Exoplanets and Stellar Physics

Abstract:

This research note presents a new planet candidate orbiting the K-dwarf TIC 392664683, which was discovered by citizen scientists as part of the Planet Hunter TESS (PHT) project. Three transit-like events were observed in Sectors 38, 65 and 102; the available data imply that the orbital period must be either 23 or 46 days. The transit depth of 1964−212+433 ppm and duration of 5.4−0.2+0.4 hr are consistent across the three sectors spanning almost five years of TESS data. Using a stellar radius of 0.7740 ± 0.0168R⊙ we derive a planetary radius between 3.5 and 4.2 R⊕, approximately the same as Neptune. Comprehensive vetting mitigates the risk that this is a false positive and supports the candidate’s likely planetary nature.

Abstract:

Abstract Ultrashort period (USP) rocky planets are expected to be depleted of any substantial gaseous envelope due to the intense irradiation they receive from their host star, making the recent detection of an atmosphere around TOI-561 b particularly surprising. That finding was based on the planet’s bulk density and dayside emission spectrum, but full-orbit phase-curve observations offer a more comprehensive way to constrain the presence and characteristics of a planet’s atmosphere. In this paper, we map TOI-561 b’s 3–5  μ m emission using JWST/NIRSpec multiorbit spectroscopic phase curves, explicitly accounting for the curvature of the out-of-eclipse baseline and possible hotspot offsets. From a simple energy balance argument as well as comparing to general circulation models, we find the phase-curve observations are best explained with high Bond albedo and moderate global heat transfer. Our 37 hr continuous observation provides a long baseline that allows us to model stellar granulation, the planetary phase curve, and dayside emission simultaneously, effectively disentangling these signals and yielding a dayside emission spectrum that is more robust against stellar variability and consistent with previous eclipse-only fits. Using general circulation model outputs, we constrain where clouds can plausibly form and test candidate compositions, finding that silicate clouds, such as SiO 2 and MgSiO 3 , can form on the dayside near the terminator and explain the observed albedo. Our findings confirm that TOI-561 b appears to have a global reflective atmosphere, suggesting exchange of volatiles with the interior to maintain it.

Abstract:

In just over 3 yr of operation, JWST has observed all seven planets of the TRAPPIST-1 system. The two innermost planets were found to have little to no atmosphere, barring the presence of high-altitude aerosols. Here we present the first JWST transit spectra of the habitable-zone exoplanet TRAPPIST-1 f, which were obtained with NIRISS SOSS over the course of five transits. At least one stellar flare occurred in each visit, but unlike observations of closer-in TRAPPIST-1 planets, no evidence for contamination of the transit spectra from unocculted stellar surface heterogeneities was found. This nondetection does not guarantee the absence of unocculted heterogeneities in all future transit observations of this planet, and it could be explained by the transit chord of TRAPPIST-1 f having properties similar to the average, out-of-transit, visible stellar hemisphere at the time of observation. The transit spectra exhibit slopes ranging from −365 ppm μm−1 down to 15 ppm μm−1, which we attribute to stellar variability, that is, flares and/or smaller-scale events. The visits least affected by flares rule out H2/He-dominated atmospheres with surface pressures higher than about 20 mbar at 95% confidence. For high-mean-molecular-mass atmospheres, the exact upper limits on surface pressures depend on the reduction pipeline and on the treatment of the residual slopes in the transit spectra.

Abstract:

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 CH4 and the photochemical product CS2. For the first time in a transit spectrum, NH3 is visually evident, while subtler features from H2O and CO2 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 NH3 and CO2 features relative to the CH4 and H2O 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.

Abstract:

Sulfur chemistry has emerged as an important probe of exoplanet atmospheres in the JWST era, although observational constraints have thus far been largely limited to SO2 and H2S in warm and hot exoplanets. Recent JWST observations have revealed CS2 in several cooler gas giant exoplanets, yielding a new tracer of sulfur chemistry. However, the detailed chemical pathways responsible for the formation of CS2 remain poorly understood. Here, we use TOI-6894 b, a temperate gas giant with evidence for CS2, as a test case for one-dimensional photochemical kinetic-transport modeling and sensitivity analyses of CS2 chemistry. We show that CS2 is produced through coupled thermochemical and photochemical processes involving CH4 and H2S as the primary carbon and sulfur reservoirs, with S2 photolysis driving disequilibrium sulfur chemistry. Our models provide a physically consistent explanation for the observed CS2 feature in TOI-6894 b. Extending our analysis to gas giant exoplanets spanning a wide range of Teq, we find that CS2 abundance peaks in temperate to warm atmospheres (Teq ∼ 500–700 K), and declines toward both lower and higher temperatures. This temperature dependence provides a unified framework for interpreting current CS2 observations, accounting for reported detections in temperate to warm planets and the lack of detections in colder and hotter giant exoplanets. Our results establish CS2 as a complementary probe of sulfur inventories and atmospheric metallicity in cool gas giant exoplanets.