arXiv subjects

Antonio García Muñoz

Publications and source records attributed to Antonio García Muñoz.

At least 19 recordsLinked to original sources

arXiv2026

The SPACE Program II: No discernible spectral features in the transmission spectrum of the sub-Neptune HD 191939 b observed with HST/WFC3

The atmospheres of sub-Neptunes provide a window into their internal structure and history, shedding light on the origin of this common, but enigmatic, class of exoplanets. However, the physical and chemical processes that shape sub-Neptunes' transmission spectra, in particular cloud and haze formation, are not well understood. To identify possible correlations between transmission spectra and UV irradiation, the SPACE (Sub-neptune Planetary Atmosphere Characterization Experiment) Program observed an array of sub-Neptunes and their host stars using the Hubble Space Telescope (HST), measuring the planets' transmission spectra between $1.1\,μ$m and $1.7\,μ$m with the Wide Field Camera 3 (WFC3) and the stars' UV spectra with the Space Telescope Imaging Spectrograph (STIS). Here, we present the observations of HD 191939 b carried out as part of the SPACE Program, which reveal no significant spectral features in the transmission spectrum. The data deliver moderate evidence at significance levels between $2.0\,σ$ and $3.2\,σ$ against a cloud-free atmosphere with solar metallicity, rendering this scenario unlikely, but still possible. A super-solar metallicity of HD 191939 b might be consistent with the known trend of increasing atmospheric metallicity with decreasing planet mass. Both hydrocarbon haze formation and cloud condensation can be efficient at HD 191939 b's zero-albedo equilibrium temperature of $(880\pm 20)\,$K, particularly in atmospheres with super-solar metallicity, possibly additionally muting absorption features.

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arXiv2026

Star Planet Interactions

Star-planet interactions (SPIs) describe the continuous exchange of energy, momentum, and mass between exoplanets and their host stars through radiative, tidal, magnetic, and particle-driven processes. Together, these interactions shape the structure, evolution, and observable properties of exoplanetary systems. In this review, we bring together current theoretical and observational understanding of SPIs, highlighting how stellar radiation, winds, and magnetic activity influence planetary atmospheres, interiors, and orbital evolution, while using the Solar System as a valuable reference for interpreting these processes. High-energy stellar radiation, particularly in the far- and extreme-ultraviolet and X-ray bands, drives atmospheric heating, photochemistry, ionisation, and escape. These effects are further influenced by stellar winds and magnetic interactions, which can either protect planetary atmospheres or accelerate their loss over time. Tidal interactions redistribute energy and angular momentum, producing internal heating and driving orbital migration and circularisation. Magnetic star-planet coupling provides additional pathways for energy transfer through reconnection and current systems, potentially enhancing atmospheric escape, heating planetary ionospheres and interiors, and generating observable signatures such as radio emission and enhanced stellar activity. We discuss how these processes work together, emphasising that their long-term impact depends on stellar evolution, planetary properties, atmospheric structure, and magnetic field strength. By presenting radiative, tidal, and magnetic interactions within a unified framework, this review highlights the physical mechanisms that shape planetary environments and identifies the key observational signatures that will complement future studies of exoplanet evolution and habitability.

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arXiv2026

The role of the Hubble Space Telescope in advancing our understanding of atmospheric escape in exoplanets

An important evolutionary pathway for planetary atmospheres is escape to space, which has been studied on Earth and Mars for several decades and more recently in exoplanets. A particularly important regime is the hydrodynamic escape, wherein atmospheric mass escapes the planet at high rates in a collisional fluid outflow. This process is used to partly explain the early evolution of rocky planets in and out of the Solar System, as well as key aspects of exoplanet demographics. Hydrodynamic escape is not occurring in the Solar System planets, so our only option for such observations is through exoplanets. The ultraviolet (UV) capabilities of the Hubble Space Telescope (HST) are fundamental to detect hydrodynamic escape and measure the resulting mass-loss rates for a range of planetary systems and to identify targets for surveys with the Habitable Worlds Observatory. We discuss here what kinds of observations and instrument modes are necessary to continue studying atmospheric escape in exoplanets for the next decade, as well as how to advance our understanding of planetary evolution and habitability.

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arXiv2025

Direct imaging characterization of cool gaseous planets

Cool gas giant exoplanets, particularly those with properties similar to those of Jupiter and Saturn, remain poorly characterized due to current observational limitations. This white paper outlines the transformative science case for the Habitable Worlds Observatory (HWO) to directly image and spectroscopically characterize a broad range of gaseous exoplanets with effective temperatures below 400 K. The study focuses on determining key atmospheric properties, including molecular composition, cloud and haze characteristics, and temperature structure, across planets of varying sizes and orbital separations. Leveraging reflected light spectroscopy and polarimetry, HWO will enable comparative planetology of cool gas giants orbiting both solar-type and M-dwarf stars, bridging the observational gap between hot exoplanets and Solar System giants. We present observational requirements and survey strategies necessary to uncover correlations between atmospheric properties and planetary or stellar parameters. This effort will establish critical constraints on planetary formation, cloud microphysics, and the role of photochemistry under diverse irradiation conditions. The unique capabilities of HWO will make it the first facility capable of characterizing true exo-Jupiters in reflected light, thus offering an unprecedented opportunity to place the Solar System in a broader galactic context.

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arXiv2025

Vibrationally excited H2 mutes the He i triplet line at 1.08 μm on warm exo-Neptunes

Context. Neptune-sized exoplanets or exo-Neptunes are fundamental in the description of exoplanet diversity. Their evolution is sculpted by atmospheric escape, often traced by absorption in the H i Lyman-α line at 1,216 Å and the He i triplet line at 1.08 μm. On warm exo-Neptunes HAT-P-11 b, GJ 3470 b and GJ 436 b, H i Lyman-α absorption causes extreme in-transit obscuration of their host stars. This suggests that He i triplet line absorption will also be strong, yet it has only been identified on two of these planets. Aims. We explore previously unaccounted for processes that might attenuate the He i triplet line on warm exo-Neptunes. In particular, we assess the role of vibrationally excited H2 to remove the He+ ion that acts as precursor of the absorbing He(2$^3$S ). Methods. We formed thermal rate coefficients for this chemical process, leveraging the available theoretical and experimental data. The process becomes notably fast at the temperatures expected in the atmospheric layers probed by the He i triplet line. Results. Our simulations show that the proposed process severely mutes the line on GJ 3470 b and causes the nondetection on GJ 436 b. The overall efficiency of this mechanism is connected to where in the atmosphere the H2-to-H transition occurs and, ultimately, to the amount of high-energy radiation received by the planet. The process will be more significant on small exoplanets than on hotter or more massive ones, as for the latter the H2-to-H transition generally occurs deeper in the atmosphere. Conclusions. Weak He i triplet line absorption need not imply the lack of a primordial, H2-He-dominated atmosphere, an idea to bear in mind when interpreting the observations of other small exoplanets.

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arXiv2025

Data availability and requirements relevant for the Ariel space mission and other exoplanet atmosphere applications

The goal of this white paper is to provide a snapshot of the data availability and data needs primarily for the Ariel space mission, but also for related atmospheric studies of exoplanets and brown dwarfs. It covers the following data-related topics: molecular and atomic line lists, line profiles, computed cross-sections and opacities, collision-induced absorption and other continuum data, optical properties of aerosols and surfaces, atmospheric chemistry, UV photodissociation and photoabsorption cross-sections, and standards in the description and format of such data. These data aspects are discussed by addressing the following questions for each topic, based on the experience of the "data-provider" and "data-user" communities: (1) what are the types and sources of currently available data, (2) what work is currently in progress, and (3) what are the current and anticipated data needs. We present a GitHub platform for Ariel-related data, with the goal to provide a go-to place for both data-users and data-providers, for the users to make requests for their data needs and for the data-providers to link to their available data. Our aim throughout the paper is to provide practical information on existing sources of data whether in databases, theoretical, or literature sources.

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arXiv2025

Testing the performance of cross-correlation techniques to search for molecular features in JWST NIRSpec G395H observations of transiting exoplanets

Cross-correlations techniques offer an alternative method to search for molecular species in JWST observations of exoplanet atmospheres. In a previous article, we applied cross-correlation functions for the first time to JWST NIRSpec/G395H observations of exoplanet atmospheres, resulting in a detection of CO in the transmission spectrum of WASP-39b and a tentative detection of CO isotopologues. Here we present an improved version of our cross-correlation technique and an investigation into how efficient the technique is when searching for other molecules in JWST NIRSpec/G395H data. Our search results in the detection of more molecules via cross-correlations in the atmosphere of WASP-39b, including $\rm H_{2}O$ and $\rm CO_{2}$, and confirms the CO detection. This result proves that cross-correlations are a robust and computationally cheap alternative method to search for molecular species in transmission spectra observed with JWST. We also searched for other molecules ($\rm CH_{4}$, $\rm NH_{3}$, $\rm SO_{2}$, $\rm N_{2}O$, $\rm H_{2}S$, $\rm PH_{3}$, $\rm O_{3}$ and $\rm C_{2}H_{2}$) that were not detected, for which we provide the definition of their cross-correlation baselines for future searches of those molecules in other targets. We find that that the cross-correlation search of each molecule is more efficient over limited wavelength regions of the spectrum, where the signal for that molecule dominates over other molecules, than over broad wavelength ranges. In general we also find that Gaussian normalization is the most efficient normalization mode for the generation of the molecular templates.

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arXiv2025

The SPACE Program. I. The featureless spectrum of HD 86226 c challenges sub-Neptune atmosphere trends

Sub-Neptune exoplanets are the most abundant type of planet known today. As they do not have a Solar System counterpart, many open questions exist about their composition and formation. Previous spectroscopic studies rule out aerosol-free hydrogen-helium-dominated atmospheres for many characterized sub-Neptunes but are inconclusive about their exact atmospheric compositions. Here we characterize the hot (Teq=1311K) sub-Neptune HD 86226 c, which orbits its G-type host star. Its high equilibrium temperature prohibits methane-based haze formation, increasing the chances for a clear atmosphere on this planet. We use HST data taken with WFC3 and STIS from the Sub-neptune Planetary Atmosphere Characterization Experiment (SPACE) Program to perform near-infrared 1.1-1.7micrometer transmission spectroscopy and UV characterization of the host star. We report a featureless transmission spectrum that is consistent within 0.4 sigma with a constant transit depth of 418+-14ppm. The amplitude of this spectrum is only 0.01 scale heights for a H/He-dominated atmosphere, excluding a cloud-free solar-metallicity atmosphere on HD 86226 c with a confidence of 6.5 sigma. Based on an atmospheric retrieval analysis and forward models of cloud and haze formation, we find that the featureless spectrum could be due to a metal enrichment [M/H] above 2.3 (3 sigma confidence lower limit) of a cloudless atmosphere, or silicate (MgSiO3), iron (Fe), or manganese sulfide (MnS) clouds. For these species, we perform an investigation of cloud formation in high-metallicity, high-temperature atmospheres. Our results highlight that HD 86226c does not follow the aerosol trend of sub-Neptunes found by previous studies. Follow-up observations with the JWST could determine whether this planet aligns with the recent detections of metal-enriched atmospheres or if it harbors a cloud species otherwise atypical for sub-Neptunes.

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arXiv2025

Exoplanet Atmospheric Escape Observations with the Habitable Worlds Observatory

The Decadal Survey on Astronomy and Astrophysics 2020 highlights the importance of advancing research focused on discovering and characterizing habitable worlds. In line with this priority, our goal is to investigate how planetary systems evolve through atmospheric escape and to develop methods for identifying potentially Earth-like planets. By leveraging the ultraviolet (UV) capabilities of the Habitable Worlds Observatory (HWO), we can use transit spectroscopy to observe atmospheric escape in exoplanets and explore the processes that shape their evolution, assess the ability of small planets to retain their atmospheres, and search for signs of Earth-like atmospheres. To achieve this, we support the development of a UV spectrograph with moderate- to high-resolution capabilities for point-source observations, coverage of key spectral features in the 100-300 nm range, and detectors that can register high count rates reliably. This article is an adaptation of a science case document developed for the Characterizing Exoplanets Steering Committee within HWO's Solar Systems in Context Working Group.

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arXiv2025

High-resolution Ultraviolet-to-nearinfrared Characterization of Exoplanet Atmospheres

The Habitable Worlds Observatory (HWO) offers a unique opportunity to revolutionize our understanding of planetary formation and evolution. The goal of this Science Case Development Document (SCDD) is to investigate the physical and chemical processes that shape the composition and atmospheric mass loss in exoplanets. We review the key observables currently known as diagnostics of mass loss via transit observations, i.e., absorption lines of escaping hydrogen (Lyman-alpha), helium, and metals (Fe, Mg, C, O). We also explore the challenges to infer planetary formation processes based on atmospheric composition characterization. HWO could enable a broad, continuous coverage from far-ultraviolet to near-infrared spectroscopy (~100--1600 nm) at high resolution (R > 60, 000), which is essential to make these measurements, disentangle their planetary origin from stellar activity, and ultimately, contextualize the escape rates by simultaneously characterizing the composition, cloud predominance, and thermal structure of exoplanet atmospheres.

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arXiv2025

Heating, Excitation, Dissociation, and Ionization of Molecules by High-Energy Photons in Planetary Atmospheres

Photoionization by high-energy photons creates nonthermal electrons with a broad range of energies that heat and chemically transform the atmospheres of planets. The specifics of the interactions are notably different when the gas is atomic or molecular. Motivated by the idea that molecules survive to high altitudes in some exoplanets, we built a model for the energy transfer from nonthermal electrons to the H2O, H2, and O2 molecules. Our calculations show that the primary electrons of energy above about a hundred eV, a likely outcome from X-ray photoionization at moderately high atmospheric densities, expend most of their energy in ionization, dissociation, and electronic excitation collisions. In contrast, the primary electrons of less than about ten eV, such as those produced by extreme-ultraviolet photons at low densities, expend most of their energy in momentum transfer (heating), rotational, and vibrational excitation collisions. The partitioning between channels with weak thresholds is particularly sensitive to local fractional ionization. The transition between these two situations introduces a parallel transition in the way that the stellar energy is deposited in the atmosphere. Our calculations show that the nonthermal electrons enhance the ionization rate by a factor of a few or more with respect to photoionization alone but may not greatly contribute to the direct dissociation of molecules unless the local flux of far-ultraviolet photons is relatively weak. These findings highlight the importance of tracking the energy from the incident photons to the nonthermal electrons and onto the gas for problems concerned with the remote sensing and energy balance of exoplanet atmospheres.

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arXiv2025

Modelling helium in exoplanet atmospheres. A revised network with photoelectron-driven processes

The He I line at 1.08 $μ$m is a valuable tracer of atmospheric escape in exoplanet atmospheres. We expand past networks used to predict the absorbing He(2$^3S$) by including, firstly, processes that involve H$_2$ and some molecular ions and, secondly, the interaction of photoelectrons with the atmosphere. We survey the literature on the chemical-collisional-radiative processes that govern the production-loss of He(2$^3S$). We simulate the atmospheric outflow from the Neptune-sized GJ 436 b by coupling a hydrodynamic model that solves the bulk properties of the gas and a Monte Carlo model that tracks the energy degradation of the photoelectrons. We identify Penning ionization of H as a key He(2$^3S$) loss process at GJ 436 b and update its rate coefficient to a value consistent with the most recent available cross sections. The update affects notably the predicted strength of the He I line. For GJ 436 b, photoelectron-driven processes (mainly ionization and excitation) modify the He(2$^3S$) population in layers too deep to affect the in-transit spectrum. The situation might be different for other atmospheres though. The spectral energy distribution of GJ 436 has a strong effect on the predicted in-transit signal. The published non-detections of the He I line for GJ 436 b are reasonably consistent with our model predictions for a solar-metallicity atmosphere when the model adopts a recently proposed spectral energy distribution. The interpretation of the He I line at 1.08 $μ$m is model-dependent. Our revised network provides a general framework to extract more robust conclusions from measurements of this line, especially in atmospheres where H$_2$ remains abundant to high altitudes. We will explore additional, previously-ignored processes in future work.

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arXiv2025

Secondary ionisation in hot atmospheres and interactions between planetary and stellar winds

The loss of close-in planetary atmospheres is influenced by various physical processes, such as photoionisation, which could potentially affect the atmosphere survivability on a secular timescale. The amount of stellar radiation converted into heat depends on the energy of the primary electrons produced by photoionisation and the local ionisation fraction. The Lyman-alpha line is an excellent probe for atmospheric escape. We study the interaction between the planetary and the stellar wind, the difference of the predicted mass-loss rates between 1D and 2D models, the signal of Ly-a and the impact of stellar flares. Using the PLUTO code, we perform 2D hydrodynamics simulations for four different planets. We consider planets in the size range from Neptune to Jupiter. We produce synthetic Ly-a profiles to comprehend the origin of the signal, and in particular its high velocity Doppler shift. Our results indicate a trend similar to the 1D models, with a decrease in the planetary mass-loss rate for all systems when secondary ionisation is taken into account. The mass-loss rates are found to decrease by 48% for the least massive planet when secondary ionisation is accounted for. We find nevertheless a decrease that is less pronounced in 2D than in 1D. We observe differences in the Ly-a profile between the different cases and significant asymmetries in all of them, especially for the lower mass planets. Finally, we observe that stellar flares do not affect the mass-loss rate because they act, in general, on a timescale that is too short. We find velocities in the escaping atmosphere up to 100 km/s, with the gas moving away from the star, which could be the result of the interaction with the stellar wind. Furthermore, we find that stellar flares generally occur on a timescale that is too short to have a visible impact on the mass-loss rate of the atmosphere.

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arXiv2024

Database of Candidate Targets for the LIFE Mission

We present the database of potential targets for the Large Interferometer For Exoplanets (LIFE), a space-based mid-infrared nulling interferometer mission proposed for the Voyage 2050 science program of the European Space Agency (ESA). The database features stars, their planets and disks, main astrophysical parameters, and ancillary observations. It allows users to create target lists based on various criteria to predict, for instance, exoplanet detection yields for the LIFE mission. As such, it enables mission design trade-offs, provides context for the analysis of data obtained by LIFE, and flags critical missing data. Work on the database is in progress, but given its relevance to LIFE and other space missions, including the Habitable Worlds Observatory (HWO), we present its main features here. A preliminary version of the LIFE database is publicly available on the German Astrophysical Virtual Observatory (GAVO).

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arXiv2023

Dynamics and Clouds in Planetary Atmospheres from Telescopic Observations

This review presents an insight into our current knowledge of the atmospheres of the planets Venus, Mars, Jupiter, Saturn, Uranus and Neptune, the satellite Titan, and those of exoplanets. It deals with the thermal structure, aerosol properties (hazes and clouds, dust in the case of Mars), chemical composition, global winds and selected dynamical phenomena in these objects. Our understanding of atmospheres is greatly benefitting from the discovery in the last three decades of thousands of exoplanets. The exoplanet properties span a broad range of conditions, and it is fair to expect as much variety for their atmospheres. This complexity is driving unprecedented investigations of the atmospheres, where those of the solar systems bodies are the obvious reference. We are witnessing a significant transfer of knowledge in both directions between the investigations dedicated to Solar System and exoplanet atmospheres, and there are reasons to think that this exchange will intensity in the future. We identify and select a list of research subjects that can be conducted at optical and infrared wavelengths with future and currently available ground-based and space-based telescopes, but excluding those from the space missions to solar system bodies.

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arXiv2023

Detection of Carbon Monoxide in the Atmosphere of WASP-39b Applying Standard Cross-Correlation Techniques to JWST NIRSpec G395H Data

Carbon monoxide was recently reported in the atmosphere of the hot Jupiter WASP-39b using the NIRSpec PRISM transit observation of this planet, collected as part of the JWST Transiting Exoplanet Community Early Release Science (JTEC ERS) Program. This detection, however, could not be confidently confirmed in the initial analysis of the higher resolution observations with NIRSpec G395H disperser. Here we confirm the detection of CO in the atmosphere of WASP-39b using the NIRSpec G395H data and cross-correlation techniques. We do this by searching for the CO signal in the unbinned transmission spectrum of the planet between 4.6 and 5.0 $μ$m, where the contribution of CO is expected to be higher than that of other anticipated molecules in the planet's atmosphere. Our search results in a detection of CO with a cross-correlation function (CCF) significance of $6.6 σ$ when using a template with only ${\rm ^{12}C^{16}O}$ lines. The CCF significance of the CO signal increases to $7.5 σ$ when including in the template lines from additional CO isotopologues, with the largest contribution being from ${\rm ^{13}C^{16}O}$. Our results highlight how cross-correlation techniques can be a powerful tool for unveiling the chemical composition of exoplanetary atmospheres from medium-resolution transmission spectra, including the detection of isotopologues.

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arXiv2023

Large Interferometer For Exoplanets (LIFE). X. Detectability of currently known exoplanets and synergies with future IR/O/UV reflected-starlight imaging missions

The next generation of space-based observatories will characterize the atmospheres of low-mass, temperate exoplanets with the direct-imaging technique. This will be a major step forward in our understanding of exoplanet diversity and the prevalence of potentially habitable conditions beyond the Earth. We compute a list of currently known exoplanets detectable with the mid-infrared Large Interferometer For Exoplanets (LIFE) in thermal emission. We also compute the list of known exoplanets accessible to a notional design of the Habitable Worlds Observatory (HWO), observing in reflected starlight. With a pre-existing method, we processed the NASA Exoplanet Archive and computed orbital realizations for each known exoplanet. We derived their mass, radius, equilibrium temperature, and planet-star angular separation. We used the LIFEsim simulator to compute the integration time ($t_{int}$) required to detect each planet with LIFE. A planet is considered detectable if a broadband signal-to-noise ratio $S/N$=7 is achieved over the spectral range $4-18.5μ$m in $t_{int}\leq$100 hours. We tested whether the planet is accessible to HWO in reflected starlight based on its notional inner and outer working angles, and minimum planet-to-star contrast. LIFE's reference configuration (four 2-m telescopes with 5% throughput and a nulling baseline between 10-100 m) can detect 212 known planets within 20 pc. Of these, 55 are also accessible to HWO in reflected starlight, offering a unique opportunity for synergies in atmospheric characterization. LIFE can also detect 32 known transiting exoplanets. Furthermore, 38 LIFE-detectable planets orbit in the habitable zone, of which 13 with $M_p<5M_\oplus$ and 8 with $5M_\oplus<M_p<10M_\oplus$. LIFE already has enough targets to perform ground-breaking analyses of low-mass, habitable-zone exoplanets, a fraction of which will also be accessible to other instruments.

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arXiv2022

An efficient Monte Carlo model for the slowing down of photoelectrons. Application to H-$α$ in exoplanet atmospheres

Photoelectrons, the fast electrons produced in the photoionization of planetary atmospheres, drive transformations in the atmospheric gas that are often inhibited by energy considerations for thermal electrons. The transformations include excitation and ionization of atoms and molecules, which affect the detectability of these gases and constrain the fraction of incident stellar radiation that transforms into heat. To gain insight into these important questions, we build a Monte Carlo model that solves the slowing down of photoelectrons in a gas with arbitrary amounts of H and He atoms and thermal electrons. Our novel multi-score scheme differs from similar tools in that it efficiently handles rare collisional channels, as in the case of low-abundance excited atoms that undergo superelastic and inelastic collisions. The model is validated and its performance demonstrated. Further, we investigate whether photoelectrons might affect the population of the excited hydrogen H(2) detected at some exoplanet atmospheres by transmission spectroscopy in the H-$α$ line. For the ultra-hot Jupiter HAT-P-32b, we find that photoelectron-driven excitation of H(2) is inefficient at the pressures probed by the line core but becomes significant (yet sub-dominant) deeper in the atmosphere where the line wings form. The contribution of photoelectrons to the destruction of H(2) either by collisional deexcitation or ionization is entirely negligible, a conclusion likely to hold for exoplanet atmospheres at large. Importantly, photoelectrons dominate the gas ionization at the altitudes probed by the H-$α$ line, a fact that will likely affect, even if indirectly, the population of H(2) and other tracers such as metastable helium. Future modeling of these excited levels should incorporate photoelectron-driven ionization.

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