Exoplanets and Stellar Physics

Abstract:

The search for atmospheres on rocky exoplanets is a crucial step in understanding the processes driving atmosphere formation, retention, and loss. Past studies have revealed the existence of planets interior to the radius valley with densities lower than would be expected for pure-rock compositions, indicative of the presence of large volatile inventories, which could facilitate atmosphere retention. Here, we present an analysis of the JWST/NIRSpec G395H transmission spectrum of the warm ( Teq, AB=0=569 K) super-Earth TOI-270 b (Rp = 1.306 R⊕), captured alongside the transit of TOI-270 d. The JWST white light-curve transit depth updates TOI-270 b’s density to ρp = 3.7 ± 0.5 g cm−3, inconsistent at 4.4σ with an Earth-like composition. Instead, the planet is best explained by a nonzero, percent-level water mass fraction, possibly residing on the surface or stored within the interior. The JWST transmission spectrum shows possible spectroscopic evidence for the presence of this water as part of an atmosphere on TOI-270 b, favoring an H2O-rich steam atmosphere model over a flat spectrum ( lnB=0.3–3.2 , inconclusive to moderate), with the exact significance depending on whether an offset parameter between the NIRSpec detectors is included. We leverage the transit of the twice-larger TOI-270 d crossing the stellar disk almost simultaneously to rule out the alternative hypothesis that the transit light source effect could have caused the water feature in TOI-270 b’s observed transmission spectrum. Planetary evolution modeling furthermore shows that TOI-270 b could sustain a significant atmosphere on gigayear timescales, despite its high stellar irradiation, if it formed with a large initial volatile inventory.

Abstract:

Thedistribution of close-in exoplanets is shaped by a complex interplay betweenatmospheric and dynamical processes. The Desert, Ridge, and Savanna(respectively a lack, overoccurence, and mild deficit of Neptunes withincreasing periods) illustrate the sensitivity of these worlds to suchprocesses, making them ideal targets to disentangle their roles. Determininghow many Neptunes are brought close-in by early disk-driven migration (DDM;expected to maintain primordial spin-orbit alignment) or late high-eccentricitytidal migration (HEM; expected to generate large misalignments) is essential tounderstanding how much atmosphere they lost. In this paper, we propose aunified view of the exo-Neptunian landscape to guide its exploration andspeculate that the Ridge is a hot spot for evolutionary processes. Low-densityNeptunes would mainly undergo DDM, becoming fully eroded at shorter periodsthan the Ridge. This is in contrast to denser Neptunes, which would be broughtto the Ridge and Desert by HEM. We embark on this exploration via the ATREIDES(Ancestry, Traits, and Relations of Exoplanets Inhabiting the Desert Edges andSavanna) collaboration, which relies on spectroscopic and photometricobservations of ~60 close-in Neptunes, their reduction with robust pipelines,and their interpretation through internal structure, atmospheric, andevolutionary models. We carried out a systematic Rossiter-McLaughlin censuswith VLT/ESPRESSO to measure the distribution of 3D spin-orbit angles,correlate its shape with the system properties (orbit, density, evaporation),and thus relate the fraction of aligned-misaligned Neptunian systems to DDM,HEM, and atmospheric erosion. The first ATREIDES target, TOI-421 c, lies in theSavanna with a neighboring sub-Neptune TOI-421 b. We measured for the firsttime their 3D spin-orbit angles (ψb = 57−15+11∘; ψc = 44.9−4.1+4.4∘). Together with the eccentricity and possibly large mutualinclination of their orbits, this hints at a chaotic dynamical origin thatcould result from DDM followed by HEM. Our program will provide the communitywith a wealth of constraints for formation and evolution models, and we welcomecollaborations that will contribute to pushing our understanding of theexo-Neptunian landscape forward.

Abstract:

High-resolution spectroscopy (HRS) of exoplanet atmospheres has successfully detected many chemical species and is quickly moving toward detailed characterization of the chemical abundances and dynamics. HRS is highly sensitive to the line shape and position; thus, it can detect three-dimensional (3D) effects such as winds, rotation, and spatial variation of atmospheric conditions. At the same time, retrieval frameworks are increasingly deployed to constrain chemical abundances, pressure–temperature (P–T) structures, orbital parameters, and rotational broadening. To explore the multidimensional parameter space, we need computationally fast models, which are consequently mostly one-dimensional (1D). However, this approach risks introducing interpretation bias since the planet’s true nature is 3D. We investigate the robustness of this methodology at high spectral resolution by running 1D retrievals on simulated observations in emission within an observational framework using 3D global circulation models of the quintessential HJ WASP-76 b. We find that the retrieval broadly recovers conditions present in the atmosphere, but that the retrieved P–T and chemical profiles are not a homogeneous average of all spatial and phase-dependent information. Instead, they are most sensitive to spatial regions with large thermal gradients, which do not necessarily coincide with the strongest emitting regions. Our results further suggest that the choice of parameterization for the P–T and chemical profiles, as well as Doppler offsets among opacity sources, impact the retrieval results. These factors should be carefully considered in future retrieval analyses.