arXiv subjects

Heather A. Knutson

Publications and source records attributed to Heather A. Knutson.

At least 37 records · Page 2Linked to original sources

arXiv2025

Constraining atmospheric composition from the outflow: helium observations reveal the fundamental properties of two planets straddling the radius gap

TOI-836 is a ~2-3 Gyr K dwarf with an inner super Earth ($R=1.7 R_\oplus$, $P=3.8$ d) and an outer mini Neptune ($R=2.6 R_\oplus$, $P=8.6$ d). JWST/NIRSpec 2.8--5.2 $μ$m transmission spectra are flat for both planets. We present Keck/NIRSPEC observations of escaping helium for super-Earth b, which shows no excess absorption in the 1083 nm triplet to deep limits (<0.2%), and mini-Neptune c, which shows strong (0.7%) excess absorption in both visits. These results demonstrate that planet c retains at least some primordial atmosphere, while planet b is consistent with having lost its entire primordial envelope. Self-consistent 1D radiative-hydrodynamic models of planet c reveal that the helium excess absorption signal is highly sensitive to metallicity: its equivalent width collapses by a factor of 13 as metallicity increases from 10x to 100x solar, and by a further factor of 12 as it increases to 200x solar. The observed equivalent width is 88% the model prediction for 100x metallicity, suggesting an atmospheric metallicity similar to K2-18b and TOI-270d, the first two mini-Neptunes with detected absorption features in JWST transmission spectra. We highlight the helium triplet as a potentially powerful probe of atmospheric composition, with complementary strengths and weaknesses to atmospheric retrievals. The main strength is its extreme sensitivity to metallicity in the scientifically significant range of 10--200x solar, and the main weakness is the enormous model uncertainties in outflow suppression and confinement mechanisms, such as magnetic fields and stellar winds, which can suppress the signal by at least a factor of ~several.

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arXiv2025

TOI-6324b: An Earth-Mass Ultra-Short-Period Planet Transiting a Nearby M Dwarf

We report the confirmation of TOI-6324 b, an Earth-sized (1.059 $\pm$ 0.041 R$_\oplus$) ultra-short-period (USP) planet orbiting a nearby ($\sim$20 pc) M dwarf. Using the newly commissioned Keck Planet Finder (KPF) spectrograph, we have measured the mass of TOI-6324 b 1.17 $\pm$ 0.22 M$_\oplus$. Because of its extremely short orbit of just $\sim$6.7 hours, TOI-6324 b is intensely irradiated by its M dwarf host, and is expected to be stripped of any thick, H/He envelope. We were able to constrain its interior composition and found an iron core mass fraction (CMF = 27$\pm$37%) consistent with that of Earth ($\sim$33%) and other confirmed USPs. TOI-6324 b is the closest to Earth-sized USP confirmed to date. TOI-6324 b is a promising target for JWST phase curve and secondary eclipse observations (Emission Spectroscopy Metric = 25) which may reveal its surface mineralogy, day-night temperature contrast, and possible tidal deformation. From 7 sectors of TESS data, we report a tentative detection of the optical phase curve variation with an amplitude of 42$\pm$28 ppm.

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arXiv2025

Atmospheric abundances and bulk properties of the binary brown dwarf Gliese 229 Bab from JWST/MIRI spectroscopy

We present JWST/MIRI low-resolution spectroscopy ($4.75-14~μ$m) of the first known substellar companion, Gliese 229 Bab, which was recently resolved into a tight binary brown dwarf. Previous atmospheric retrieval studies modeling Gliese 229 B as a single brown dwarf have reported anomalously high carbon-to-oxygen ratios (C/O) of $\approx 1.1$ using $1-5~μ$m ground-based spectra. Here, we fit the MIRI spectrum of Gliese 229 Bab with a two-component binary model using the Sonora Elf Owl grid and additionally account for the observed $K$ band flux ratio of the binary brown dwarf. Assuming the two brown dwarfs share the same abundances, we obtain $\rm C/O=0.65\pm0.05$ and $\rm [M/H]=0.00^{+0.04}_{-0.03}$ as their abundances ($2σ$ statistical errors), which are fully consistent with the host star abundances. We also recover the same abundances if we fit the MIRI spectrum with a single brown dwarf model, indicating that binarity does not strongly affect inferred abundances from mid-infrared data when the $T_\rm{eff}$ are similar between components of the binary. We measure $T_\rm{eff}=900^{+78}_{-29}~$K and $T_\rm{eff}=775^{+20}_{-33}~$K for the two brown dwarfs. We find that the vertical diffusion coefficients of $\log{K_\rm{zz}} \approx4.0$ are identical between the two brown dwarfs and in line with $\log{K_\rm{zz}}$ values inferred for isolated brown dwarfs with similar $T_\rm{eff}$. Our results demonstrate the power of mid-infrared spectroscopy in providing robust atmospheric abundance measurements for brown dwarf companions and by extension, giant planets.

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arXiv2025

A New Spectral Library for Modeling the Surfaces of Hot, Rocky Exoplanets

JWST's MIRI LRS provides the first opportunity to spectroscopically characterize the surface compositions of close-in terrestrial exoplanets. Models for the bare-rock spectra of these planets often utilize a spectral library from R. Hu et al., which is based on room temperature reflectance measurements of materials that represent archetypes of rocky planet surfaces. Here we present an expanded library that includes hemispherical reflectance measurements for a greater variety of compositions, varying textures (solid slab, coarsely crushed, and fine powder), as well as high temperature (500-800 K) emissivity measurements for select samples. We incorporate this new library into version 6.3 of the retrieval package PLATON and use it to show that surfaces with similar compositions can have widely varying albedos and surface temperatures. We additionally demonstrate that changing the texture of a material can significantly alter its albedo, making albedo a poor proxy for surface composition. We identify key spectral features -- the 5.6 \textmu{m} olivine feature, the transparency feature, the Si-O stretching feature, and the Christiansen feature -- that indicate silicate abundance and surface texture. We quantify the number of JWST observations needed to detect these features in the spectrum of the most favorable super-Earth target, LHS 3844 b, and revisit the interpretation of its Spitzer photometry. Lastly, we show that temperature-dependent changes in spectral features are likely undetectable at the precision of current exoplanet observations. Our results illustrate the importance of spectroscopically-resolved thermal emission measurements, as distinct from surface albedo constraints, for characterizing the surface compositions of hot, rocky exoplanets.

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arXiv2025

Updated Mass, Eccentricity, and Tidal Heating Constraints for the Earth-sized Planet LP 791-18 d

LP 791-18 d is a temperate Earth-sized planet orbiting a late M dwarf, surrounded by an interior super-Earth (LP 791-18 b, $R_P$ = 1.2 $R_{\oplus}$, $P=0.95$ days) and an exterior sub-Neptune (LP 791-18 c, $R_P$ = 2.5 $R_{\oplus}$, $P=4.99$ days). Dynamical interactions between LP 791-18 d and c produce transit timing variations (TTVs) that can be used to constrain the planet masses and eccentricities. These interactions can also force a non-zero eccentricity for LP 791-18 d, which raises its internal temperature through tidal heating and could drive volcanic outgassing. We present three new transit observations of LP 791-18 c with Palomar/WIRC, including the most precise TTV measurements ($<$ 6 seconds) of this planet to date. We fit these times with a TTV model to obtain updated constraints on the mass, eccentricity, and tidal heat flux of LP 791-18 d. We reduce the mass uncertainty by more than a factor of two ($M_d$ = 0.91 $\pm$ 0.19 $M_{\oplus}$). We perform an updated fit assuming tidally damped free eccentricities and find $e_d = 0.0011^{+0.0010}_{-0.0008}$ and $e_c = 0.0001 \pm 0.0001$, consistent with circular orbits. We find that the observed TTVs are not sensitive to $e \leq$ $\sim$0.01. Without a tidally damped eccentricity prior, $e_d = 0.056^{+0.015}_{-0.014}$, much higher than the eccentricity predicted by n-body simulations incorporating the effects of dynamical excitation and tidal damping. We predict the timing of upcoming JWST secondary eclipse observations for LP 791-18 d, which could tightly constrain the eccentricity and tidal quality factor of this Earth-sized exoplanet.

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arXiv2025

Atmospheric Mass Loss from TOI-1259 A b, a Gas Giant Planet With a White Dwarf Companion

The lack of close-in Neptune-mass exoplanets evident in transit surveys has largely been attributed either to photoevaporative mass loss or high-eccentricity migration. To distinguish between these two possibilities, we investigate the origins of TOI-1259 A b, a Saturn-mass (0.4 M$_J$, 1.0 R$_J$) exoplanet lying along the upper edge of the Neptune desert. TOI-1259 A b's close-in ($P$ = 3.48 days) orbit and low bulk density make the planet particularly vulnerable to photoevaporation. Using transits observed in the 1083 nm metastable helium line, we probe the upper atmosphere of TOI-1259 A b with the Hale Telescope at Palomar Observatory and the Near-Infrared Spectrograph on Keck II. We report an excess absorption of $0.395\pm{0.072}\%$ with Palomar and a blueshifted absorption of $2.4\pm0.52\%$ with Keck, consistent with an extended escaping atmosphere. Fitting this signal with a Parker wind model, we determine a corresponding atmospheric mass loss rate of log($\dot{M}$) = $10.33\pm 0.13$ g/s for a thermosphere temperature of $8400^{+1200}_{-1000}$ K based on the Palomar absorption and log($\dot{M}$) = $10.0\pm 0.1$ g/s for a thermosphere temperature of $8200^{+1000}_{-900}$ K based on the Keck absorption. This relatively low rate suggests that this planet would not have been significantly altered by mass loss even if it formed in-situ. However, the presence of a white dwarf companion, TOI-1259 B, hints that this planet may not have formed close-in, but rather migrated inward relatively late. Given the estimated parameters of the proto-white dwarf companion, we find that high-eccentricity migration is possible for the system.

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arXiv2024

A Global Perspective with Updated Constraints on the Ultra-hot Jupiter WASP-19b: Atmospheric Properties and Stellar Activity

We present a detailed reanalysis of the atmospheric properties of WASP-19b, an ultra-hot Jupiter (1.14 M Jup, 1.41 R Jup) orbiting an active Sun-like star every 0.79 day. We reanalyze a transit and secondary eclipse of WASP-19b observed by the Hubble Space Telescope's Wide Field Camera 3 spectrograph (1.1 - 1.7 microns). When combined with Spitzer photometry at longer wavelengths, our analyses indicate the presence of water absorption features in both the planet's transmission and emission spectra, consistent with results from previously published studies. We jointly fit WASP-19b's dayside emission and transmission spectra with a retrieval model in order to constrain its atmospheric composition, and explore the effect of stellar activity on its transmission spectrum in greater depth. We also compare our dayside emission spectrum to predictions from a general circulation model, and conclude that magnetic drag appears to be relatively unimportant in shaping WASP-19b's atmospheric circulation. Lastly, we compare the size of WASP-19b's dayside water absorption feature to the population of hot Jupiters with similar measurements, and show that it is located in the transitional irradiation regime where temperature inversions first begin to emerge. As in previous studies, we find that the current observations provide relatively weak constraints on this planet's atmospheric properties. These constraints could be significantly improved by the addition of spectroscopically resolved observations at longer wavelengths with JWST/NIRSpec PRISM.

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arXiv2024

A Fourth Planet in the Kepler-51 System Revealed by Transit Timing Variations

Kepler-51 is a $\lesssim 1\,\mathrm{Gyr}$-old Sun-like star hosting three transiting planets with radii $\approx 6$-$9\,R_\oplus$ and orbital periods $\approx 45$-$130\,\mathrm{days}$. Transit timing variations (TTVs) measured with past Kepler and Hubble Space Telescope (HST) observations have been successfully modeled by considering gravitational interactions between the three transiting planets, yielding low masses and low mean densities ($\lesssim 0.1\,\mathrm{g/cm^3}$) for all three planets. However, the transit time of the outermost transiting planet Kepler-51d recently measured by the James Webb Space Telescope (JWST) 10 years after the Kepler observations is significantly discrepant from the prediction made by the three-planet TTV model, which we confirmed with ground-based and follow-up HST observations. We show that the departure from the three-planet model is explained by including a fourth outer planet, Kepler-51e, in the TTV model. A wide range of masses ($\lesssim M_\mathrm{Jup}$) and orbital periods ($\lesssim 10\,\mathrm{yr}$) are possible for Kepler-51e. Nevertheless, all the coplanar solutions found from our brute-force search imply masses $\lesssim 10\,M_\oplus$ for the inner transiting planets. Thus their densities remain low, though with larger uncertainties than previously estimated. Unlike other possible solutions, the one in which Kepler-51e is around the $2:1$ mean motion resonance with Kepler-51d implies low orbital eccentricities ($\lesssim 0.05$) and comparable masses ($\sim 5\,M_\oplus$) for all four planets, as is seen in other compact multi-planet systems. This work demonstrates the importance of long-term follow-up of TTV systems for probing longer period planets in a system.

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arXiv2024

JWST/NIRISS reveals the water-rich "steam world" atmosphere of GJ 9827 d

With sizable volatile envelopes but smaller radii than the solar system ice giants, sub-Neptunes have been revealed as one of the most common types of planet in the galaxy. While the spectroscopic characterization of larger sub-Neptunes (2.5-4R$_\oplus$) has revealed hydrogen-dominated atmospheres, smaller sub-Neptunes (1.6--2.5R$_\oplus$) could either host thin, rapidly evaporating hydrogen-rich atmospheres or be stable metal-rich "water worlds" with high mean molecular weight atmospheres and a fundamentally different formation and evolutionary history. Here, we present the 0.6--2.8$μ$m JWST NIRISS/SOSS transmission spectrum of GJ 9827 d, the smallest (1.98 R$_\oplus$) warm (T$_\mathrm{eq, A_B=0.3} \sim 620$K) sub-Neptune where atmospheric absorbers have been detected to date. Our two transit observations with NIRISS/SOSS, combined with the existing HST/WFC3 spectrum, enable us to break the clouds-metallicity degeneracy. We detect water in a highly metal-enriched "steam world" atmosphere (O/H of $\sim 4$ by mass and H$_2$O found to be the background gas with a volume mixing ratio of >31%). We further show that these results are robust to stellar contamination through the transit light source effect. We do not detect escaping metastable He, which, combined with previous nondetections of escaping He and H, supports the steam atmosphere scenario. In water-rich atmospheres, hydrogen loss driven by water photolysis happens predominantly in the ionized form which eludes observational constraints. We also detect several flares in the NIRISS/SOSS light-curves with far-UV energies of the order of 10$^{30}$ erg, highlighting the active nature of the star. Further atmospheric characterization of GJ 9827 d probing carbon or sulfur species could reveal the origin of its high metal enrichment.

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arXiv2024

The ESO SupJup Survey III: Confirmation of 13CO in YSES 1 b and Atmospheric Detection of YSES 1 c with CRIRES+

High-resolution spectroscopic characterization of young super-Jovian planets enables precise constraints on elemental and isotopic abundances of their atmospheres. As part of the ESO SupJup Survey, we present high-resolution spectral observations of two wide-orbit super-Jupiters in YSES 1 (or TYC 8998-760-1) using the upgraded VLT/CRIRES+ (R~100,000) in K-band. We carry out free atmospheric retrieval analyses to constrain chemical and isotopic abundances, temperature structures, rotation velocities, and radial velocities. We confirm the previous detection of 13CO in YSES 1 b at a higher significance of 12.6σ, but point to a higher 12CO/13CO ratio of 88+/-13 (1σ confidence interval), consistent with the primary's isotope ratio 66+/-5. We retrieve a solar-like composition in YSES 1 b with a C/O=0.57+/-0.01, indicating a formation via gravitational instability or core accretion beyond the CO iceline. Additionally, the observations lead to detections of H2O and CO in the outer planet YSES 1 c at 7.3σ and 5.7σ, respectively. We constrain the atmospheric C/O ratio of YSES 1 c to be either solar or subsolar (C/O=0.36+/-0.15), indicating the accretion of oxygen-rich solids. The two companions have distinct vsini, 5.34+/-0.14 km/s for YSES 1 b and 11.3+/-2.1 km/s for YSES 1 c, despite their similar natal environments. This may indicate different spin axis inclinations or effective magnetic braking by the long-lived circumplanetary disk around YSES 1 b. YSES 1 represents an intriguing system for comparative studies of super-Jovian companions and linking present atmospheres to formation histories.

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arXiv2024

Quartz Clouds in the Dayside Atmosphere of the Quintessential Hot Jupiter HD 189733 b

Recent mid-infrared observations with JWST/MIRI have resulted in the first direct detections of absorption features from silicate clouds in the transmission spectra of two transiting exoplanets, WASP-17 b and WASP-107 b. In this paper, we measure the mid-infrared ($5-12$ $μ$m) dayside emission spectrum of the benchmark hot Jupiter HD 189733 b with MIRI LRS by combining data from two secondary eclipse observations. We confirm the previous detection of H$_2$O absorption at 6.5 $μ$m from Spitzer/IRS and additionally detect H$_2$S as well as an absorption feature at 8.7 $μ$m in both secondary eclipse observations. The excess absorption at 8.7 $μ$m can be explained by the presence of small ($\sim$0.01 $μ$m) grains of SiO$_2$[s] in the uppermost layers of HD 189733 b's dayside atmosphere. This is the first direct detection of silicate clouds in HD 189733 b's atmosphere, and the first detection of a distinct absorption feature from silicate clouds on the day side of any hot Jupiter. We find that models including SiO$_2$[s] are preferred by $6-7σ$ over clear models and those with other potential cloud species. The high altitude location of these silicate particles is best explained by formation in the hottest regions of HD 189733 b's dayside atmosphere near the substellar point. We additionally find that HD 189733 b's emission spectrum longward of 9 $μ$m displays residual features not well captured by our current atmospheric models. When combined with other JWST observations of HD 189733 b's transmission and emission spectrum at shorter wavelengths, these observations will provide us with the most detailed picture to date of the atmospheric composition and cloud properties of this benchmark hot Jupiter.

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arXiv2024

A Survey of Protoplanetary Disks Using the Keck/NIRC2 Vortex Coronagraph

Recent Atacama Large Millimeter/submillimeter Array (ALMA) observations of protoplanetary disks in the millimeter continuum have shown a variety of radial gaps, cavities, and spiral features. These substructures may be signposts for ongoing planet formation, and therefore these systems are promising targets for direct imaging planet searches in the near-infrared. To this end, we present results from a deep imaging survey in the $L'$-band (3.8 $μ$m) with the Keck/NIRC2 vortex coronagraph to search for young planets in 43 disks with resolved features in the millimeter continuum or evidence for gaps/central cavities from their spectral energy distributions. Although we do not detect any new point sources, using the vortex coronagraph allows for high sensitivity to faint sources at small angular separations (down to ${\sim}$0$^{\prime\prime}$.1), allowing us to place strong upper limits on the masses of potential gas giant planets. We compare our mass sensitivities to the masses of planets derived using ALMA observations, and while we are sensitive to $\sim$1 M$_{Jup}$ planets in the gaps in some of our systems, we are generally not sensitive to planets of the masses expected from the ALMA observations. In addition to placing upper limits on the masses of gas giant planets that could be interacting with the dust in the disks to form the observed millimeter substructures, we are also able to map the micron-sized dust as seen in scattered light for 8 of these systems. Our large sample of systems also allows us to investigate limits on planetary accretion rates and disk viscosities.

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arXiv2024

An Earth-sized Planet on the Verge of Tidal Disruption

TOI-6255~b (GJ 4256) is an Earth-sized planet (1.079$\pm0.065$ $R_\oplus$) with an orbital period of only 5.7 hours. With the newly commissioned Keck Planet Finder (KPF) and CARMENES spectrographs, we determined the planet's mass to be 1.44$\pm$0.14 $M_{\oplus}$. The planet is just outside the Roche limit, with $P_{\rm orb}/P_{\rm Roche}$ = 1.13 $\pm0.10$. The strong tidal force likely deforms the planet into a triaxial ellipsoid with a long axis that is $\sim$10\% longer than the short axis. Assuming a reduced stellar tidal quality factor $Q_\star^\prime \approx10^7$, we predict that tidal orbital decay will cause TOI-6255 to reach the Roche limit in roughly 400 Myr. Such tidal disruptions may produce the possible signatures of planet engulfment that have been on stars with anomalously high refractory elemental abundances compared to its conatal binary companion. TOI-6255 b is also a favorable target for searching for star-planet magnetic interactions, which might cause interior melting and hasten orbital decay. TOI-6255 b is a top target (Emission Spectroscopy Metric of about 24) for phase curve observations with the James Webb Space Telescope.

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arXiv2024

Atmospheric characterization of the super-Jupiter HIP 99770 b with KPIC

Young, self-luminous super-Jovian companions discovered by direct imaging provide a challenging test of planet formation and evolution theories. By spectroscopically characterizing the atmospheric compositions of these super-Jupiters, we can constrain their formation histories. Here we present studies of the recently discovered HIP 99770 b, a 16 MJup high-contrast companion on a 17 au orbit, using the fiber-fed high-resolution spectrograph KPIC (R~35,000) on the Keck II telescope. Our K-band observations led to detections of H2O and CO in the atmosphere of HIP 99770 b. We carried out free retrieval analyses using petitRADTRANS to measure its chemical abundances, including the metallicity and C/O ratio, projected rotation velocity (vsini), and radial velocity (RV). We found that the companion's atmosphere has C/O=0.55(-0.04/+0.06) and [M/H]=0.26(-0.23/+0.24) (1σ confidence intervals), values consistent with those of the Sun and with a companion formation via gravitational instability or core accretion. The projected rotation velocity < 7.8 km/s is small relative to other directly imaged companions with similar masses and ages. This may imply a near pole-on orientation or effective magnetic braking by a circumplanetary disk. In addition, we added the companion-to-primary relative RV measurement to the orbital fitting and obtained updated constraints on orbital parameters. Detailed characterization of super-Jovian companions within 20 au like HIP 99770 b is critical for understanding the formation histories of this population.

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arXiv2024

TOI-1408: Discovery and Photodynamical Modeling of a Small Inner Companion to a Hot Jupiter Revealed by TTVs

We report the discovery and characterization of a small planet, TOI-1408 c, on a 2.2-day orbit located interior to a previously known hot Jupiter, TOI-1408 b ($P=4.42$ d, $M=1.86\pm0.02\,M_\mathrm{Jup}$, $R=2.4\pm0.5\,R_\mathrm{Jup}$) that exhibits grazing transits. The two planets are near 2:1 period commensurability, resulting in significant transit timing variations (TTVs) for both planets and transit duration variations (TDVs) for the inner planet. The TTV amplitude for TOI-1408 c is 15% of the planet's orbital period, marking the largest TTV amplitude relative to the orbital period measured to date. Photodynamical modeling of ground-based radial velocity (RV) observations and transit light curves obtained with the Transiting Exoplanet Survey Satellite (TESS) and ground-based facilities leads to an inner planet radius of $2.22\pm0.06\,R_\oplus$ and mass of $7.6\pm0.2\,M_\oplus$ that locates the planet into the Sub-Neptune regime. The proximity to the 2:1 period commensurability leads to the libration of the resonant argument of the inner planet. The RV measurements support the existence of a third body with an orbital period of several thousand days. This discovery places the system among the rare systems featuring a hot Jupiter accompanied by an inner low-mass planet.

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arXiv2024

Hydrogen sulfide and metal-enriched atmosphere for a Jupiter-mass exoplanet

As the closest transiting hot Jupiter to Earth, HD 189733b has been the benchmark planet for atmospheric characterization. It has also been the anchor point for much of our theoretical understanding of exoplanet atmospheres from composition, chemistry, aerosols to atmospheric dynamics, escape, and modeling techniques. Prior studies of HD 189733b have detected carbon and oxygen-bearing molecules H2O and CO in the atmosphere. The presence of CO2 and CH4 has been claimed but later disputed. The inferred metallicity based on these measurements, a key parameter in tracing planet formation locations, varies from depletion to enhancement, hindered by limited wavelength coverage and precision of the observations. Here we report detections of H2O (13.4 sigma), CO2 (11.2 sigma), CO (5 sigma), and H2S (4.5 sigma) in the transmission spectrum (2.4-5 micron) of HD 189733b. With an equilibrium temperature of ~1200K, H2O, CO, and H2S are the main reservoirs for oxygen, carbon, and sulfur. Based on the measured abundances of these three major volatile elements, we infer an atmospheric metallicity of 3-5 times stellar. The upper limit on the methane abundance at 5 sigma is 0.1 ppm which indicates a low carbon-to-oxygen ratio (<0.2), suggesting formation through the accretion of water-rich icy planetesimals. The low oxygen-to-sulfur and carbon-to-sulfur ratios also support the planetesimal accretion formation pathway.

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arXiv2024

Exoplanet Aeronomy: A Case Study of WASP-69b's Variable Thermosphere

Aeronomy, the study of Earth's upper atmosphere and its interaction with the local space environment, has long traced changes in the thermospheres of Earth and other solar system planets to solar variability in the X-ray and extreme ultraviolet (collectively, "XUV") bands. Extending comparative aeronomy to the short-period extrasolar planets may illuminate whether stellar XUV irradiation powers atmospheric outflows that change planetary radii on astronomical timescales. In recent years, near-infrared transit spectroscopy of metastable HeI has been a prolific tracer of high-altitude planetary gas. We present a case study of exoplanet aeronomy using metastable HeI transit observations from Palomar/WIRC and follow-up high-energy data from the Neil Gehrels Swift Observatory that were taken within one month of the WASP-69 system, a K-type main sequence star with a well-studied hot Jupiter companion. Supplemented by archival data, we find that WASP-69's X-ray flux in 2023 was less than 50% of what was recorded in 2016 and that the metastable HeI absorption from WASP-69b was lower in 2023 versus past epochs from 2017-2019. Via atmospheric modeling, we show that this time-variable metastable HeI signal is in the expected direction given the observed change in stellar XUV, possibly stemming from WASP-69's magnetic activity cycle. Our results underscore the ability of multi-epoch, multi-wavelength observations to paint a cohesive picture of the interaction between an exoplanet's atmosphere and its host star.

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arXiv2024

Are these planets or brown dwarfs? Broadly solar compositions from high-resolution atmospheric retrievals of ~10-30 $M_\textrm{Jup}$ companions

Using Keck Planet Imager and Characterizer (KPIC) high-resolution ($R$~35000) spectroscopy from 2.29-2.49 $μ$m, we present uniform atmospheric retrievals for eight young substellar companions with masses of ~10-30 $M_\textrm{Jup}$, orbital separations spanning ~50-360 au, and $T_\textrm{eff}$ between ~1500-2600 K. We find that all companions have solar C/O ratios, and metallicities, to within the 1-2$σ$ level, with the measurements clustered around solar composition. Stars in the same stellar associations as our systems have near-solar abundances, so these results indicate that this population of companions is consistent with formation via direct gravitational collapse. Alternatively, core accretion outside the CO snowline would be compatible with our measurements, though the high mass ratios of most systems would require rapid core assembly and gas accretion in massive disks. On a population level, our findings can be contrasted with abundance measurements for directly imaged planets with m<10 $M_\textrm{Jup}$, which show tentative atmospheric metal enrichment. In addition, the atmospheric compositions of our sample of companions are distinct from those of hot Jupiters, which most likely form via core accretion. For two companions with $T_\textrm{eff}$~1700-2000 K (kap And b and GSC 6214-210 b), our best-fit models prefer a non-gray cloud model with >3$σ$ significance. The cloudy models yield 2-3$σ$ lower $T_\textrm{eff}$ for these companions, though the C/O and [C/H] still agree between cloudy and clear models at the $1σ$ level. Finally, we constrain 12CO/13CO for three companions with the highest S/N data (GQ Lup b, HIP 79098 b, and DH Tau b), and report $v$sin($i$) and radial velocities for all companions.

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