arXiv subjects

Peter Gao

Publications and source records attributed to Peter Gao.

At least 37 records · Page 2Linked to original sources

arXiv2026

GEMS JWST: Transmission spectroscopy of TOI-5205b reveals significant stellar contamination and a metal-poor atmosphere

Recent discoveries of transiting giant exoplanets ($R_p\gtrsim8\mathrm{~R_\oplus}$) around M dwarfs (GEMS) present an opportunity to investigate their atmospheric compositions and explore how such massive planets form around low-mass stars contrary to the prediction from formation models. We present the first transmission spectra of TOI-5205b, a short-period ($P=1.63~\mathrm{days}$) Jupiter-like planet ($M_p=1.08~\mathrm{M_J}$ and $R_p=0.94~\mathrm{R_J}$) orbiting an M4 dwarf ($M_\star=0.392~\mathrm{M_\odot}$, $R_\star=0.394~\mathrm{R_\odot}$). We obtained three transits using the PRISM mode of the JWST Near Infrared Spectrograph (NIRSpec) spanning $0.6-5.3$ um. The data reveal significant stellar contamination that is evident in the light curves as spot-crossing events and in the transmission spectra as a larger transit depth at bluer wavelengths. Atmospheric retrievals demonstrate that stellar contamination from unocculted starspots and faculae is the dominant component of the transmission spectrum at wavelengths $λ\lesssim3.0$ um, reducing the sensitivity to the presence of clouds or hazes in our models and preventing detection of $\mathrm{H_2O}$. The wavelength coverage enabled a robust detection of $\mathrm{CH_4}$ and $\mathrm{H_2S}$, which have detectable molecular features between $3.0-5.0$ um. For both clear or cloudy atmospheres, Bayesian retrievals consistently favored an atmosphere with sub-solar metallicity ($3σ$ upper limit of $\log\mathrm{[M/H]}\lesssim-1.24$) and super-solar C/O ratio ($3σ$ lower limit of $\log\mathrm{[C/O]}\gtrsim0.09$), although this may partly be driven by the non-detection of water due to stellar contamination. Planetary interior models predict a bulk metallicity of 10--20\%, which is larger than the atmospheric metallicity and suggests that the interior of TOI-5205b is decoupled from its atmosphere.

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arXiv2025

A carbon-rich atmosphere on a windy pulsar planet

A handful of enigmatic Jupiter-mass objects have been discovered orbiting pulsars. One such object, PSR\,J2322-2650b, uniquely resembles a hot Jupiter exoplanet due to its minimum density of 1.8 g/cm^3 and its ~1900 K equilibrium temperature. We use JWST to observe PSR J2322-2650b's emission spectrum across an entire orbit. In stark contrast to every known exoplanet orbiting a main-sequence star, we find an atmosphere rich in molecular carbon (C3, C2) with strong westward winds. Our observations open up new exoplanetary chemical (ultra-high C/O and C/N ratios of >100 and >10,000, respectively) and dynamical regimes (ultra-fast rotation with external irradiation) to observational study. The extreme carbon enrichment poses a severe challenge to the current understanding of ``black widow'' companions, which were expected to consist of a wider range of elements due to their origins as stripped stellar cores.

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arXiv2025

Condensation Clouds in Substellar Atmospheres with Virga

Here we present an open-source cloud model for substellar atmospheres, called Virga. The Virga-v0 series has already been widely adopted in the literature. It is written in Python and has heritage from the Ackerman & Marley (2001) model (often referred to as eddysed), used to study clouds on both exoplanets and brown dwarfs. In the development of the official Virga-v1 we have retained all the original functionality of eddysed and updated/expanded several components including the back-end optical constants data, calculations of the Mie properties, available condensate species, saturation vapor pressure curves and formalism for fall speeds calculations. Here we benchmark Virga by reproducing key results in the literature, including the SiO2 cloud detection in WASP-17 b and the brown dwarf Diamondback-Sonora model series. Development of Virga is ongoing, with future versions already planned and ready for release. We encourage community feedback and collaborations within the GitHub code repository.

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arXiv2025

JWST COMPASS: A NIRSpec G395H Transmission Spectrum of the Super-Earth GJ 357 b

We present JWST NIRSpec/G395H transmission spectroscopy observations of GJ 357 b, a warm ($T_{\mathrm{eq}} \approx 525$ K) super-Earth ($1.2\ \mathrm{R_{\oplus}} $, $1.84\ \mathrm{M_{\oplus}} $) orbiting a nearby M3-type star, with a median precision of 18 ppm and 27 ppm in NRS1 and NRS2, respectively. These precisions are obtained by binning the spectrum into 53 spectroscopic channels with a resolution of 60 pixels (around 0.02 $μ$m) each. Our analysis of the transmission spectrum reveals no detectable atmospheric spectral features. By comparing the observed spectrum with 1D forward models, we rule out atmospheres with mean molecular weights (MMW) lower than 8 g/mol to $3 σ$, as well as atmospheres with metallicities less than 300x solar. The lack of a low MMW primary atmosphere is consistent with a primordial H$_2$ rich atmosphere having escaped, given the planet's $\gtrsim5$ Gyr age, relatively low surface gravity (log g = 3.09), and its likely history of substantial incident extreme ultraviolet radiation. We conclude that GJ 357 b most likely possesses either a high-MMW secondary atmosphere, perhaps rich in oxidized gases like CO$_2$, or is a bare rock with no atmosphere. Upcoming scheduled JWST thermal emission observations could help distinguish between these scenarios by detecting signatures indicative of atmospheric heat redistribution or molecular absorption.

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arXiv2025

The Effects of Cuboid Particle Scattering on Reflected Light Phase Curves: Insights from Laboratory Data and Theory

Understanding the optical properties of exoplanet cloud particles is a top priority. Many cloud condensates form as nonspherical particles and their optical properties can be very different from those of spheres. In this study, we focus on KCl particles, which likely form as cuboids in warm (T=500-1000K) exoplanet atmospheres. We compare the phase functions (at 532 nm wavelength) of KCl particles computed with Mie theory, the two-term Henyey-Greenstein (TTHG) approximation, laboratory data, and the discrete dipole approximation (DDA). Mie theory assumes scattering from spheres, while TTHG functions are used to approximate cloud scattering in two-stream radiative transfer models like PICASO. Laboratory measurements and DDA allow for a robust understanding of scattering from cuboid and deformed cuboid particle shapes. We input these phase functions into PICASO using cloud distributions from the cloud model Virga, to determine how different phase functions can impact the reflected-light intensities of the benchmark sub-Neptune, GJ 1214b. Simulated reflected light phase curves of GJ1214b produced using the TTHG, laboratory, and DDA phase curves differ by less than 3ppm. Our findings suggest that TTHG phase functions may be useful for approximating the scattering intensity of certain cuboid and irregular particle shapes. Future work should expand upon the wavelengths and particles considered to better determine when scattering approximations, like TTHG, may be useful in lieu of more accurate, but time-consuming laboratory measurements and/or nonspherical scattering theory.

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arXiv2025

The James Webb Space Telescope NIRSpec-PRISM Transmission Spectrum of the Super-Puff, Kepler-51d

Kepler-51 is a 500 Myr G dwarf hosting three "super-puffs" and one low-mass non-transiting planet. Kepler-51d, the coolest (T_eq ~ 350 K) transiting planet in this system, is also one of the lowest density super-puffs known to date (rho_p = 0.038 +/- 0.009 g/cm^3). With a planetary mass of Mp = 5.6 +/- 1.2 Earth masses and a radius of Rp = 9.32 +/- 0.18 Earth radii, the observed properties of this planet are not readily explained by most planet formation theories. Hypotheses explaining Kepler-51d's low density range from a substantial H/He envelope comprising more than 30% of its mass, to a high-altitude haze layer, to a tilted ring system. To test these hypotheses, we present the NIRSpec-PRISM 0.6-5.3 micron transmission spectrum of Kepler-51d observed by the James Webb Space Telescope. We find a spectrum best fit by a sloped line covering the entire wavelength range. Based on forward modeling and atmosphere retrievals, Kepler-51d likely possesses a low-metallicity atmosphere with high-altitude hazes of submicron particle sizes spanning pressures of 1-100 microbars. However, the spectrum could also be explained by a tilted ring with an estimated lifetime on the order of ~0.1 Myr. We also investigate the stellar activity of this young Sun-like star, extracting a spot temperature significantly hotter than sunspots and spot covering fractions on the order of 0.1-10%, depending on the assumed spot parameters.

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arXiv2025

Far-ultraviolet flares and variability of the young M dwarf AU Mic: a non-detection of planet c in transit at Lyman-alpha

Atmospheric escape's potential to shape the exoplanet population motivates detailed observations of systems actively undergoing escape. AU Mic is a young and active M dwarf hosting two close-in transiting sub- to Neptune-sized planets. Atmospheric escape was previously detected on the inner planet b, with radially-blown neutral hydrogen producing ~30% blue-shifted absorption in Lyman-alpha. We obtained one HST/STIS transit of the outer planet c, to search for the planet's escaping atmosphere in transmission at Lyman-alpha and compare with AU Mic b. We detected 6 short-duration flares in Si IV and C IV, of which only one corresponded to a Lyman-alpha flare. We identified longer-duration stellar variability at the tens of percent level for lines less sensitive to stellar activity, including O I, C II and Lyman-alpha, which inhibits detection of an exosphere. We do not report absorption associated with an exosphere containing neutral hydrogen or any metals detectable in the far-ultraviolet, and discuss the implications of the non-detection. This work highlights the importance of 1) careful consideration of stellar variability in atmospheric escape observations, and 2) the dual-influence of photoionization and stellar wind when interpreting and modeling atmospheric escape.

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arXiv2025

The radiative effects of photochemical hazes on the atmospheric circulation and phase curves of sub-Neptunes

Measuring the atmospheric composition of hazy sub-Neptunes like GJ~1214b through transmission spectroscopy is difficult because of the degeneracy between mean molecular weight and haziness. It has been proposed that phase curve observations can break this degeneracy because of the relationship between mean molecular weight (MMW) and phase curve amplitude. However, photochemical hazes can strongly affect phase curve amplitudes as well. We present a large set of GCM simulations of the sub-Neptune GJ~1214b that include photochemical hazes with varying atmospheric composition, haze opacity and haze optical properties. In our simulations, photochemical hazes cause temperature changes of up to 200~K, producing thermal inversions and cooling deeper regions. This results in increased phase curve amplitudes and adds a considerable scatter to the phase curve amplitude--metallicity relationship. However, we find that if the haze production rate is high enough to significantly alter the phase curve, the secondary eclipse spectrum will exhibit either emission features or strongly muted absorption features. Thus, the combination of a white-light phase curve and a secondary eclipse spectrum can successfully distinguish between a hazy, lower MMW and a clear, high MMW scenario.

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arXiv2025

Clouds and Hazes in GJ 1214b's Metal-Rich Atmosphere

The sub-Neptune GJ 1214b has an infamously flat transmission spectrum, likely due to thick aerosols in its atmosphere. A recent JWST MIRI spectroscopic phase curve of GJ 1214 b added to this picture, suggesting a highly reflective and metal-rich atmosphere. Using a 3D General Circulation Model with both photochemical hazes and condensate clouds, we characterize how different aerosol types affect the atmospheric structure of GJ 1214 b and manifest in its spectroscopic phase curve. Additionally, we reanalyze the original GJ 1214 b JWST phase curve. The reanalysis shows a hotter nightside, similar dayside temperature, and a lower, but still elevated, Bond albedo (0.42 +/- 0.11) than the original results. We find that a scenario with both clouds and hazes is most consistent with the JWST phase curve. Reflective clouds or hazes are needed to explain the large Bond albedo, and hazes or a super-solar metallicity help account for the several hundred Kelvin day-night temperature difference measured by the phase curve.

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arXiv2025

JWST COMPASS: NIRSpec/G395H Transmission Observations of TOI-776 c, a 2 Rearth M Dwarf Planet

The atmospheres of planets between the size of Earth and Neptune at short orbital periods have been under intense scrutiny. Of the ~dozen planets in this regime with atmospheres studied so far, a few appear to have prominent molecular features while others appear relatively void of detectable atmospheres. Further work is therefore needed to understand the atmospheres of these planets, starting with observing a larger sample. To this end, we present the 3-5 micron transmission spectrum of TOI-776 c, a warm (Teq ~420 K), ~2 Rearth, ~7 Mearth planet orbiting an M1V star, measured with JWST NIRSpec/G395H. By combining two visits, we measure a median transit precision of ~18 ppm and ~32 ppm in the NRS1 and NRS2 detectors, respectively. We compare the transmission spectrum to both non-physical and physical models, and find no strong evidence for molecular features. For cloud-top pressures larger than 10^-3 bar, we rule out atmospheric metallicities less than 180-240x solar (depending on the reduction and modeling technique), which corresponds to a mean molecular weight of ~6-8 g/mol. However, we find simple atmosphere mixture models (H2O+H2/He or CO2+H2/He) give more pessimistic constraints, and caution that mean molecular weight inferences are model dependent. We compare TOI-776 c to the similar planet TOI-270 d, and discuss possible options for further constraining TOI-776 c's atmospheric composition. Overall, we suggest these TOI-776 c observations may represent a combination of planetary and stellar parameters that fall just below the threshold of detectable features in small planet spectra; finding this boundary is one of the main goals of the COMPASS program.

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arXiv2025

Exact variance estimation for model-assisted survey estimators using U- and V-statistics

Model-assisted estimation combines sample survey data with auxiliary information to increase precision when estimating finite population quantities. Accurately estimating the variance of model-assisted estimators is challenging: the classical approach ignores uncertainty from estimating the working model for the functional relationship between survey and auxiliary variables. This approach may be asymptotically valid, but can underestimate variance in practical settings with limited sample sizes. In this work, we develop a connection between model-assisted estimation and the theory of U- and V-statistics. We demonstrate that when predictions from the working model for the variable of interest can be represented as a U- or V-statistic, the resulting model-assisted estimator also admits a U- or V-statistic representation. We exploit this connection to derive an improved estimator of the exact variance of such model-assisted estimators. The class of working models for which this strategy can be used is broad, ranging from linear models to modern ensemble methods. We apply our approach to the model-assisted estimator constructed with a linear regression working model, commonly referred to as the generalized regression estimator, show that it can be re-written as a U-statistic, and propose an estimator of its exact variance. We illustrate our proposal and compare it against the classical asymptotic variance estimator using household survey data from the American Community Survey.

stat.ME↗

arXiv2025

JWST COMPASS: NIRSpec/G395H Transmission Observations of the Super-Earth TOI-776b

We present two transit observations of the $\sim$520K, 1.85R$_\oplus$, 4.0M$_\oplus$ super-Earth TOI-776b with JWST NIRSpec/G395H, resulting in a 2.8-5.2$μ$m transmission spectrum. Producing reductions using the ExoTiC-JEDI and Eureka! pipelines, we obtain a median transit depth precision of 34ppm for both visits and both reductions in spectroscopic channels 30 pixels wide ($\sim$0.02$μ$m). We find that our independent reductions produce consistent transmission spectra, however, each visit shows differing overall structure. For both reductions, a flat line is preferred for Visit 1 while a flat line with an offset between the NRS1 and NRS2 detectors is preferred for Visit 2; however, we are able to correct for this offset during our modeling analysis following methods outlined in previous literature. Using picaso forward models, we can rule out metallicities up to at least 100$\times$ solar with an opaque pressure of 10$^{-3}$ bar to $\geq$3$σ$ in all cases, however, the exact lower limit varies between the visits, with Visit 1 ruling out $\lesssim$100$\times$ solar while the lower limits for Visit 2 extend beyond $\sim$350$\times$ solar. Our results add to the growing list of super-Earth atmospheric constraints by JWST, which provide critical insight into the diversity and challenges of characterizing terrestrial planets.

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arXiv2025

Statistical trends in JWST transiting exoplanet atmospheres

Our brains are hardwired for pattern recognition as correlations are useful for predicting and understanding nature. As more exoplanet atmospheres are being characterized with JWST, we are starting to unveil their properties on a population level. Here we present a framework for comparing exoplanet transmission spectroscopy from 3 to 5$μ$m with four bands: L (2.9 - 3.7$μ$m), SO$_2$ (3.95 - 4.1$μ$m), CO$_2$ (4.25 - 4.4$μ$m) and CO (4.5 - 4.9$μ$m). Together, the four bands cover the major carbon, oxygen, nitrogen, and sulfur-bearing molecules including H$_2$O, CH$_4$, NH$_3$, H$_2$S, SO$_2$, CO$_2$, and CO. Among the eight high-precision gas giant exoplanet planet spectra we collected, we found strong correlations between the SO$_2$-L index and planet mass (r=-0.41$\pm$0.09) and temperature (r=-0.64$\pm$0.08), indicating SO$_2$ preferably exists (SO$_2$-L$>$-0.5) among low mass ($\sim<$0.3M$_J$) and cooler ($\sim<$1200K) targets. We also observe strong temperature dependency for both CO$_2$-L and CO-L indices. Under equilibrium chemistry and isothermal thermal structure assumptions, we find that the planet sample favors super-solar metallicity and low C/O ratio ($<$0.7). In addition, the presence of a mass-metallicity correlation is favored over uniform metallicity with the eight planets. We further introduce the SO$_2$-L versus CO$_2$-L diagram alike the color-magnitude diagram for stars and brown dwarfs. All reported trends here will be testable and be further quantified with existing and future JWST observations within the next few years.

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arXiv2025

TOI-421 b: A Hot Sub-Neptune with a Haze-Free, Low Mean Molecular Weight Atmosphere

Common features of sub-Neptunes atmospheres observed to date include signatures of aerosols at moderate equilibrium temperatures (~500-800 K), and a prevalence of high mean molecular weight atmospheres, perhaps indicating novel classes of planets such as water worlds. Here we present a 0.83-5 micron JWST transmission spectrum of the sub-Neptune TOI-421 b. This planet is unique among previously observed counterparts in its high equilibrium temperature ($T_{eq} \approx 920$) and its Sun-like host star. We find marked differences between the atmosphere of TOI-421 b and those of sub-Neptunes previously characterized with JWST, which all orbit M stars. Specifically, water features in the NIRISS/SOSS bandpass indicate a low mean molecular weight atmosphere consistent with solar metallicity, and no appreciable aerosol coverage. Hints of SO$_2$ and CO (but not CO$_2$ or CH$_4$) also exist in our NIRSpec/G395M observations, but not at sufficient signal-to-noise to draw firm conclusions. Our results support a picture in which sub-Neptunes hotter than ~850 K do not form hydrocarbon hazes due to a lack of methane to photolyze. TOI-421 b additionally fits the paradigm of the radius valley for planets orbiting FGK stars being sculpted by mass loss processes, which would leave behind primordial atmospheres overlying rock/iron interiors. Further observations of TOI-421 b and similar hot sub-Neptunes will confirm whether haze-free atmospheres and low mean molecular weights are universal characteristics of such objects.

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arXiv2024

Reflected-light Phase Curves with PICASO: A Kepler-7b Case Study

Examining reflected light from exoplanets aids in our understanding of the scattering properties of their atmospheres and will be a primary task of future flagship space- and ground-based telescopes. We introduce an enhanced capability of Planetary Intensity Code for Atmospheric Scattering Observations (PICASO), an open-source radiative transfer model used for exoplanet and brown dwarf atmospheres, to produce reflected light phase curves from three-dimensional atmospheric models. Since PICASO is coupled to the cloud code Virga, we produce phase curves for different cloud condensate species and varying sedimentation efficiencies (fsed) and apply this new functionality to Kepler-7b, a hot Jupiter with phase curve measurements dominated by reflected starlight. We model three different cloud scenarios for Kepler-7b: MgSiO3 clouds only, Mg2SiO4 clouds only, and Mg2SiO4, Al2O3, and TiO2 clouds. All our Virga models reproduce the cloudy region west of the substellar point expected from previous studies, as well as clouds at high latitudes and near the eastern limb, which are primarily composed of magnesium silicates. Al2O3 and TiO2 clouds dominate near the substellar point. We then compare our modeled reflected light phase curves to Kepler observations and find that models with all three cloud condensate species and low sedimentation efficiencies (0.03 - 0.1) match best, though our reflected light phase curves show intensities approximately one-third of those observed by Kepler. We conclude that a better understanding of zonal transport, cloud radiative feedback, and particle scattering properties is needed to further explain the differences between the modeled and observed reflected light fluxes.

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arXiv2024

Clouds and Hazes in the Atmospheres of Triton and Pluto

Clouds and hazes are abundant in the thin and cold atmospheres of Triton and Pluto, where they are thought to be produced by interactions between atmospheric gases and ultraviolet photons from the Sun and those scattered by the local interstellar medium. These interactions lead to a rich network of chemical reactions that produces higher order hydrocarbons and nitriles that condense out to form ice clouds, and ultimately complex haze particles that rain down onto the surface that impact the atmospheric thermal structure, gas chemistry, and surface evolution. In this chapter, we will review the observational evidence for clouds and hazes in the atmospheres of Triton and Pluto and theoretical interpretations thereof, and the emerging set of experiments aiming to produce Triton and Pluto clouds and hazes in the lab to learn about them in detail.

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arXiv2024

JWST COMPASS: The first near- to mid-infrared transmission spectrum of the hot super-Earth L 168-9 b

We present the first broadband near- to mid-infrared (3-12 microns) transmission spectrum of the highly-irradiated (T_eq = 981 K) M dwarf rocky planet L 168-9 b (TOI-134 b) observed with the NIRSpec and MIRI instruments aboard JWST. We measure the near-infrared transit depths to a combined median precision of 20 ppm across the three visits in 54 spectroscopic channels with uniform widths of 60 pixels (~0.2 microns wide; R~100), and the mid-infrared transit depths to 61 ppm median precision in 48 wavelength bins (~0.15 microns wide; R~50). We compare the transmission spectrum of L 168-9 b to a grid of 1D thermochemical equilibrium forward models, and rule out atmospheric metallicities of less than 100x solar (mean molecular weights <4 g mol$^{-1}$) to 3-sigma confidence assuming high surface pressure (>1 bar), cloudless atmospheres. Based on photoevaporation models for L 168-9 b with initial atmospheric mass fractions ranging from 2-100%, we find that this planet could not have retained a primordial H/He atmosphere beyond the first 200 Myr of its lifetime. Follow-up MIRI eclipse observations at 15 microns could make it possible to confidently identify a CO2-dominated atmosphere on this planet if one exists.

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arXiv2024

Light Scattering Measurements of KCl Particles as an Exoplanet Cloud Analog

Salt clouds are predicted to be common on warm exoplanets, but their optical properties are uncertain. The Exoplanet Cloud Ensemble Scattering System (ExCESS), a new apparatus to measure the scattering intensity and degree of linear polarization (DOLP) for an ensemble of particles, is introduced here and used to study the light scattering properties of KCl cloud analogs. ExCESS illuminates particles with a polarized laser beam (532 nm) and uses a photomultiplier tube detector to sweep the plane of illumination. Scattering measurements for KCl particles were collected for three size distributions representative of modeled clouds for the warm exoplanet GJ 1214b. Our measurements show that Lorenz-Mie calculations, commonly used to estimate the light scattering properties of assumedly spherical cloud particles, offer an inaccurate depiction of cubic and cuboid shaped KCl particles. All of our measurements indicate that Lorenz-Mie scattering overestimates the backscattering intensity of our cloud analogs and incorrectly predicts the scattering at mid-phase angles (~90 degrees) and preferential polarization state of KCl scattered light. Our results align with the general scattering properties of non-spherical particles and underscore the importance of further understanding the effects that such particles will have on radiative transfer models of exoplanet atmospheres and reflected light observations of exoplanets by the upcoming Nancy Grace Roman Space Telescope and Habitable Worlds Observatory.

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