, like books on a shelf.
Theory B: The Bumpy Ride (High-Eccentricity Migration). Imagine the planets were thrown around like pinballs, crashing into each other or getting kicked by gravity, before finally settling down close to the star. This usually leaves the planets with messy, tilted orbits.

The Paper's Conclusion:
The "Hot Jupiter" (TOI-375 b) has a very neat, circular orbit with almost no tilt. This suggests it likely took the Smooth Slide (Theory A). It probably formed far out and gently drifted inward without getting into a fight with its siblings. The fact that it has two other giant siblings living peacefully nearby supports this "neat and tidy" story.

Why Is This Discovery Special?

This is a rare find for two reasons:

The "Three-Giant" Club: Finding a system with three fully grown gas giants is rare. Finding one where the star is also old and changing is even rarer.
The "Transit" Bonus: Usually, when we find planets far away, we can only guess their size. But because the inner planet (TOI-375 b) passes in front of the star, we know its exact size. This makes the whole system a perfect "test case" for scientists to check their theories on how planets are born and grow up.

The Takeaway

This paper tells us that even as a star gets old and changes, it can still host a complex family of giant planets. The family of TOI-375 seems to have grown up in a relatively calm environment, drifting gently into their current spots rather than fighting their way there. It's a snapshot of a planetary family that helps us understand how our own cosmic neighborhood might have formed billions of years ago.

Problem and Context
Hot Jupiters, gas giants orbiting close to their host stars with periods under 10 days, present a challenge to standard planet formation theories which predict inefficient formation in the inner disk. Two primary migration mechanisms are proposed to explain their existence: smooth disk-driven migration, which typically results in low eccentricities and co-planar systems, and high-eccentricity migration (via planet-planet scattering or Kozai-Lidov cycles), which often produces high eccentricities, misaligned orbits, and isolated hot Jupiters. Well-characterized multi-planet systems are crucial for distinguishing between these pathways. However, systems hosting three or more fully evolved Jupiter-mass planets, particularly those with at least one transiting planet, are extremely rare.

Methodology
The authors present the discovery and characterization of the TOI-375 system, an intermediate-mass post-main-sequence star ( $M_\star \approx 1.44 M_\odot$ ). The study utilizes a multi-faceted observational approach:

Transit Photometry: Data from the TESS mission (across multiple sectors with 30-minute and 2-minute cadences) and ground-based follow-up from the LCOGT network and MEarth-South were analyzed. Light curves were detrended using Gaussian processes and linear models to account for systematic noise and atmospheric effects.
Radial Velocity (RV) Measurements: 66 high-resolution spectra were obtained using the FEROS spectrograph (MPG 2.2m telescope) and the CHIRON spectrograph (CTIO 1.5m telescope) between 2018 and 2019. RVs were derived using cross-correlation techniques with the CERES pipeline (for FEROS) and standard cross-correlation (for CHIRON).
Stellar Characterization: The stellar parameters were derived using the Species code, incorporating Gaia DR3 parallaxes and magnitudes, and comparing the spectrum against synthetic grids and MIST isochrones.
Global Modeling: The juliet library was employed to jointly model the photometric and RV data. Bayesian model comparison (using AIC and BIC) was used to select the best-fitting orbital configuration, testing for circular vs. eccentric orbits and linear trends.
Interior Modeling: The physical properties of the innermost planet were analyzed using MESA to compute interior models based on core accretion scenarios.

Key Results
The analysis confirms a unique three-planet system orbiting TOI-375:

TOI-375 b: A transiting hot Jupiter with an orbital period of $9.45469 \pm 0.00002$ days. It has a mass of $0.745 \pm 0.053 M_J$ , a radius of $0.961 \pm 0.043 R_J$ , and a low eccentricity of $0.087 \pm 0.042$ . Its equilibrium temperature is approximately 1373 K.
TOI-375 c: A warm-cold Jupiter detected via RV with a period of $115.5^{+2.0}_{-1.6}$ days and a minimum mass ( $M \sin i$ ) of $2.11 \pm 0.22 M_J$ .
TOI-375 d: A cold Jupiter detected via RV with a period of $297.9^{+28.9}_{-18.6}$ days and a minimum mass of $1.40 \pm 0.28 M_J$ .

Stellar analysis places TOI-375 at the end of the sub-giant branch, with an age of approximately 2.94 Gyr. No significant stellar activity indicators (bisector, H $\alpha$ , S-index) correlate with the RV signals, supporting the planetary origin of the variations. Interior modeling of TOI-375 b suggests a heavy-element enrichment ( $Z_p/Z_\star \approx 10$ ) consistent with the core accretion model, with no evidence of radius inflation despite high irradiation.

Significance and Claims
The paper claims that TOI-375 is the first known system to host three or more fully evolved gas giant planets with at least one transiting member. This architecture provides a rare constraint on formation and migration theories. The low eccentricity of the inner planet and the presence of outer companions are consistent with a disk-driven migration scenario, though high-eccentricity migration cannot be entirely ruled out without precise measurements of mutual inclinations and spin-orbit alignment. The system serves as a valuable case study for understanding the dynamical evolution of multi-giant systems around evolved stars.

The Landlord: An Aging Star

The Tenants: Three Giant Planets

The Big Mystery: How Did They Get There?

Why Is This Discovery Special?

The Takeaway

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