Leonardos Gkouvelis

The first observations of short-period rocky exoplanets have initiated the era of terrestrial exoplanet characterization. The interpretation of these observations relies on forward models used in atmospheric and surface retrieval frameworks. While atmospheric modeling is supported by extensive laboratory datasets of molecular opacities, the experimental basis for rocky surface characterization remains limited. Most reflectance libraries of planetary analog materials provide spectra measured at a single illumination and viewing geometry, constraining the spectral behavior but not its full angular scattering properties. We show that incomplete angular coverage in laboratory bidirectional reflectance measurements introduces an intrinsic systematic uncertainty in the interpretation of rocky exoplanet observations. Using the Hapke photometric framework, we demonstrate that multiple combinations of photometric parameters can reproduce the same laboratory reflectance spectrum while predicting different angular scattering behaviors and disk-integrated observables. We propagate this nonuniqueness to reflected-light phase curves, geometric albedos, and planet–star contrast ratios between 0.5 and 1.7 μ m, representative of the spectral range targeted by future direct-imaging missions such as the Habitable Worlds Observatory. We find that single-geometry laboratory spectra produce large uncertainties in predicted signals, particularly at small phase angles where opposition effects dominate. Simplified assumptions commonly adopted in exoplanet studies, such as isotropic scattering or neglecting opposition effects, can additionally introduce large systematic biases. Using multiangular laboratory measurements from the Bloomsburg University Goniometer dataset, we show that the uncertainty in predicted observables collapses once angular scattering behavior is directly constrained. Our results demonstrate that multiangular measurements are required to construct physically predictive reflected-light models of rocky exoplanets.

Context. Short-orbit gaseous exoplanets are the best targets to study atmospheric dynamics. Their hot atmospheres generate emission that is strong enough to be detected and analyzed in detail. A time series of such emission observations collected at various photometric filters – phase curves – provides insights into atmospheric flows. Modern observations reveal a wide variety of phase curves, but their utility for probing atmospheric circulation as a function of altitude has not yet been explored in detail. Aims. We aim to understand the properties of phase curves and their connection to underlying atmospheric flows, as well as to define a set of multiwavelength observations that could be used to resolve these flows as a function of altitude. Methods. We utilized a subset of the solar metallicity models from the grid of hot Jupiters calculated by ADAM (former SPARC/MITgcm) to predict phase curves. To do this we used state-of-the-art radiative transfer codes and the latest opacity sources. We made predictions for a variety of photometric filters on board the S PITZER , TESS, CHEOPS, HST, and JWST missions, and explored the sensitivity of each filter to flows at various atmospheric depths. Results. Our calculations show that the main parameter that regulates the phase-curve offsets in our models is the atmospheric temperature, where hotter planets tend to have smaller offsets, although high metallicity can also have strong impact by reducing phase-curve offsets. This is not fully supported by available observations, which possibly indicates a missing physical process in the models. The predicted contribution functions suggest that the best combination of photometric filters to study atmospheric flows is JWST/NIRCam filters because they are sensitive to a wide range of pressures between 10 bar and 10 ⁻⁴ bar depending on planet temperature, respectively. High-resolution spectroscopy is predicted to detect differential Doppler shifts of ∆υ≈1–4 km s ⁻¹ between molecular bands formed at different altitudes, providing an independent probe of vertical circulations.

Observations of the metastable He i 10830 Å triplet have revealed escaping exoplanet atmospheres whose inferred helium abundances range from nearly nebular compositions to strongly helium-depleted winds. Such depletion is commonly interpreted as evidence for atmospheric evolution, preferential escape, or departures from primordial composition. We present a closed-form analytic transport-retention theory for the atmospheric transition region between the homopause and the base of the planetary wind. The theory quantifies the competition between upward transport and molecular separation, demonstrating that transport physics alone can substantially deplete the helium abundance supplied to the escaping flow. Our solution yields a retention factor, χ _He , that measures the fraction of helium supplied to the base of the hydrodynamic wind relative to the deep atmospheric abundance. Combining the analytic framework with numerical forward models of the He i 10830 Å absorption and a sample of 12 helium-observed sub-Neptunes and mini-Neptunes, we find that observed systems span the full range of predicted retention states, from nearly complete retention to strong helium depletion. These forward models use the helium abundance supplied by the analytic framework as the lower-boundary condition to predict the corresponding He i 10830 Å absorption. We further show that increasing helium retention systematically strengthens the expected absorption signal by increasing the helium reservoir available to the upper atmosphere. These results suggest that helium depletion does not necessarily imply intrinsically helium-poor atmospheres or evolutionary transitions toward secondary compositions, but may instead be a natural consequence of transport physics in escaping atmospheres.

Planet-encircling dust storms fundamentally reshape Martian weather and the near-surface electrostatic environment, yet the evolution of electric fields on global scales has not been previously investigated with physically consistent triboelectric modeling. We investigate the generation and evolution of electric fields during global dust storms (GDSs) using bimodal dust size distributions from the NASA Ames Mars Global Climate Model (GCM), coupled with a triboelectric charging and electrostatic diagnostic scheme that links charging to the local dynamical state of the atmosphere. Focusing on the dust-lifting and buildup phase and its subsequent evolution, we quantify the electric-field energy density and discharge characteristics, including onset thresholds, event frequency, and spatial clustering. The simulations reveal broad storm-active belts of enhanced electrification, with the most frequent threshold exceedances occurring in southern low-to-mid latitudes and secondary activity in northern low-to-mid latitudes. Modeled near-surface electric fields reach 10 ² −10 ³ V m ⁻¹ , with domain-maximum values of several 10 ⁴ V m ⁻¹ , comparable to values inferred for smaller-scale dust phenomena. The results indicate that electric-field generation is controlled by the interplay between dust loading, turbulence-driven collisional activity, and conductivity-dependent charge relaxation, with diurnal conductivity variations strongly suppressing daytime electric-field buildup and most events remaining in the weak glow or Townsend discharge regime. While the model captures the large-scale distribution of electrically favorable conditions, the predicted spatial extent of activity likely represents an upper bound, as small-scale turbulent structures are not fully resolved. These results provide a quantitative framework to identify regions where electrostatic discharges are most likely during GDSs and to inform instrument design, power-system protection, and operations planning for future robotic and human missions.

Transmission spectroscopy is a key technique in the characterization of exoplanet atmospheres and has been widely applied to planets undergoing hydrodynamic escape. While a robust analytic theory exists for transmission spectra of hydrostatic atmospheres, the corresponding interpretation for escaping atmospheres has so far relied on numerical modeling, despite the growing number of observations of planetary winds. In this work, a theory of transmission spectroscopy in hydrodynamically escaping atmospheres is developed by coupling the standard transmission geometry to a steady-state, spherically symmetric, isothermal outflow. This approach yields closed-form expressions for the chord optical depth and effective transit radius of a planetary wind and allows the optical depth inversion problem to be examined. The analytic solution reveals that transmission spectroscopy of planetary winds naturally separates into two regimes. In an opacity-limited regime, transmission depths retain sensitivity to the atmospheric mass-loss rate, Ṁ . Beyond a critical threshold, however, spectral-line cores become saturated and no longer provide a unique constraint on the escape rate. This transition is marked by a sharp analytic boundary of the form σ ( λ ) Ṁ ≤ C _sat , where σ ( λ ) is the line absorption cross section and C _sat is a constant set by the thermodynamic and geometric properties of the wind. This condition specifies when the inversion between transmission depth and mass-loss rate admits a real solution. Once it is violated, the effective transit radius is no longer controlled by opacity or mass loss, but by the geometric extent of the absorbing wind. These results demonstrate that spectral-line saturation in transmission spectroscopy corresponds to a fundamental loss of invertibility between absorption and atmospheric mass loss, rather than a gradual weakening of sensitivity. The theory provides a physically transparent explanation for why strong transmission line cores, such as the He triplet or Ly α , may lose unique sensitivity to mass-loss rates once they enter the saturation regime, while weaker lines and the wings of strong lines can remain diagnostic when observationally accessible.

The Transiting Exoplanet Survey Satellite (TESS) has delivered a large number of transiting planet candidates around nearby stars by identifying periodic decreases in stellar brightness. Establishing the planetary nature of these signals and determining their fundamental properties is a necessary step toward detailed studies of their internal structure, atmospheres, and formation pathways. In this work, we investigate the planetary nature of the TOI–1752 system (M1 V, 103.02 ± 0.34 pc), which hosts two TESS candidates: TOI–1752 b, a short-period object consistent with a lava-world scenario, and TOI–1752 c, a sub-Neptune-size planet candidate located in the optimistic habitable zone. We obtained ground-based multi-color photometric follow-up observations of TOI–1752, which we combined with TESS photometry to assess the nature of both signals. We performed a formal statistical validation using the TRICERATOPS framework, while independently vetting the candidates with the neural-network-based classifier WATSON-Net, which provides a machine-learning assessment of their planetary likelihood based on light-curve morphology, centroid diagnostics, and auxiliary vetting features. We validate TOI–1752 b as a bona fide planet with a radius of 1.69 ± 0.07R_⊕ and an orbital period of |$0.935186^{+0.000001}_{-0.000002}$| days, and TOI–1752 c with a radius of |$2.29^{+0.13}_{-0.14} R_{\oplus }$| and an orbital period of 32.7144 ± 0.0004 days. The combined analysis confirms TOI–1752 as a new planetary system, places TOI–1752 c within the optimistic habitable zone of its host star, and identifies TOI–1752 b as a promising target for atmospheric characterization, with an estimated emission spectroscopy metric (ESM) of up to ~8.

Context . The global circulation regime of the coolest class of brown dwarfs, known as the Y dwarfs, remains largely unexplored. Aims . We aim to investigate the interplay between convection, rotation, and cloud thermal feedback through a selected sample of Y dwarf atmospheric models. We explore a range of effective temperatures 400 K ≤ T _eff ≤ 600 K and rotation rates P _rot = 2.5–20 h. In this temperature range, salt and sulfide condensates are expected to form. We include KCl, Na ₂ S, and MnS clouds in our simulations to study their effect on the atmosphere. Our goal is to identify circulation regimes and emergent trends across this space, providing insights into the dynamical processes governing Y dwarf atmospheres. Methods . We ran a suite of twelve general circulation models (GCMs) across the outlined parameter grid. For this purpose, we developed additional physics modules for the THOR GCM to model brown dwarf atmospheres. The THOR dynamical core was coupled to modules for interior thermal perturbations near the radiative–convective boundary, a mixing-length convection scheme, a gray two-stream radiative transfer module using Rosseland-mean opacities, and simple cloud tracers with thermal feedback and scattering. Results . Across all simulations, the circulation resides in a radiative-forcing-dominated regime with weak winds, minimal horizontal temperature contrasts, and no persistent jets. Convection controls vertical mixing and sets the extent of the salt and sulfide cloud layers that form below the photosphere. Thermal structures equilibrate quickly and cloud radiative feedback remains insignificant, with limited variability. Conclusions . Y dwarf atmospheres in this parameter range, within the gray radiative transfer framework adopted in this work, remain controlled by thermal radiation from the interior, with small variability primarily set by rotation and clouds playing a secondary role. As our single-band approach does not capture spectral windows that could probe deeper into the cloud-rich layers, our constraints on cloud radiative feedback are likely conservative, and we outline possible causes and pathways toward more active regimes.

Analytical models are essential for building physical intuition and guiding the interpretation of exoplanet observations by clarifying the dependencies that shape atmospheric signatures. I present a generalization of the classical isothermal, isobaric transmission model by allowing the opacity to vary with pressure as a power law, κ ∝ P ⁿ , and explicitly defining the reference opacity κ ₀ at a chosen pressure P ₀ . By treating the slant optical depth as an Abel transform of the radial absorption coefficient, I derive a closed-form expression for the effective transit radius in a hydrostatic, isothermal atmosphere with pressure-dependent opacity. The solution provides a compact framework for exploring nonisobaric effects and explicitly links the vertical opacity gradient to observable spectral features. I benchmark the model against empirical transmission spectra of Earth and the hot Jupiter WASP-39b, finding a significantly improved fit relative to the isobaric formula. This generalized expression offers a physically interpretable foundation for analyzing high-precision spectra from JWST and upcoming ARIEL observations, and can serve as a basis for semianalytical retrieval approaches optimized for computational efficiency.

Synthetic spectra of airless rocky exoplanets rely on laboratory measurements of planetary analogue materials to determine their optical and photometric properties. Realistic surface modeling requires several wavelength-dependent parameters, including the single-scattering albedo, phase-function asymmetry factor, and the amplitude and width of the opposition surge. However, existing laboratory datasets generally provide reflectance only at a fixed incidence-emission geometry, leaving these key photometric quantities highly degenerate or unconstrained. As a result, current surface models resort to simplified scalings to approximate reflectance and emissivity. With this Note, I draw attention to the urgent need for multi-angular, multi-spectral bidirectional reflectance distribution function measurements of planetary-analogue materials. Such datasets will be essential for interpreting reflected and emitted spectra of rocky exoplanets with upcoming facilities such as HWO, LIFE, and future direct-imaging missions.

Recent advances in space-based and ground-based facilities now allow the atmospheric characterization of a selected sample of rocky exoplanets. These atmospheres offer key insights into planetary formation and evolution, but their interpretation requires models that couple atmospheric processes with both the planetary interior and the surrounding space environment. This work focuses on the Earth-size planet LP 791-18 d, which is estimated to receive continuous tidal heating due to the orbital configuration of the system; thus, it is expected to exhibit volcanic activity. Using a 1D radiative-convective model coupled with chemical kinetics and an outgassing scheme at the lower boundary, we simulated the planet’s atmospheric composition across a range of oxygen fugacities, surface pressures, and graphite activities. We estimated the mantle temperature of ≈1680–1880 K, balancing the competing contributions of interior tidal heating and convective cooling. Our results show that the atmospheric mean molecular weight gradient is controlled by oxygen fugacity rather than bulk metallicity. Furthermore, we used the atmospheric steady-state solutions produced from the interior redox state versus surface pressure parameter space, and explored their atmospheric stability. We find that stability is achieved only in highly oxidized scenarios, fO₂ − IW ≳ 2, while reduced interior states fall into the hydrodynamic escape regime with mass loss rates on the order of ≈10⁵−10⁸ kg/s. We argue that scenarios with reduced interior states are likely to have exhausted their volatile budget during the planet’s lifetime. Furthermore, we predict the atmospheric footprint of the planet’s interior based on its oxidation state and assess its detectability using current or forthcoming tools to constrain the internal and atmospheric composition. We show that the degeneracy between bare rock surfaces and thick atmospheres can be resolved by using three photometric bands to construct a color-color diagram that accounts for potential effects from photochemical hazes and clouds. For JWST/MIRI, this discrimination is possible only in the case of highly oxidized atmospheres. The case of LP 791-18 d enables the investigation of secondary atmosphere formation through outgassing, with implications for similar rocky exoplanets. Our modeling approach connects interior and atmospheric processes, providing a basis for exploring volatile evolution and potential habitability.