arXiv subjects

Michiel Min

Publications and source records attributed to Michiel Min.

At least 19 recordsLinked to original sources

arXiv2026

Early Exploration of the Scientific Discovery Space for the Habitable Worlds Observatory

The Habitable Worlds Observatory (HWO) is a future NASA flagship mission concept identified by the Astro2020 Decadal Survey as the highest priority for large space missions. HWO should conduct "transformative astrophysics" and search for biosignatures in the atmospheres of approximately 25 potentially Earth-like planets. To further the early-stage development of HWO, NASA formed the Science, Technology, Architecture Review Team (START). In turn, START invited the scientific community to join working groups to explore the potential discovery space. In this paper, we present 70 science cases that resulted from this process. The cases address four scientific pillars: growth of galaxies (15 cases), evolution of the elements (13 cases), solar systems in context (32 cases), and living worlds (10 cases). Combined, they would address 27 of the 30 science questions and discovery areas identified by Astro2020. The 140 observing programs needed for the 70 investigations encompass a rich variety of spectroscopic (for 87% of science cases) and photometric (for 30%) observations extending from the UV to the NIR. Additionally, high-contrast and polarimetric capabilities would be needed for 34% and 27% of science cases, respectively. Access to UV wavelengths is critical: 83% of science cases need data at wavelengths <400 nm, and 26% extend to <100 nm. In the NIR, 26% of science cases need observations at wavelengths >=2000 nm. Pursuing the full portfolio of science would also necessitate precise astrometry for planet mass measurement, rapid response capabilities, a large instantaneous field of regard, non-sidereal tracking, saturation mitigation strategies, and high dynamic range.

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arXiv2026

A new window in time: a mid-infrared slit spectroscopy mode for precision time-series astronomy with JWST/MIRI

The Mid-Infrared Instrument (MIRI) on board the James Webb Space Telescope (JWST) provides Low Resolution Spectroscopy (LRS) over 5-12 um at a resolving power of R ~ 100. To date, all MIRI LRS time-series observations (TSOs) have been carried out in slitless mode, since long-duration pointing stability within the narrow 4.7'' x 0.51'' slit could not previously be guaranteed, leading to the possibility of degraded TSOs due to jitter. Commissioning activities established the telescope jitter to be less than 1 milliarcsecond (mas), four times less than the initial requirement. It was, therefore, worth assessing the suitability of the MIRI LRS slit as a TSO mode for precision time-domain science; this is the aim of the Cycle 3 program (PID 6219, PI: A. Dyrek). We observed a transit of the exoplanet HAT-P-12b over ~10 hours and compared the results to archival slitless observations of the same target (PID 1281, PI: P.-O. Lagage). Two independent data reductions of the transit spectra of both slit and slitless configurations are consistent within 1 sigma, validating the feasibility of the new mode. A joint fit of the slit and slitless observations confirms the presence of a spectral feature near 7.5 um. Using measured JWST pointing variations, we estimated slit-loss variations to be smaller than 40 ppm at 10 um. The drawback of the slitless mode is a higher background. Compellingly, the slit background is ~ 38 times lower on average than in slitless mode, improving sensitivity for faint targets (Jmag ~ 13-15). We also identified time-correlated noise unique to the slit dataset at long wavelengths, which requires further investigation. This new capability of TSOs in slit mode, which will be supported in Cycle 7 (in 2028), opens a new avenue for precision time-series astronomy for faint targets with JWST/MIRI.

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arXiv2026

Characterizing the oxidation state of rocky exoplanets with the Large Interferometer for Exoplanets (LIFE)

The oxidation state of rocky exoplanets is expected to play a fundamental role in shaping the chemical composition of their secondary atmospheres by influencing the chemical composition of volcanically released gasses. Distinguishing planetary redox states through direct atmospheric characterization would offer insight into the formation and evolution of secondary atmospheres on exoplanets and inform the background chemistry of putative biosignatures. The Large Interferometer For Exoplanets (LIFE) mission concept aims to employ a space-based mid-infrared nulling interferometer to characterize exoplanetary atmospheres. In this work, we assess LIFE's performance in distinguishing the redox states of rocky exoplanets by direct spectroscopic measurements. We focus on the observability and spectral features of redox-sensitive molecules in secondary atmospheres of Earth-sized exoplanets. We develop and apply a retrieval framework based on the ARtful modeling Code for exoplanet Science (ARCiS) and the LIFE mission simulator (LIFEsim) to simulate observations of Earth-sized planets with atmospheres from a range of plausible mantle redox conditions. Our simulations show that LIFE in its baseline configuration can successfully constrain dominant atmospheric species (e.g. CO2, CH4 and NH3) with sufficient accuracy to distinguish redox states for planets orbiting a Sun-like star at 10 pc. Retrieved redox-sensitive molecules show clear trends across oxidation states, with CO2 dominating in oxidizing (with oxygen fugacity fO2 $\sim$ IW+2 to IW+6, where IW is the iron-w$\"u$stite buffer) environments and NH3 in reducing (fO2 $\sim$ IW-2 to IW-6) environments, and CH4 serving as a strong tracer among intermediate (fO2 $\sim$ IW+4 to IW-4) oxidation states.

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arXiv2026

A Cloudy Fit to the Atmosphere of WASP-107 b

Context. WASP-107 b has been observed comprehensively by JWST in the near- and mid-IR bands, making it an ideal planet to probe the composition and internal dynamics. Recent analysis reveals a 8-10 um silicate feature, but it still remains uncertain how silicate clouds form on this planet. Aims. We aim at fitting the complete JWST spectrum of WASP-107 b, from 0.9 um to 12 um with a physically motivated cloud model and self-consistent temperature profile. Methods. Two-stream radiative transfer is coupled to a cloud formation model until convergence between cloud and temperature profiles is reached. We search a model grid spanning metallicity, turbulent diffusivity, internal heat flux and nucleation parameters to find the best fit model. Results. The silicate cloud feature at 10 um and the near-IR molecular band strength can be simultaneously and naturally explained without assuming a parametrized temperature profile. A moderate vertical diffusivity of Kzz = 10^9 cm^2 s^-1 is needed to bring the cloud particles to the upper atmosphere of WASP-107 b. This Kzz is favored by the joint fitting of the near-IR water feature and mid-IR silicate feature -- both sensitive to clouds. From the strength of H2O and CO2 bands, our model suggests a metallicity 17 times solar. Conclusions. Even in warm planets such as WASP-107 b, silicate clouds can form in the relatively cool upper atmosphere because turbulence uplifts vapor and cloud particles. Despite having considerably fewer degrees of freedom, the self-consistent modeling approach successfully fits WASP-107 b's multi-wavelength data, instilling confidence in the derived physical parameters.

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arXiv2026

Hot and cloudy: High temperature clouds in super-Earths and sub-Neptunes

JWST observations provide for the first time evidence for an atmosphere on a rocky exoplanet - 55 Cnc e. The atmosphere of 55 Cnc e is hot with $\text{T}_{\text{eq}}>2000$K and shows strong variability, for which cloud formation above a molten crust could be one possible explanation. The composition of the atmosphere of 55 Cnc e is still unknown but suggests the presence of volatiles. We have run cloud formation models on a grid of N-dominated, O-dominated, C-dominated and H-dominated atmospheres to investigate which type of cloud we could expect on hot super-Earths and hot sub-Neptunes ($1000$K $<$ T $<$ $3000$K). Our models combine radiative transfer with equilibrium chemistry of the gaseous and condensed phases, vertical mixing of condensable species, sedimentation, nucleation and coagulation. We find that the condensability of species is highly dependent on the oxygen abundance of an atmosphere. Oxygen poor atmospheres can be heated by UV and optical absorbers PS, TiO and CN which create temperature inversions. These inhibit condensation. Oxygen rich atmospheres are colder without temperature inversions, and are therefore more favourable environments for cloud formation. The major expected cloud component in O-dominated atmospheres with solar refractory abundance is TiO$_2$(s). Spectral features of clouds in these worlds are stronger in transmission than in emission, in particular at short wavelengths. We find a lack of optical data of solid species in comparison to the variety of stable cloud components which can form on hot, rocky planets.

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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

Non-detection of J1407b in ALMA Band 7 Observations

We report on ALMA Band 7 continuum observations towards the star 1SWASP J140747.93-394542.6 taken in mid 2024. These observations were a follow-up of a previous detection of an emission source in the J1407 field of view at an unexpected position in 2017. Proper motion analysis indicated that if this were the object responsible for the 2007 eclipse of J1407, it would be detectable at a new position, but still within an ALMA field of view in 2024. Here we present the non-detection of emission in the ALMA field of view (to a 1{\sigma} upper limit of 17.5 {\mu}Jy),at both the 2017 position, and the position expected from proper motions. We place upper limits on a source at the proper motion corrected expected location in this 2024 data, and rule out the possibility of a dusty object being responsible for the 2007 eclipse

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arXiv2025

Identifying rocky planets and water worlds among sub-Neptune-sized exoplanets with the Habitable Worlds Observatory

Astronomers are debating whether the plentiful "sub-Neptune" exoplanets -- worlds a bit larger than Earth but smaller than Neptune -- are predominantly rocky planets, water-rich "ocean worlds," or gas-enshrouded mini-Neptunes. This question is crucial because such sub-Neptune-sized planets are among the most common in our galaxy, yet we have no analog in our own solar system, making them a key to understanding planet formation and diversity. It also directly impacts the search for habitable worlds: larger-than-Earth planets with solid surfaces or oceans could support life, whereas gas-rich mini-Neptunes likely cannot. However, distinguishing these types using only a planet's mass and radius is very challenging, because different compositions can produce similar densities, leaving a world's nature ambiguous with current data. The proposed Habitable Worlds Observatory (HWO), a future NASA flagship telescope, offers a solution. HWO could directly image and spectroscopically analyze starlight reflected from 50~100 sub-Neptunes around nearby stars, aiming to reveal their atmospheric compositions and potential surfaces. Using visible and near-infrared spectroscopy along with sensitive polarimetry, HWO would detect atmospheric gases (such as water vapor, methane, and carbon dioxide) and search for telltale surface signatures, including rock absorption features and the characteristic reflectivity patterns of oceans. By analyzing these signals, we could determine whether sub-Neptunes are large rocky planets or water worlds rather than gas-dominated mini-Neptunes. Crucially, expanding the search beyond Earth-sized planets to include these abundant sub-Neptunes may uncover entirely new classes of potentially habitable worlds, directly advancing HWO's mission to identify and characterize planets that could support life.

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arXiv2025

Characterizing the Dynamics and Chemistry of Transiting Exoplanets with the Habitable World Observatory

The primary scientific objective of this Habitable Worlds Observatory (HWO) Science Case Development Document (SCDD) is to measure planetary rotation rates of transiting exoplanets to determine the structure, composition, circulation, and aerosol properties of their planetary atmospheres. For this analysis, HWO would obtain spectroscopic phase curves for planets with orbital periods of 5 - 20+ days, to assess tidal locking radius assumptions. Extending phase curve studies out to longer orbital periods than accessible with current and near-future telescopes will enable detailed investigation of atmospheric structure, composition, and circulation for planets that are much cooler than the more highly irradiated planets accessible with JWST phase curve observations (i.e., Teq < 500 K for HWO versus 1400 K <= Teq <= 2600 K for JWST). Broad wavelength coverage extending from the UV to the NIR would capture both reflected light and thermal emission, enabling HWO to conduct comprehensive characterization of planetary atmospheres. UV observations would probe high altitudes, thereby providing valuable insights into atmospheric (dis)equilibrium, aerosol properties, and the effects of photochemical processes on atmospheric composition. We also discuss the role of polarimetry in the classification of aerosols and the associated role they play in the atmospheric energy budget that directly ties them to the chemistry and circulation structure of the atmosphere.

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arXiv2024

Combined analysis of the 12.8 and 15 $\mu m$ JWST/MIRI eclipse observations of TRAPPIST-1 b

The first JWST/MIRI photometric observations of TRAPPIST-1 b allowed for the detection of the thermal emission of the planet at 15 $\mu m$, suggesting that the planet could be a bare rock with a zero albedo and no redistribution of heat. These observations at 15 $\mu m$ were acquired as part of GTO time that included a twin program at 12.8 $\mu m$ in order to have a measurement in and outside the CO$_2$ absorption band. Here we present five new occultations of TRAPPIST-1 b observed with MIRI in an additional photometric band at 12.8 $\mu m$. We perform a global fit of the 10 eclipses and derive a planet-to-star flux ratio and 1-$\sigma$ error of 452 $\pm$ 86 ppm and 775 $\pm$ 90 ppm at 12.8 $\mu m$ and 15 $\mu m$, respectively. We find that two main scenarios emerge. An airless planet model with an unweathered (fresh) ultramafic surface, that could be indicative of relatively recent geological processes fits well the data. Alternatively, a thick, pure-CO2 atmosphere with photochemical hazes that create a temperature inversion and result in the CO2 feature being seen in emission also works, although with some caveats. Our results highlight the challenges in accurately determining a planet's atmospheric or surface nature solely from broadband filter measurements of its emission, but also point towards two very interesting scenarios that will be further investigated with the forthcoming phase curve of TRAPPIST-1 b.

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arXiv2024

A new pathway to SO$_2$: Revealing the NUV driven sulfur chemistry in hot gas giants

Context. Photochemistry is a key process driving planetary atmospheres away from local thermodynamic equilibrium. Recent observations of the H$_2$ dominated atmospheres of hot gas giants have detected SO$_2$ as one of the major products of this process. Aims. We investigate which chemical pathways lead to the formation of SO$_2$ in an atmosphere, and we investigate which part of the flux from the host star is necessary to initiate SO$_2$ production. Methods. We use the publicly available S-N-C-H-O photochemical network in the VULCAN chemical kinetics code to compute the disequilibrium chemistry of an exoplanetary atmosphere. Results. We find that there are two distinct chemical pathways that lead to the formation of SO$_2$. The formation of SO$_2$ at higher pressures is initiated by stellar flux >200 nm, whereas the formation of SO$_2$ at lower pressures is initiated by stellar flux <200 nm. In deeper layers of the atmosphere, OH is provided by the hydrogen abstraction of H$_2$O, and sulfur is provided by the photodissociation of SH and S$_2$, which leads to a positive feedback cycle that liberates sulfur from the stable H$_2$S molecule. In higher layers of the atmosphere, OH is provided by the photodissociation of H$_2$O, and sulfur can be liberated from H$_2$S by either photodissociation of SH and S$_2$, or by the hydrogen abstraction of SH. Conclusions. We conclude that the stellar flux in the 200-350 nm wavelength range as well as the ratio of NUV/UV radiation are important parameters determining the observability of SO$_2$. In addition we find that there is a diversity of chemical pathways to the formation of SO$_2$. This is crucial for the interpretation of SO$_2$ detections and derived elemental abundance ratios and overall metallicities.

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arXiv2024

The effect of dynamic temperatures on pebble dynamics and planet formation

Context. To date, more than 5000 exoplanets have been discovered. The large majority of these planets have a mass between 1 and 17 {M_\oplus}, making them so-called super-Earths and mini-Neptunes. The exact formation process for this abundant planet population has not yet been fully constrained. Aims. Recent studies on the formation of these planets make various assumptions with regard to the disk. The primary mass budget, held in pebbles, is either assumed to have a constant size or is parametrized as a flux. Simplifications of the temperature structure, in the form of a static power law, do not consider the temperature evolution and high magnitudes of heating in the inner part of the disk. In this study, we aim to investigate the effect these simplifications of temperature and pebble sizes have on the pebble densities and resulting planet populations. Methods. To constrain the timescales needed to form super-Earths, we developed a model for exploring a large parameter space. We included the effect of two different temperature prescriptions on a viscously accreting and spreading disk. We formed a pebble reservoir utilizing a simplified conversion timescale with a time- and radially dependent Stokes number for the dust. We then tracked the temporal evolution of the surface densities of gas, dust, and pebbles. Pebbles were allowed to drift and be accreted onto a growing protoplanet. As a planet grows, it exerts a torque on the disk, carving out a gap and affecting the pebble drift, before halting the growth of the planet.

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arXiv2024

Large Interferometer For Exoplanets (LIFE). XIV. Finding terrestrial protoplanets in the galactic neighborhood

The increased brightness temperature of young rocky protoplanets during their magma ocean epoch makes them potentially amenable to atmospheric characterization to distances from the solar system far greater than thermally equilibrated terrestrial exoplanets, offering observational opportunities for unique insights into the origin of secondary atmospheres and the near surface conditions of prebiotic environments. The Large Interferometer For Exoplanets (LIFE) mission will employ a space-based mid-infrared nulling interferometer to directly measure the thermal emission of terrestrial exoplanets. Here, we seek to assess the capabilities of various instrumental design choices of the LIFE mission concept for the detection of cooling protoplanets with transient high-temperature magma ocean atmospheres, in young stellar associations in particular. Using the LIFE mission instrument simulator (LIFEsim) we assess how specific instrumental parameters and design choices, such as wavelength coverage, aperture diameter, and photon throughput, facilitate or disadvantage the detection of protoplanets. We focus on the observational sensitivities of distance to the observed planetary system, protoplanet brightness temperature using a blackbody assumption, and orbital distance of the potential protoplanets around both G- and M-dwarf stars. Our simulations suggest that LIFE will be able to detect (S/N $\geq$ 7) hot protoplanets in young stellar associations up to distances of $\approx$100 pc from the solar system for reasonable integration times (up to $\sim$hours). Detection of an Earth-sized protoplanet orbiting a solar-sized host star at 1 AU requires less than 30 minutes of integration time. M-dwarfs generally need shorter integration times. The contribution from wavelength regions $<$6 $\mu$m is important for decreasing the detection threshold and discriminating emission temperatures.

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arXiv2024

ExoLyn: a golden mean approach to multi-species cloud modelling in atmospheric retrieval

Context. Clouds are ubiquitous in exoplanets' atmospheres and play an important role in setting the opacity and chemical inventory of the atmosphere. Understanding clouds is a critical step in interpreting exoplanets' spectroscopic data. Aims. The aim is to model the multi-species nature of clouds in atmospheric retrieval studies. To this end, we develop ExoLyn - a 1D cloud model that balances physical consistency with computational efficiency. Methods. ExoLyn solves the transport equation of cloud particles and vapor under cloud condensation rates that are self-consistently calculated from thermodynamics. ExoLyn is a standalone, open source package capable to be combined with \texttt{optool} to calculate solid opacities and with \texttt{petitRADTRANS} to generate transmission or emission spectra. Results. With ExoLyn we find that the compositional structure of clouds in hot Jupiter planets' atmospheres is layered with a cloud dominated by magnesium-silicates on top of an iron cloud. This finding is consistent with more complex cloud formation models but can be obtained with ExoLyn in only a few seconds. The composition of the cloud particles can be constrained from the spectrum, for example, MgSiO3 and Mg2SiO4 components give rise to an absorption feature at 8 - 10 um. We investigate the dependence of the cloud structure on the bulk elemental composition of the planet and find that SiO2-dominated clouds forms on metal-rich planet and Fe clouds with strong extinction effect forms on C-rich planet. Conclusions. Designed towards maximum flexibility, ExoLyn can also be used in retrieval analysis of sub-Neptunes and self-luminous planets. The efficiency of ExoLyn opens the possibility of joint retrieval of exoplanets' gas and cloud components.

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arXiv2024

Why heterogeneous cloud particles matter: Iron-bearing species and cloud particle morphology affects exoplanet transmission spectra

The possibility of observing spectral features in exoplanet atmospheres with space missions like JWST and ARIEL necessitates the accurate modelling of cloud particle opacities. In exoplanet atmospheres, cloud particles can be made from multiple materials and be considerably chemically heterogeneous. Therefore, assumptions on the morphology of cloud particles are required to calculate their opacities. The aim of this work is to analyse how different approaches to calculate the opacities of heterogeneous cloud particles affect cloud particle optical properties. We calculate cloud particle optical properties using seven different mixing treatments: four effective medium theories (EMTs: Bruggeman, Landau-Lifshitz-Looyenga (LLL), Maxwell-Garnett, and Linear), core-shell, and two homogeneous cloud particle approximations. We study the mixing behaviour of 21 commonly considered cloud particle materials for exoplanets. To analyse the impact on observations, we study the transmission spectra of HATS-6b, WASP-39b, WASP-76b, and WASP-107b.Materials with large refractive indices, like iron-bearing species or carbon, can change the optical properties of cloud particles when they comprise less than 1\% of the total particle volume. The mixing treatment of heterogeneous cloud particles also has an observable effect on transmission spectroscopy. Assuming core-shell or homogeneous cloud particles results in less muting of molecular features and retains the cloud spectral features of the individual cloud particle materials. The predicted transit depth for core-shell and homogeneous cloud particle materials are similar for all planets used in this work. If EMTs are used, cloud spectral features are broader and cloud spectral features of the individual cloud particle materials are not retained. Using LLL leads to less molecular features in transmission spectra compared to Bruggeman.

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arXiv2024

Dust mineralogy and variability of the inner PDS 70 disk

The inner disk of the young star PDS 70 may be a site of rocky planet formation, with two giant planets detected further out. Solids in the inner disk may inform us about the origin of this inner disk water and nature of the dust in the rocky planet-forming regions. We aim to constrain the chemical composition, lattice structure, and grain sizes of small silicate grains in the inner disk of PDS 70, observed both in JWST/MIRI MRS and Spitzer IRS. We use a dust fitting model, called DuCK, based on a two-layer disk model. We use Gaussian Random Field and Distribution of Hollow Spheres models to obtain two sets of dust opacities. The third set of opacities is obtained from aerosol spectroscopy. We use stoichiometric amorphous silicates, forsterite, and enstatite in our analysis. We also used iron-rich and magnesium-rich amorphous silicate and fayalite dust species to study the iron content. The Gaussian Random Field opacity agrees well with the observed spectrum. In both MIRI and Spitzer spectra, amorphous silicates are the dominant dust species. Crystalline silicates are dominated by iron-poor olivine. We do not find strong evidence for enstatite. Moreover, the MIRI spectrum indicates larger grain sizes than the Spitzer spectrum, indicating a time-variable small grain reservoir. The inner PDS 70 disk is dominated by a variable reservoir of optically thin warm amorphous silicates. We suggest that the small grains detected in the inner PDS 70 disk are likely transported inward from the outer disk as a result of filtration and fragmentation at the ice line. In addition, the variation between MIRI and Spitzer data can be explained by the grain growth over 15 years and a dynamical inner disk where opacity changes occur resulting from the highly variable hot innermost dust reservoir.

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arXiv2024

The PLATO Mission

PLATO (PLAnetary Transits and Oscillations of stars) is ESA's M3 mission designed to detect and characterise extrasolar planets and perform asteroseismic monitoring of a large number of stars. PLATO will detect small planets (down to <2 R_(Earth)) around bright stars (<11 mag), including terrestrial planets in the habitable zone of solar-like stars. With the complement of radial velocity observations from the ground, planets will be characterised for their radius, mass, and age with high accuracy (5 %, 10 %, 10 % for an Earth-Sun combination respectively). PLATO will provide us with a large-scale catalogue of well-characterised small planets up to intermediate orbital periods, relevant for a meaningful comparison to planet formation theories and to better understand planet evolution. It will make possible comparative exoplanetology to place our Solar System planets in a broader context. In parallel, PLATO will study (host) stars using asteroseismology, allowing us to determine the stellar properties with high accuracy, substantially enhancing our knowledge of stellar structure and evolution. The payload instrument consists of 26 cameras with 12cm aperture each. For at least four years, the mission will perform high-precision photometric measurements. Here we review the science objectives, present PLATO's target samples and fields, provide an overview of expected core science performance as well as a description of the instrument and the mission profile at the beginning of the serial production of the flight cameras. PLATO is scheduled for a launch date end 2026. This overview therefore provides a summary of the mission to the community in preparation of the upcoming operational phases.

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arXiv2024

FlopPITy: Enabling self-consistent exoplanet atmospheric retrievals with machine learning

Interpreting the observations of exoplanet atmospheres to constrain physical and chemical properties is typically done using Bayesian retrieval techniques. Because these methods require many model computations, a compromise is made between model complexity and run time. Reaching this compromise leads to the simplification of many physical and chemical processes (e.g. parameterised temperature structure). Here we implement and test sequential neural posterior estimation (SNPE), a machine learning inference algorithm, for exoplanet atmospheric retrievals. The goal is to speed up retrievals so they can be run with more computationally expensive atmospheric models, such as those computing the temperature structure using radiative transfer. We generate 100 synthetic observations using ARCiS (ARtful Modeling Code for exoplanet Science, an atmospheric modelling code with the flexibility to compute models in varying degrees of complexity) and perform retrievals on them to test the faithfulness of the SNPE posteriors. The faithfulness quantifies whether the posteriors contain the ground truth as often as we expect. We also generate a synthetic observation of a cool brown dwarf using the self-consistent capabilities of ARCiS and run a retrieval with self-consistent models to showcase the possibilities that SNPE opens. We find that SNPE provides faithful posteriors and is therefore a reliable tool for exoplanet atmospheric retrievals. We are able to run a self-consistent retrieval of a synthetic brown dwarf spectrum using only 50,000 forward model evaluations. We find that SNPE can speed up retrievals between $\sim2\times$ and $\geq10\times$ depending on the computational load of the forward model, the dimensionality of the observation, and the signal-to-noise ratio of the observation. We make the code publicly available for the community on Github.

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