arXiv subjects

Giulia Roccetti

Publications and source records attributed to Giulia Roccetti.

9 recordsLinked to original sources

arXiv2026

Habitability of Tidally Heated H$_2$-Dominated Exomoons around Free-Floating Planets

Exomoons around free-floating planets (FFPs) can survive their host planet's ejection. Such ejections can increase their orbital eccentricity, providing significant tidal heating in the absence of any stellar energy source. Previous studies suggested that liquid water could exist on such moons under thick CO$_2$-dominated atmospheres, but these models faced challenges with CO$_2$ condensation and atmospheric collapse, particularly in the high-pressure regimes that favoured long-term habitability. To address this, we employ a self-consistent model, including radiative transfer and equilibrium chemistry with condensation, to simulate a more stable hydrogen-dominated atmosphere for a range of initial chemical compositions, including C, O, and N. We find that such atmospheres can effectively trap heat via collision-induced absorption of H$_2$, maintaining surface temperatures suitable for liquid water for time-scales of up to 4.3 Gyr, depending on the surface pressure, while not prone to condensation-induced collapse. Wet-dry cycling caused by the strong tides together with the alkalinity of dissolved NH$_3$ could create favourable conditions for RNA polymerisation and thus support the emergence of life.

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arXiv2026

Habitable Worlds Observatory Living Worlds Working Group: Surface Biosignatures on Potentially Habitable Exoplanets

The Habitable Worlds Observatory (HWO) is the first NASA Astrophysics flagship mission with a key science goal of searching for signs of life on rocky habitable exoplanets beyond our solar system. The Living Worlds Community Working Group was charged with investigating how HWO could characterize planets orbiting stars in the solar neighborhood, search for signs of life, and interpret potential biosignatures within a false positive and false negative framework. The Surface Biosignatures Task assessed the measurement requirements and instrument needs to detect these biosignatures under an 'Earth through time' scenario. Surface biosignatures are planetary-scale spectral features resulting from absorption and/or scattering of radiation by organisms containing photosynthetic and non-photosynthetic pigments. This secondary class of biosignature can be used to corroborate atmospheric biosignatures by providing multiple lines of evidence to aid in assessing their biogenicity. Furthermore, surface biopigments are the only way to detect more primitive forms of anoxygenic photosynthesis if oxygenic photosynthesis never evolved. Key Findings: To detect biopigments on the surface of planets under Archean, Proterozoic, and Modern atmospheric compositions (15 percent coverage, 50 percent cloud cover), an SNR of 20-40 would be needed over 500-1100 nm. However, there may be some cases in which lower SNR is required; studies are ongoing. Coronagraph requirements: (1) The detection of surface biosignatures would be greatly enhanced by having as many parallel coronagraph channels as possible across the entire wavelength range with no or minimal gaps between channels. (2) Retrieval studies revealed that restricted wavelength ranges (e.g., 0.4 - 0.7 microns), such as may be used during initial survey strategies, are not sufficient to deconvolve the biopigment features from the abiotic background.

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arXiv2025

Planet Earth in reflected and polarized light -- III. Modeling and analysis of a decade-long catalog of Earthshine observations

Earthshine observations offer a unique opportunity to study Earth as an exoplanet seen from the Moon. As the Sun-Earth-Moon geometry changes, Earth can be observed as a spatially unresolved exoplanet at different phase angles, providing important context for future observations of Earth-like exoplanets. Here, we present a catalog of Earthshine polarization spectra obtained with FORS2 on the VLT, covering diverse scenes, surface conditions, cloud properties, and weather patterns for over a decade. For the first time, we model this extensive dataset in detail using a homogeneous modeling framework. Previous efforts to model some of these spectra struggled to reproduce the observed polarization continuum, even with advanced 3D radiative transfer models incorporating satellite-derived surface and atmospheric data. We improve upon this with a 3D model that includes subgrid cloud variability, wavelength-dependent surface albedo maps, and an accurate treatment of ocean glint. Our simulations successfully reproduce most observed spectra to a much higher precision than previously possible. Our statistical analysis reveals that the spectral slope in the visible can distinguish between ocean and mixed surfaces in both reflected and polarized light, which is not possible using broadband filters alone. Polarized light at large phase angles, beyond the Rayleigh scattering regime, is particularly effective in differentiating oceans from land, unlike reflected light. We also identify correlations between cloud optical thickness and the polarized spectral slope, and between cloud cover and broadband B-R differences in reflected light, demonstrating the diagnostic power of these observations. This work highlight the potential of polarization for characterizing Earth-like exoplanets. From polarization alone, we can infer oceans, vegetation, and an active water cycle, key indicators of a habitable planet.

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arXiv2025

Detecting alien living worlds and photosynthetic life using imaging polarimetry with the HWO coronagraph

Our Earth, being the only living planet that we know, provides us with clues that photosynthetic life-forms may be dominant on other exoplanets for billions of years. Spectropolarimetric signatures of the terrestrial photosynthetic life (PSLife) are well studied in the lab and remotely sensed with space and airborne instrumentation. An astonishing biosignature revealed by these measurements is an extremely strong linear polarization (tens \%) associated with broad absorption bands of biological pigments (biopigments) driving photosynthesis in various organisms. Also, unique circular-polarization signatures are associated with biopigments and other complex macromolecules as a sign of homochirality which is ubiquitous in terrestrial life forms. Thus, low-resolution spectro- or multi-band polarimetry of exoplanets directly imaged at an unprecedented contrast using the HWO coronagraph is a novel opportunity for a robust discovery of life on exoplanets. Here we propose to carry out two surveys and two follow-up observing programs. Survey 1 will identify potentially habitable planets (PHPs) through detection of atmospheres, clouds and liquid surface water (ocean) using linear polarimetry. Survey 2 will identify Living World (LW) candidates among PHPs by searching for strong linear polarization signatures associated with strong and broad absorption bands reminiscent of terrestrial biopigments. Follow-up program 3 will obtain multi-color surface maps of LWs, determine the distribution and abundance of alien photosynthetic organisms with exo-biopigments (exoBPs) and correlate their properties with the atmospheric and surface compositions. Follow-up program 4 will employ circular polarization to verify homochirality of exoBPs. This comprehensive approach aims at providing a quantitative answer to the ultimate question "Are we are alone in the Universe?".

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arXiv2025

Planet Earth in reflected and polarized light: II. Refining contrast estimates for rocky exoplanets with ELT and HWO

The characterization of nearby rocky exoplanets will become feasible with the next generation of telescopes, such as the Extremely Large Telescope (ELT) and the mission concept Habitable Worlds Observatory (HWO). Using an improved model setup, we aim to refine the estimates of reflected and polarized light contrast for a selected sample of rocky exoplanets in the habitable zones of nearby stars. We perform advanced 3D radiative transfer simulations for Earth-like planets orbiting G-type and M-type stars. Our simulations incorporate realistic, wavelength-dependent surface albedo maps and a detailed cloud treatment, including 3D cloud structures and inhomogeneities, to better capture their radiative response. These improvements are based on Earth observations. We present models of increasing complexity, ranging from simple homogeneous representations to a detailed Earth-as-an-exoplanet model. Our results show that averaging homogeneous models fails to capture Earth's full complexity, especially in polarization. Moreover, simplistic cloud models distort the representation of absorption lines at high spectral resolutions, particularly in water bands, potentially biasing atmospheric chemical abundance estimates. Additionally, we provide updated contrast estimates for observing rocky exoplanets around nearby stars with upcoming instruments such as ANDES and PCS at the ELT. Compared to previous studies, our results indicate that reflected light contrast estimates are overestimated by a factor of two when simplified cloud and surface models are used. Instead, measuring the fractional polarization in the continuum and in high-contrast, high-resolution spectra may be more effective for characterizing nearby Earth-like exoplanets. These refined estimates are essential for guiding the design of future ELT instruments and the HWO mission concept.

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arXiv2025

Planet Earth in reflected and polarized light I. 3D radiative transfer simulations of realistic surface-atmosphere systems

Future telescopes will characterize rocky exoplanets in reflected light, revealing their albedo, which depends on surface, cloud, and atmospheric properties. Identifying these features is crucial for assessing habitability. We present reference spectra and phase curves for an unresolved Earth-like exoplanet in reflected and polarized light, showing how phase- and wavelength-dependent reflectance reveals key planetary properties. Using the 3D radiative transfer code MYSTIC, we enhance surface and cloud modeling with validated, wavelength-dependent albedo maps of Earth's seasonal and spectral features, alongside a novel treatment of sub-grid cloud variability using ERA5 reanalysis data. Our models, incorporating high-resolution 3D cloud structures, show that sub-grid cloud variability reduces total reflectance and increases phase curve variability, especially at large phase angles where ocean glint dominates. We also find that neglecting wavelength-dependent albedo maps overestimates the vegetation red edge in spectra. Comparing an Ocean planet to an Earth-like planet with seasonal cloud variability, we show that polarization is more sensitive than intensity alone in distinguishing both cases. Moreover, polarization captures richer surface details, making it a crucial tool for resolving retrieval degeneracies. Our simulations serve as a reference for observing Earth as an exoplanet and provide benchmarks for optimizing observational strategies and retrieval frameworks for future telescopes targeting small, rocky exoplanets.

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arXiv2025

Vertical structure of an exoplanet's atmospheric jet stream

Ultra-hot Jupiters, an extreme class of planets not found in our solar system, provide a unique window into atmospheric processes. The extreme temperature contrasts between their day- and night-sides pose a fundamental climate puzzle: how is energy distributed? To address this, we must observe the 3D structure of these atmospheres, particularly their vertical circulation patterns, which can serve as a testbed for advanced Global Circulation Models (GCM) [e.g. 1]. Here, we show a dramatic shift in atmospheric circulation in an ultra-hot Jupiter: a unilateral flow from the hot star-facing side to the cooler space-facing side of the planet sits below an equatorial super-rotational jet stream. By resolving the vertical structure of atmospheric dynamics, we move beyond integrated global snapshots of the atmosphere, enabling more accurate identification of flow patterns and allowing for a more nuanced comparison to models. Global circulation models based on first principles struggle to replicate the observed circulation pattern [3], underscoring a critical gap between theoretical understanding of atmospheric flows and observational evidence. This work serves as a testbed to develop more comprehensive models applicable beyond our Solar System as we prepare for the next generation of giant telescopes.

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arXiv2024

HAMSTER: Hyperspectral Albedo Maps dataset with high Spatial and TEmporal Resolution

Surface albedo is an important parameter in radiative-transfer simulations of the Earth's system as it is fundamental for correctly calculating the energy budget of the planet. The Moderate Resolution Imaging Spectroradiometer (MODIS) instruments on NASA's Terra and Aqua satellites continuously monitor daily and yearly changes in reflection at the planetary surface. The MODIS Surface Reflectance Black-Sky Albedo dataset (version 6.1 of MCD43D) provides detailed albedo maps for seven spectral bands in the visible and near-infrared range. These albedo maps allow us to classify different Lambertian surface types and their seasonal and yearly variability and change, albeit only into seven spectral bands. However, a complete set of albedo maps covering the entire wavelength range is required to simulate radiance spectra and correctly retrieve atmospheric and cloud properties from remote sensing observations of the Earth. We use a principal component analysis (PCA) regression algorithm to generate hyperspectral albedo maps of the Earth. By combining different datasets containing laboratory measurements of hyperspectral reflectance for various dry soils, vegetation surfaces, and mixtures of both, we reconstruct albedo maps across the entire wavelength range from 400 to 2500 nm. The PCA method is trained with a 10-year average of MODIS data for each day of the year. We obtain hyperspectral albedo maps with a spatial resolution of 0.05{\deg} in latitude and longitude, a spectral resolution of 10 nm, and a temporal resolution of 1 d (day). Using the hyperspectral albedo maps, we estimate the spectral profiles of different land surfaces, such as forests, deserts, cities, and icy surfaces, and study their seasonal variability. These albedo maps will enable us to refine calculations of the Earth's energy budget and its seasonal variability and improve climate simulations.

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arXiv2023

Presence of liquid water during the evolution of exomoons orbiting ejected free-floating planets

Free-floating planets (FFPs) can result from dynamical scattering processes happening in the first few million years of a planetary system's life. Several models predict the possibility, for these isolated planetary-mass objects, to retain exomoons after their ejection. The tidal heating mechanism and the presence of an atmosphere with a relatively high optical thickness may support the formation and maintenance of oceans of liquid water on the surface of these satellites. In order to study the timescales over which liquid water can be maintained, we perform dynamical simulations of the ejection process and infer the resulting statistics of the population of surviving exomoons around free-floating planets. The subsequent tidal evolution of the moons' orbital parameters is a pivotal step to determine when the orbits will circularize, with a consequential decay of the tidal heating. We find that close-in ($a \lesssim 25 $R$_{\rm J}$) Earth-mass moons with CO$_2$-dominated atmospheres could retain liquid water on their surfaces for long timescales, depending on the mass of the atmospheric envelope and the surface pressure assumed. Massive atmospheres are needed to trap the heat produced by tidal friction that makes these moons habitable. For Earth-like pressure conditions ($p_0$ = 1 bar), satellites could sustain liquid water on their surfaces up to 52 Myr. For higher surface pressures (10 and 100 bar), moons could be habitable up to 276 Myr and 1.6 Gyr, respectively. Close-in satellites experience habitable conditions for long timescales, and during the ejection of the FFP remain bound with the escaping planet, being less affected by the close encounter.

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