arXiv subjects

Isabel Rebollido

Publications and source records attributed to Isabel Rebollido.

At least 19 recordsLinked to original sources

arXiv2026

That's no moon: Exomoon survival and detection limitations

While moons are ubiquitous in the Solar System, exomoons have not yet been unambiguously confirmed despite the discov- ery of over 5,600 exoplanets. We investigated the reasons for this lack of detections by reviewing high-impact literature searches, assessing dynamical stability and detectability constraints, and comparing these with the moon population in the Solar System. We compiled a homogeneous catalogue of 24 proposed exomoon candidates, including stellar, planetary, and inferred lunar parameters. For each case we applied Hill, Roche, and Laplace stability criteria and estimated detectability limits for current and forth- coming instruments. In addition, we tested 6038 confirmed exoplanets against the same stability and mission-specific observational filters for a given range of moon parameters. A large fraction of the potential exomoon candidates investigated in the literature are either inconsistent with long-term stability or produce signals below Kepler sensitivity. Only 8 of the 13 candidates meet at least one of the stability criteria. Our extended analysis highlights 27 promising targets for exomoon searches with PLATO, while moons with Solar-System-like properties remain below current thresholds. The lack of exomoon detections is likely due to a combination of the inefficient formation of stable massive moons and limited instrumental sensitivity. Other processes, such as early moon loss during planetary migration or giant impacts, may also influence moon survival. Future searches should prioritise systems that combine long-term dynamical stability with favourable observational conditions for next-generation photometric and astrometric facilities.

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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

The JWST Early Release Science Program for Direct Observations of Exoplanetary Systems VIII: patchy forsterite and enstatite clouds in the atmosphere of VHS 1256 b, retrieval lessons learned and outlook to the future

JWST defines a new era for the data-driven approach of retrieval modelling, which has become a cornerstone tool for the statistical inference of exoplanetary and brown dwarf properties. The Early Release Science program #1386 observations of VHS 1256 b represent a huge jump in data quality, data quantity and spectral coverage for such objects. VHS 1256 b is a young, planetary mass and extremely variable companion that populates the enigmatic L/T cohort of substellar atmospheres. In this first retrieval analysis of the full 1 - 18 micron dataset, we apply the Brewster retrieval framework to the NIRSpec and MIRI spectroscopic observations of VHS 1256 b, exploring a variety of cloud species and structures. Using Delta(BIC) we find that the data is best described by a forsterite (Mg$_{2}$SiO$_{4}$) and enstatite (MgSiO$_{3}$) cloud combination. Our analysis shows a strong preference for patchy silicate cloud coverage, which aligns with VHS 1256 b's extensive and well documented spectral variability. Our retrieval is able to place constraints on the abundances of H$_{2}$O, CO, CO$_{2}$, CH$_{4}$ as well as NH$_{3}$. We also show that the retrieved parameters are sensitive to the data used and the relative signal-to-noise ratios between data from different instruments. We conclude with the next steps for the wider retrieval community to better understand young and cloudy exoplanetary atmospheres.

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arXiv2026

JWST NIRCam Reveals the Largest Known M-dwarf Debris Disk Around TWA 10 and New Scattered-Light Observations of the TWA 25 Debris Disk

We present JWST NIRCam observations of two M-dwarf systems located in the TW Hydra association, TWA 10 and TWA 25. Both systems harbor detected debris disks in the F200W and F444W filters. Whereas the TWA 25 disk has been previously imaged, these observations represent the discovery and first images of the TWA 10 disk. In addition to planet searches within these systems, we also conduct an analysis of each debris disk, where the TWA 10 debris disk is characterized for the first time. We find that the TWA 10 debris disk is very large, with a radius of $\sim$191 au, significantly greater than other known M-dwarf debris disks. The TWA 25 disk hosts a sharp inner dust surface density power-law and a moderate brightness asymmetry present at 2 $\mu$m, suggesting potential sculpting from inner planets and potentially enhanced collisional activity. Finally, we find one potential companion candidate within the TWA 10 system and two within the TWA 25 system, although the measured F200W-F444W color suggests that these candidates are likely background objects. Both systems do not have measured IR-excesses in their SEDs, where radiative-transfer modeling suggests that these disks (and potentially more M-dwarf disks) were likely missed by previous disk detection surveys due to having low luminosity fractions.

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arXiv2026

Photometric Variability and Rotation of Beta Pictoris b from JWST NIRCam Coronagraphic Imaging

We report the detection of photometric variability in the directly imaged super-Jupiter $\beta$ Pictoris b. Using JWST NIRCam dual-band coronagraphic imaging, we conducted a 16-hour continuous photometric monitoring campaign in the F210M and F410M filters. We developed and validated a time-series photometry framework that combines PSF subtraction, principal component analysis for systematic noise removal, and injection-and-recovery tests to confirm signal fidelity. Both light curves show consistent sinusoidal variability at $\sim$5$\sigma$ and $\gg 5\sigma$ significance in the F210M and F410M bands, respectively. A joint sinusoidal fit yields a rotation period of $P_{\rm rot} = 9.00 \pm 0.13$ hr and variability amplitudes of $0.85 \pm 0.07\%$ and $0.89 \pm 0.04\%$ in F210M and F410M, respectively. The near-identical amplitudes and periods in both bands confirm a common astrophysical origin in a heterogeneous atmosphere. Combining $P_{\rm rot}$ with the previously measured projected rotational velocity, we constrain the line-of-sight spin axis inclination of $\beta$ Pic b. The result favors an equator-on viewing geometry, consistent with line-of-sight spin-orbit alignment: the planetary spin axis, orbital plane, debris disk, and stellar equator are all mutually aligned. This stands in sharp contrast to the large obliquities of wide-separation companions that are likely formed via gravitational fragmentation. Together with the system's young age, this observation provides independent dynamical evidence that $\beta$ Pic b formed via core accretion. This result constitutes the first detection of rotational modulation in a close-in, high-contrast exoplanet that likely formed via core accretion, demonstrating that time-series coronagraphic imaging with JWST opens a powerful new window onto the rotation, atmospheric dynamics, and spin-orbit architecture of this population.

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arXiv2026

Probing the origins. III. Exoplanet demographics across Galactic birth radii

We quantify radial mixing in exoplanet hosts and explore links between birth environment, orbital evolution, planetary architecture, and Galactic habitability. We constructed a homogeneous catalogue by cross-matching the Encyclopaedia of Exoplanetary Systems with Gaia DR3 astrometry and infrared photometry from 2MASS and AllWISE. Stellar orbits were integrated using Galpy. Stellar birth radii were inferred by combining Galactic chemical enrichment models with the generalised additive model introduced in Paper I. Giant-planet hosts preferentially trace inner-Galaxy birth sites, whereas brown-dwarf hosts span a broader, less localised range of radial displacements. Rocky-only systems show smaller radial excursions and less centrally concentrated birth radii, while rocky+giant systems are intermediate, retaining a stronger link to inner-disc birth environments than rocky-only systems. We also find that outward-migrators host more compact outer detected companions than inward-migrators, with non-migrators in between. This trend remains tentative because of heterogeneous detection biases. Giant-planet hosts retain a strong connection to metal-rich inner-Galaxy birth environments, whereas brown-dwarf hosts span a broader range of radial displacements, and rocky-only systems are less centrally concentrated. The older ages of rocky and rocky+giant hosts, especially among outward migrators, make them useful reference populations for future habitability and technosignature searches. Dynamically heated outer-Galaxy-born hosts show that planet-hosting systems can survive significant Galactic perturbations, although whether their architectures retain causal imprints of this evolution remains uncertain. No clear connection is found between radial displacement and the number of detected planets.

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arXiv2026

JWST-TST High Contrast: First Direct Spectroscopy of GJ 504 b reveals Clouds and Possible Metal Enrichment

Characterizing the coldest directly imaged companions through direct spectroscopy has only recently become possible with the James Webb Space Telescope. We present moderate-resolution (R $\sim$ 2,700) spectroscopic observations of the directly imaged planetary-mass companion (PMC), GJ 504 b, using the $JWST$/NIRSpec. As the coldest imaged PMC of the pre-JWST era GJ 504 b is too faint for ground-based spectroscopy, with only photometric observations possible. Leveraging advanced post-processing techniques with a forward modeling framework, we detect the companion at high signal-to-noise (S/N$>$300). We also present the first successful PSF subtraction with angular differential imaging (ADI) in the NIRSpec point cloud, detecting GJ 504 b at S/N$>10$ and reaching contrast limits $<10^{-4}$. The extracted 2.9--5.3 $\mu m$ spectra show strong signatures of several molecular species, including H$_2$O, $^{12}$C$^{16}$O, CH$_4$, CO$_2$, NH$_3$, H$_2$S, $^{13}$C$^{16}$O, and $^{12}$C$^{18}$O. Atmospheric modeling of the spectra using \texttt{petitRADTRANS}, yields an effective temperature = 564$\pm$4 K, surface gravity $\log{g}$ = 4.87$^{+0.13}_{-0.12}$, metallicity [M/H] = 0.67$^{+0.13}_{-0.12}$, C/O ratio = 0.64$^{+0.02}_{-0.02}$, interstellar $^{12}$C/$^{13}$C and $^{16}$O/$^{18}$O isotopologue ratios, and strong evidence of disequilibrium chemistry and salt clouds. The retrieved parameters indicate a mass 25.2$^{+8.4}_{-6.0}$ $M_\mathrm{Jup}$, which is in agreement with the mass range (19--27 $M_\mathrm{Jup}$) obtained from ATMO evolutionary models, implying an age of 2.5--4.0 Gyr. Lastly, we compare the abundances of GJ 504 b to its primary, obtaining a stellar abundance of sulfur (S), super-stellar carbon (C), and possibly, oxygen (O). The observed metal enrichment tentatively supports planet-like formation, but does not entirely exclude stellar abundances for GJ 504 b.

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arXiv2026

The evolution of exocomets and their source populations

We review the current state of knowledge of the long-term evolution of the small bodies that give rise to comets and exocomets, as well as their reservoirs. The active cometary phase is only transitory, and bodies that become comets pass from a source population, such as the Kuiper Belt, Oort Cloud or their extra-solar analogues, through the active cometary phase, to eventual dormancy or destruction. We discuss dynamical delivery channels that can move comets from their source reservoirs to orbits with small periapsides, and the depletion of these reservoirs by dynamical and collisional means. We also discuss the physical evolution of cometary nuclei, especially in light of recent advances from missions to Solar System comets such as Rosetta's visit to 67P. We then describe our current knowledge of interstellar objects, which can originate from the same source regions as exocomets but be amenable to detailed study when they enter the Solar System. We include a summary of stellar winds emanating from different types of stars, which become increasingly strong once stars leave the Main Sequence. This is followed by a description of how small bodies are affected by stellar evolution, and the range of comet-like phenomena observed towards white dwarf stars. Overall, while we have an increasingly good picture of the physical and dynamical evolution of Solar System comets, a number of large gaps remain in our knowledge of the physics of exocomets, related to our inability to directly probe these bodies and many of the planets that might be affecting their orbits.

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arXiv2026

The JWST Early Release Science Program for Direct Observations of Exoplanetary Systems. VII. Molecular Mapping Performance with JWST/MIRI MRS: VHS 1256 b as a case study

VHS 1256 b was the first planetary-mass companion to be observed with the James Webb Space Telescope's Mid-Infrared Instrument (JWST/MIRI) using the Medium-Resolution Spectrometer (MRS). The MRS provides high-quality integral-field spectral data in the mid-infrared (IR) wavelengths from 4.9 to 18 um. This dataset serves as a testbed for applying cross-correlation techniques to characterize exoplanet atmospheres. We implement the so-called molecular mapping approach, which consists of performing a spectral cross-correlation between each spectral pixel and atmospheric model templates. We compare these results with those obtained from cross-correlation of the extracted spectrum. Using a self-consistent Exo-REM atmospheric model grid, we constrain the temperature, surface gravity, C/O ratio, and metallicity, finding values consistent with those obtained from other analysis methods. We detect CO (S/N $\sim$ 25) and H2O (S/N $\sim$ 76), with tentative detections of NH3 and CH4 (S/N$\sim$ 3). We test cross-correlation to measure trace-species abundances and isotopic ratios. We measure a volume mixing ratio of [NH3] =-5.73^{+0.15}_{-0.14} and an isotopic ratio $^{12}\mathrm{C}/^{13}\mathrm{C}=77.8^{+13}_{-10}$, both consistent with free-chemistry retrievals. The derived NH3 volume mixing ratio, combined with the measured temperature and radius, is consistent with VHS 1256 b having a mass above the deuterium-burning limit. These results demonstrate the diagnostic power of mid-IR spectroscopy and highlight cross-correlation as a robust method for characterizing directly imaged exoplanets, even in future higher-contrast regimes where spectral extraction becomes challenging. Future MIRI MRS observations across a wider range of temperatures and masses will further expand our understanding of planetary atmospheric chemistry.

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arXiv2026

Forecasting Catastrophe: Constraints on the Fomalhaut Main Belt Planetesimal Population from Observed Collisional Remnants

Catastrophic planetesimal disruptions offer a unique opportunity to study and characterize large planetesimal populations in exoplanetary systems that are not currently detectable by modern observatories. The unexpected discovery of a second collision event in the Fomalhaut system raises important questions about the planetesimal population and dynamical state inside the Fomalhaut main belt that led to two collisions in 20 years. We present a statistical model developed and applied to the archetypal Fomalhaut system to provide new constraints on the bulk properties of the planetesimals in Fomalhaut's main belt. Utilizing the constraints provided by the spatially resolved Fomalhaut cs1 and cs2 collision events, we retrieve the belt parameters that best reproduce the observed collision rate while remaining consistent with the system's age and dust mass. Our best-fit model suggests a total main belt mass of 200-360 $M_{\oplus}$, with the transition from a collisionally evolved to a primordial planetesimal population occurring at a radius of $115_{-10}^{+30}$ km and a maximum planetesimal radius of $380_{-202}^{+643}$ km. We estimate a catastrophic collision rate of $0.086_{-0.048}^{+0.067}$ collision events per year for planetesimals with radii $\ge$ 100 km in the region interior to the main belt. Our findings show that further observable collisions are likely, motivating continued monitoring of Fomalhaut and other nearby debris disks.

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arXiv2025

Finding New Debris Discs at Sub-millimetre Wavelengths

Debris discs reveal the architectures and dynamical histories of planetary systems. Sub-millimetre observations trace large dust grains within debris discs, revealing their bulk properties. Debris discs have so far only been detected around ~20% of stars, representing the bright end of the population. A new facility is required to reach fainter discs, overcoming the confusion limit, with multiwavelength capabilities for characterisation, sensitivity to large-scale emission for nearby targets and a large field of view for surveying distant populations. All of this is made possible with the Atacama Large Aperture Submillimetre Telescope (AtLAST).

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arXiv2025

JWST/NIRSpec Detects Warm CO Emission in the Terrestrial-Planet Zone of HD 131488

We have obtained a high-resolution, JWST NIRSpec $2.87$ -- $5.14$ $\mu$m spectrum of the debris disk around HD 131488. We discover CO fundamental emission indicating the presence of warm fluorescent gas within $\sim10$ AU of the star. The large discrepancy in CO's vibrational and rotational temperature indicates that CO is out of thermal equilibrium and is excited with UV fluorescence. Our UV fluorescence model gives a best fit of $1150\,$K with an effective temperature of $450$, $332$, and $125\,$K for the warm CO gas kinetic temperature within $0.5$, $1$, and $10\,$AU to the star and a gas vibrational temperature of $8800\,$K. The newly discovered warm CO gas population likely resides between sub-AU scales and $\sim\,10\,$AU, interior to the cold CO reservoir detected beyond $35\,$AU with HST STIS and ALMA. The discovery of warm, fluorescent gas in a debris disk is the first such detection ever made. The detection of warm CO raises the possibility of unseen molecules (H$_2$O, H$_2$, etc) as collisional partners to excite the warm gas. We estimated a lower mass limit for CO of $1.25\times 10^{-7}\text{M}_{\oplus}$, which is $10^{-5}$ of the cold CO mass detected with ALMA and HST. We demonstrate that UV fluorescence emerges as a promising avenue for detecting tenuous gas at $10^{-7}$ Earth-mass level in debris disks with JWST.

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arXiv2025

Probing the era of giant collisions: millimeter observations of the HD 166191 system

We present non-simultaneous ALMA band 7 and SMA observations of the HD 166191 disk, which was recently thought to have a collision in its terrestrial planet zone. Both observations detect dust continuum emission and the ALMA observations detect the 12CO J=3-2 line from the circumstellar disk. We do not detect SiO, a potential indicator of giant collisions, but place a limit on the total SiO mass in the system. Unlike previously observed in the infrared, we do not find evidence for variability at millimeter wavelengths when comparing the ALMA continuum observations from 2024 to the pre-collision SMA observations from 2014. We perform modeling of the CO and continuum visibilities and find that both the CO and dust are marginally spatially resolved and are contained to within 20 au from the central star. The modeling of the CO suggests that the outer regions of the disk are gas rich, although further observations are needed to confirm the total gas mass. The evolutionary state of this system has been debated in the literature, and our observations, while not definitive, are generally consistent with the idea that this disk is similar to an evolved protoplanetary or transition/hybrid disk. This could suggest that collisions in the terrestrial planet zone of HD 166191 are occurring while the disk is in a transitional phase, where the inner few au are depleted of gas. This makes HD 166191 an important object for understanding the transition between protoplanetary and debris disks and the stages at which collisions occur.

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arXiv2025

An Overview of Exocomets

We give a general overview of what the scientific community refers to as "exocomets". The general definition of exocomets, as presented in this work, is discussed and compared with Solar System comets and interstellar objects, addressing their detection around main-sequence stars as well as orbiting white dwarfs. We introduce the different types of exocomet observations, highlighting the difference between exocometary 'bodies' and exocometary 'material'. We provide a census of all exocometary system candidates detected so far, both via spectroscopy and photometry, including detections around white dwarfs.

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arXiv2025

The nature of ASASSN-24fw's occultation: modelling the event as dimming by optically thick rings around a sub-stellar companion

ASASSN-24fw is a main-sequence F-type star that experienced a rapid and long-lasting dimming event beginning in late 2024 and continuing until mid 2025. Its pre-dimming spectral energy distribution shows a persistent infrared excess with a fractional luminosity of approximately 0.5 percent. We model this excess using a two-component blackbody fit and find dust components with temperatures of about 1070 K and 390 K. Archival light curves indicate that ASASSN-24fw was photometrically stable prior to the event, suggesting that the dimming is caused by an external occulting body rather than intrinsic stellar variability. The event lasted about 275 days and exhibits a distinctive flat-bottomed profile of nearly 200 days, unlike most long-duration occultation events reported in the last decade. We analyze the light curve and spectra obtained during dimming to study the properties of both the star and the occulting material. A parametric light-curve model reveals multiple ingress phases, consistent with variations in the density and structure of the obscuring material. A second transit model favors an occulting body consistent with a gas giant or brown dwarf with a minimum mass of about 3.4 Jupiter masses and surrounded by an extended circumplanetary disk or rings of radius roughly 0.17 au. Near-infrared spectra taken during dimming show enhanced infrared excess and spectral features consistent with a late-type companion, approximately M8. We also detect variable H-alpha emission, suggesting evolving gas and dust in the occulting structure. Imaging from LCOGT identifies a nearby object within 3 arcsec, likely a bound companion at a projected separation of about 3000 au. Systems like ASASSN-24fw appear rare, and continued follow-up will help constrain the nature of the occulting body and the circumstellar environment.

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arXiv2025

Discovery of a Debris Disk Around TWA 20

We report the discovery of a debris disk surrounding the M3 star, TWA 20, revealed by JWST coronagraphic observations using the Near-infrared Camera (NIRCam). With reference-star differential imaging (RDI), we resolve the disk in scattered light in the F200W filter at a high signal-to-noise ratio and in the F444W filter at a low signal-to-noise ratio. The disk morphology and orientation are characterized via a forward modeling approach, where we determine a radius of 64.7-6.5+6.2 AU and an inclination of 70.1-3.3+2.5 deg. Utilizing our forward model, we improve the fidelity of the debris disk image using model-constrained RDI (MCRDI). The newly discovered disk is one of only 6 disks detected in scattered light that orbit M dwarf stars; it is the third largest of the 6 resolved M dwarf disks and orbits the third faintest host star. The detection of this disk exemplifies the sensitivity of JWST to debris disks around low-luminosity host stars, which have historically been difficult to detect because these disks are cool and dim. We identify a nebulous structure that cannot be explained by an axisymmetric disk. A search for companions in the TWA 20 system yields no candidates.

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arXiv2025

JWST-TST High Contrast: Medium-resolution spectroscopy reveals a carbon-rich circumplanetary disk around the young accreting exoplanet Delorme 1 AB b

Young accreting planetary-mass objects are thought to draw material from a circumplanetary disk (CPD) composed of gas and dust. While the gas within the disk is expected to disperse within the first million years, strong accretion has nonetheless been detected in older systems, including the 30--45 Myr-old planetary-mass companion Delorme 1 AB b. We conducted spectroscopic observations with JWST/MIRI to investigate the presence of circumplanetary material around this young, accreting planet, and to characterize the planet's atmospheric properties and composition. We perform forward modeling using atmospheric models to characterize the planet's atmosphere. Beyond 10 microns, the SED becomes dominated by this CPD rather than the planet itself. We detect strong emission from HCN and C$_2$H$_2$, while no O-bearing species are observed in the CPD spectrum, suggesting the gas in the CPD has an elevated C/O. We also identify spatially extended H$_2$ emission around the planet, tracing warm gas, with indications that it may be at a higher temperature than the non-extended component. The mid-infrared spectrum of the planetary-mass companion Delorme 1 AB b reveals the first detection of bright C-bearing species in a CPD, together with an outflow traced by H$_2$ extended emission, that could be interpreted as a disk wind. The hot dust continuum emission suggests an inner cavity in the CPD. The presence of warm gas in the CPD provides constraints on the disk's chemical composition and physical conditions, opening up new avenues for disk studies. The study of these long-lived "Peter Pan" disks will enhance our understanding of how accretion persists in evolved low-mass systems, shedding light on their formation, longevity, and evolutionary pathways in planetary systems.

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arXiv2025

Debris disks and their properties with the Habitable Worlds Observatory

The study of the last stages of planet formation, also known as debris disks, is fundamental to place constrains on the formation of planetary sized bodies. Debris disks are composed of dust and occasionally small amounts of gas, both released through dynamical interactions of small rocky bodies and dust particles, such as collisions and evaporation. The distribution of the dust can reveal the presence of forming planets and its composition can directly trace that of comets, asteroids and even planets. While we have been observing debris disks for 40 years now, most observations so far have been restricted to the cold outer regions of the system, and therefore information of the terrestrial zone is still missing. The improved spatial resolution, inner working angle and sensitivity that the Habitable Worlds Observatory will provide will enable a much closer look into the structure and composition of debris disks (particularly of its inner region) and enable the search for the forming rocky planets within the disk.

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