Amaury H. M. J. Triaud

Context . Of the thousands of exoplanets detected to date, only a very small fraction are classified as circumbinary planets. When considering only low-mass planets, their number becomes negligible. This rarity may be partly explained by observational biases, but also by the challenging dynamical environment that often prevents the long-term stability of planetary orbits in binary systems. Aims . It is therefore essential to investigate the conditions that enable the formation and survival of stable circumbinary planets, with particular attention to the case of low-mass planets. Methods . To this end, we performed N -body simulations coupled with a planet migration prescription to study the post-formation dynamics of two circumbinary planets of 10 Earth masses in a protoplanetary disc. This approach allowed us to estimate whether, for a given set of binary and planetary parameters, planets remain bound to their stars or become unstable and ejected. Results . Based on more than 1000 simulations, our results indicate that two-planet inward migration can, in some cases, be halted through resonance capture with the binary. In contrast to some previous purely N -body studies, such configurations appear to remain stable over long timescales. We further find that the binary parameters q _B and e _B play a key role in shaping the stability landscape of multi-planet systems. Within the explored parameter space, some regions support long-term stability, whereas others appear highly unstable. These stability regions are also highly sensitive to the migration timescales (i.e. to disc properties and planetary masses). Finally, our simulations suggest that systems in which two planets enter resonance while migrating together are more likely to form stable multi-planet configurations, particularly around highly eccentric binaries. Conclusions . This constitutes a first step towards identifying the most promising binary star systems likely to host multiple low-mass circumbinary planets. They may serve as valuable guidelines for future observational campaigns, such as those of PLATO and Gaia .

We report the confirmation and characterisation of two moderate-period sub-Jovian planets transiting the bright F-type star HD 148797 ( G = 9.4 mag, T _eff = 6441 ± 51 K). Using photometric time series from TESS and CHEOPS, we determined orbital periods of 42.1 d for HD 148797 b and 68.2 d for HD 148797 c, putting the period ratio very close to the golden mean at 1.619 and therefore near several strong harmonics, and we measured planetary radii of 8.25 ± 0.37 R _E and 8.37 ± 0.38 R _E , respectively. We detected significant anti-correlated transit-timing variations for both planets, which contain enough harmonic information to yield photodynamical masses of 39.3 _−8.5 ⁺¹³ M _E for HD 148797 b and 39.6 ± 9.3 M _E for HD 148797 c. The corresponding bulk densities, 0.39 ± 0.12 and 0.377 ± 0.084 g cm ⁻³ , place both planets among the low-density sub-Jovians of the Neptunian savanna. The architecture of HD 148797 is not unusual within this regime: we find that detected multi-system fractions in the savanna remain at ∼70–90%, and that most savanna multi-planet systems contain at least one adjacent planet pair with P _out / P _in < 3. This pattern suggests that savanna sub-Jovians are commonly found in dynamically cold systems, consistent with smoother migration pathways such as disk-driven migration rather than disruptive high-eccentricity tidal migration. As a bright, co-evolved system hosting two warm savanna sub-Jovians with similar radii and masses, HD 148797 is also a promising target for comparative atmospheric characterisation.

Short-period white-dwarf–main-sequence (WDMS) binaries provide direct constraints on the outcomes of common-envelope (CE) evolution and the survival of low-mass companions during engulfment. We report the discovery and confirmation of 10 totally eclipsing WD + cool-M-dwarf binaries identified through a search tailored to minute-scale eclipses in Transiting Exoplanet Survey Satellite (TESS) high-cadence data and validated with SPECULOOS multiband photometry. The systems have orbital periods of a few hours and companion masses of 0.094–0.175 M _⊙ . Our pilot search of ∼37,000 Gaia-selected WDs increases the number of WD+M systems first identified through TESS by an order of magnitude, demonstrating that minute-scale WD eclipses occupy a previously underexplored discovery regime for TESS. SPECULOOS follow-up establishes the WDs as the sources of the eclipse signals, while chromatic eclipse dilution and broadband spectral-energy-distribution (SED) modeling provide complementary constraints on the component properties. These systems extend the TESS-discovered eclipsing WD+M population toward lower companion masses and establish an effective, scalable framework for expanding the search from the ∼37,000 WDs analyzed here to a pool of O (10 ⁶ ) Gaia WD candidates with TESS photometry.

White dwarf–brown dwarf (WD+BD) binaries are rare laboratories for probing both substellar survival through post-main-sequence evolution and the physics of strongly irradiated atmospheres. We present time-resolved eclipse photometry and radial-velocity confirmation of J0404+1112, a compact ( P = 2.93 hr), totally eclipsing WD+BD system initially reported in Research Notes of the AAS by J. de Wit et al. New high-cadence observations resolve the total eclipse of the WD and, combined with spectroscopy, broadband photometry, and the Gaia parallax, yield a revised system architecture. We find a hot DA WD with T _eff ≃ 28,000 K, log g ≃ 8.0 , M _WD ≃ 0.6 M _⊙ , and R _WD ≃ 0.015 R _⊙ , orbited by a ∼40 M _Jup BD with radius ∼0.9 R _Jup . These parameters supersede earlier spectral energy distribution (SED)-based estimates in which the WD temperature, radius, distance, and extinction were strongly covariant. We break this degeneracy with normalized Balmer-line profile fitting and high-cadence eclipse photometry that constrains the white dwarf radius geometrically. The total eclipses enable isolation of the brown dwarf (BD) flux and constrain its nightside emission. With its short orbital period and strong irradiation, J0404+1112 provides a uniquely compact, high-sensitivity benchmark for probing atmospheric heat redistribution and photochemistry in the temperature regime of the hottest ultrahot Jupiters, spanning substellar-point and nightside temperatures of ∼3700 K and ∼1750 K within a single ∼3 hr orbit.

We report the JWST NIRSpec/PRISM transit spectrum of TOI-6894 b, an exceptional 420 K sub-Saturn that is one of the rare giant planets transiting a late M dwarf. Remarkably, both the light curve and the transit spectrum exhibit almost no stellar contamination. The spectrum is dominated by prominent absorption features from CH ₄ and the photochemical product CS ₂ . For the first time in a transit spectrum, NH ₃ is visually evident, while subtler features from H ₂ O and CO ₂ can also be seen. We significantly improve upon state-of-the-art photochemical reaction networks, and use our new network to run radiative-convective photochemical (“RCP”) models at different metallicities. These models show that the spectrum—in particular the size of the NH ₃ and CO ₂ features relative to the CH ₄ and H ₂ O features—is most consistent with a metallicity of 3–10× solar. Using a semi-free retrieval framework that perturbs the RCP model’s abundance and temperature profiles to fit the data, we find that the planet’s C/O, N/O, and S/O ratios are consistent with solar values. A grid retrieval on 1D RCP equilibrium models reveals a similar result: [M/H] = 0.46 ± 0.08 and C/O = 0.69 ± 0.06. The planet’s atmospheric metallicity, abundance ratios, and bulk metal fraction are all strikingly similar to those of Jupiter, Saturn, and other gas giant exoplanets, despite orbiting a very low-mass star.

Binary stars are ubiquitous; yet it remains unclear how wide-orbit stellar companions influence the formation of hot Jupiters, particularly around M dwarfs. Here, we first report the discovery of TOI-5628Ab, a giant planet transiting a midtype M dwarf ( M _* = 0.36 ± 0.02 M _⊙ ) every 4.34 days, accompanied by an associated white dwarf TOI-5628B ( M _WD = 0.59 ± 0.16 M _⊙ ) at a projected distance of about 2500 au. Using TESS, ground-based photometry, and SPIRou radial velocities, we constrain the planet radius to 0.74 ± 0.04 R _J and mass to 0.09 ± 0.04 M _J , with a 3 σ upper limit of 0.22 M _J . Building on this system, we further conduct a homogeneous systematic search for comoving stellar companions with projected semimajor axes between 100 and 10,000 au around all M dwarfs with confirmed giant planets with periods smaller than 10 days and radii larger than 0.7 R _J , as well as a group of field M stars with stellar properties similar to the planet sample, based on the stellar kinematics from Gaia DR3. We measure a stellar multiplicity of 34.2% ± 9.5% for M dwarfs hosting short-period giant planets, which is substantially higher than the fraction of 5.3% ± 3.7% for the field M stars by approximately a factor of 6. Our results suggest that wide-orbit stellar companions tend to promote the formation of short-period gas giants around M stars with masses 0.21 ≤ M _* ≤ 0.64 M _⊙ , and high-eccentricity migration may play an important role in producing such systems.

Long-period transiting planets are essential edge-cases for understanding exoplanet formation and evolution. Their detection is challenging due to the infrequent nature of their transits, making every observation of these objects very valuable. Our goal is to confirm and further characterise the long-period planetary candidate TOI-4409,b (initially identified by TESS) through additional observations using the Antarctica-based ASTEP telescope, supplemented with CHAT, OMES, LCOGT, and PEST light-curve data and radial-velocity (RV) measurements collected with the HARPS, FEROS, and PFS instruments. We jointly analysed the photometric data from eight TESS, four ASTEP, one CHAT, two OMES, four LCOGT, and one PEST light curve(s), and RV data from the HARPS, FEROS, and PFS instruments, covering observations from 2018 to 2024. TOI-4409,b is a bona fide puffy warm super-Neptune exoplanet with a planet radius of 7.44 ± 0.19,R_⊕, and a mass of 0.047 ± J ,(3σ upper limit <0.095,M_ J) orbiting a low-mass main-sequence star (effective temperature of 4928 ± 48, and a stellar radius of 0.720 ± 0.018, _⊙) with a period of 92.49179 ± 0.00010 days. The integration of various observational methods and consistent transit signals across multiple instruments confirms the planetary nature of TOI-4409,b. The combined transit, imaging, chromaticity, and RV evidence supports the confirmation of the planet. K R With a radius of 7.44 ± 0.19,R_⊕ and a low bulk density of 0.20 ± 0.07 -3 $, TOI-4409,b occupies a sparsely populated region of parameter space among warm, puffy super-Neptunes on long-period orbits. The detection of candidate transit timing variations with an ∼10-minute semi-amplitude hints at the presence of an additional body in the system. The favourable host-star brightness and extended transit duration ($∼7 hours) make TOI-4409,b a promising target for atmospheric characterisation with JWST and Ariel.