arXiv subjects

Brandon Park Coy

Publications and source records attributed to Brandon Park Coy.

11 recordsLinked to original sources

arXiv2026

Revealing the Shiny Nature of Lava Worlds: Small Planet Phase Curves with TESS

Recent thermal emission observations suggest that `lava worlds'--Earth-sized exoplanets hot enough to melt their silicate surfaces--have atmospheres on a population-level. This evidence comes from measured dayside brightness temperatures much lower than expected for a dark, maximally hot bare rock. However, these results are degenerate between a thick atmosphere advecting heat to the planet's nightside or a high albedo reflecting incoming radiation. This degeneracy can be resolved through visible light phase curve observations, which can constrain how much light is reflected by the planet. Here, we present a population-level analysis of the visible-light TESS phase curves of $R\lesssim5\,R_{\oplus}$ exoplanets. We select the population by defining a `Reflection Spectroscopy Metric' meant to quantify the relative signal-to-noise of reflected light observations. We report new $>3\,σ$ detections of the TESS phase curve signals of LTT 9779 b and the lava worlds TOI-2431 b, TOI-561 b, 55 Cancri e, K2-141 b, and TOI-1444 b. We use these results and previous results from Kepler and CHEOPS to homogeneously derive geometric albedos for these planets, revealing that lava worlds generally show high albedos compared to the low geometric albedos measured for hot Jupiters at similar temperatures. These results support an emerging picture of reflective silicate clouds on these worlds cooling their daysides. The upcoming PLATO mission is expected to tightly constrain the reflected light phase curves of lava worlds.

astro-ph.EP↗

arXiv2026

JWST MIRI reveals a potential atmosphere on the ultra-hot rocky planet TOI-431b

An open question in exoplanet science is whether rocky exoplanets extremely close in to their host stars, `lava worlds', can retain significant atmospheres. Thermal emission observations via secondary eclipse can be used to determine whether a rocky exoplanet possesses an atmosphere. It is expected that an atmosphere could increase the planet's albedo and/or redistribute heat away from the dayside, reducing the secondary eclipse depth measured. Recent eclipse observations of lava planets, rocky planets hot enough to have liquid magma surfaces, have suggested the presence of atmospheres. Here, we present a single JWST MIRI/LRS partial secondary eclipse of the lava planet TOI-431b. We measure an eclipse depth of 81 $\pm$ 16 ppm, which corresponds to a brightness temperature of $1967^{+237}_{-253}$ K and a brightness temperature ratio R = $0.81\pm0.10$, being 1.2 $σ$ higher than the value reported by Spitzer. The observed brightness temperature ratio is 1.8$σ$ below that of a zero-albedo, zero heat redistribution bare rock (R=1). Given that magma pools are expected to have low albedos, we find that our results are best explained by the presence of an atmosphere. Future work to measure the exact composition of TOI-431b's atmosphere and improve models of lava planet atmospheres would better constrain the nature and evolution of lava planets.

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arXiv2026

Evidence for an Atmosphere on the Ultra-Short Period super-Earth HD 3167 b

'Lava worlds'-Earth-sized planets hot enough (Teq >~ 1100 K) to melt their dayside silicate surfaces-have emerged as promising candidates for atmospheric detection and characterization. Thermal emission observations show an apparent dichotomy: the hottest lava worlds have colder daysides than the temperature of a maximally emitting bare rock, indicating the likely presence of thick and/or reflective atmospheres while the coldest ones do not. However, where in instellation flux this potential bifurcation occurs is uncertain. We present a JWST MIRI LRS eclipse of the ultra-short period (USP) lava world HD 3167 b (Teq = 1786 K, R = 1.6 Rearth, P = 0.96 d) that helps bridge this gap. We measure the white light eclipse depth to be 38 +/- 11 ppm, more than 5 sigma lower than the expected eclipse depth of a dark, maximally hot bare rock. We use this to derive a dayside brightness temperature that is best explained by the presence of an atmosphere that cools the dayside by reflecting incoming starlight and/or efficiently redistributing heat to the planet's nightside. An atmosphere is further compatible with the planet's slight under-density compared to an Earth-like composition. The corresponding dayside emission spectrum is not precise enough to constrain atmospheric composition, motivating follow-up spectroscopic observations with JWST NIRSpec. Lastly, we use our observation and existing data to refine key planetary parameters of the HD 3167 system. HD 3167 b is currently the least irradiated USP super-Earth with evidence for an atmosphere.

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arXiv2025

The Cosmic Shoreline Revisited: A Metric for Atmospheric Retention Informed by Hydrodynamic Escape

The "cosmic shoreline," a semi-empirical relation that separates airless worlds from worlds with atmospheres as proposed by K. J. Zahnle & D. C. Catling, is now guiding large-scale JWST surveys aimed at detecting rocky exoplanet atmospheres. We expand upon this framework by revisiting the shoreline using existing hydrodynamic escape models applied to Earth-like, Venus-like, and steam atmospheres for rocky exoplanets, and we estimate energy-limited escape rates for CH4 atmospheres. We determine the critical instellation required for atmospheric retention by calculating time-integrated atmospheric mass loss. Our analysis introduces a new metric for target selection in the Rocky Worlds Director's Discretionary Time and refines expectations for rocky planet atmosphere searches. Exploring initial volatile inventory ranging from 0.01% to 1% of planetary mass, we find that its variation prevents the definition of a unique clear-cut shoreline, though nonlinear escape physics can reduce this sensitivity to initial conditions. Additionally, uncertain distributions of high-energy stellar evolution and planet age further blur the critical instellations for atmospheric retention, yielding broad shorelines. Hydrodynamic escape models find atmospheric retention is markedly more favorable for higher-mass planets orbiting higher-mass stars, with carbon-rich atmospheres remaining plausible for 55 Cancri e despite its extreme instellation. We caution that our estimates are sensitive to processes with poorly understood dynamics, such as atomic line cooling. Finally, we illustrate how density measurements can be used to statistically test the existence of the cosmic shorelines, emphasizing the need for more precise mass and radius measurements.

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arXiv2025

The Role of Tectonic Luck in Long-Term Habitability of Abiotic Earth-like Planets

Carbonate-silicate weathering feedback is thought to stabilize Earth's climate on geologic timescales. If climate warms, faster mineral dissolution and increased rainfall speed up weathering, increasing CO2 drawdown and opposing the initial warming. Limits to where this feedback might operate on terrestrial exoplanets with N2-O2-CO2-H2O atmospheres are used to define the 'habitable zone'-the range of orbits around a star where liquid water can be stable on a planet's surface. However, the impacts on long-term habitability of randomly varying volcanic outgassing, tectonic collisions, and tectonic parameters (e.g., number of continental plates, size of plates, plate velocity) remain poorly understood. In this work, we present an idealized and broadly-applicable quasi-2D model of the long-term climate stability of abiotic Earth-twins. The model tracks atmospheric CO2 as 'disks' collide, promoting uplift and supplying new weatherable minerals through erosion. Without resupply, soils become less weatherable and the feedback's strength wanes, making a planet susceptible to catastrophic warming events or hard snowballs where the surface becomes frozen over. We find that tectonic uplift spurred by continental collisions cannot be the sole supplier of weatherable minerals within our model framework, as such climates either become uninhabitably hot (for complex life) as soils become leached of weatherable minerals or experience extreme swings in temperature over short timescales. This conclusion is strengthened when taking into account the destabilizing effects of outgassing variability and increasing stellar luminosity. In addition to frequent collisions, other resupply mechanisms for weatherable minerals, such as wind-driven dust transport, glacial erosion, and/or seafloor weathering, are likely required for long-term stability on Earth-like terrestrial exoplanets.

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arXiv2025

Population-level Hypothesis Testing with Rocky Planet Emission Data: A Tentative Trend in the Brightness Temperatures of M-Earths

Determining which rocky exoplanets have atmospheres, and why, is a key goal for the James Webb Space Telescope. So far, emission observations of individual rocky exoplanets orbiting M stars (M-Earths) have not provided definitive evidence for atmospheres. Here, we synthesize emission data for M-Earths and find a trend in measured brightness temperatures (ratioed to its theoretical maximum value) as a function of instellation. However, the statistical evidence of this trend is dependent on the choice of stellar model, and we consider its identification tentative. We show that this trend can be explained by either the onset of thin/tenuous (<1 bar) CO2-rich atmospheres on colder worlds, or a population of bare rocks with stronger space weathering and/or coarser regolith on closer-in worlds. Such grain coarsening may be caused by sintering near the melting point of rock or frequent volcanic resurfacing. Furthermore, we highlight considerations when testing rocky planet hypotheses at the population level, including the choice of instrument, stellar modeling, and how brightness temperatures are derived. We also find that fresh (unweathered) fine-grained surfaces can serve as a false positive to the detection of moderate atmospheric heat redistribution through eclipse observations. However, we argue that such surfaces are unlikely given the ubiquity of space weathering in the Solar System, the low albedo of Solar System airless bodies, and the high stellar wind environments of M-Earths. Emission data from a larger sample of M-Earths will be able to confirm or reject this tentative trend and diagnose its cause through spectral characterization.

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arXiv2025

The Atmosphere of Titan in Late Northern Summer from JWST and Keck Observations

Saturn's moon Titan undergoes a long annual cycle of 29.45 Earth years. Titan's northern winter and spring were investigated in detail by the Cassini-Huygens spacecraft (2004-2017), but the northern summer season remains sparsely studied. Here we present new observations from the James Webb Space Telescope (JWST) and Keck II telescope made in 2022 and 2023 during Titan's late northern summer. Using JWST's mid-infrared instrument, we spectroscopically detected the methyl radical, the primary product of methane break-up and key to the formation of ethane and heavier molecules. Using the near-infrared spectrograph onboard JWST, we detected several non-local thermodynamic equilibrium CO and CO2 emission bands, which allowed us to measure these species over a wide altitude range. Lastly, using the near-infrared camera onboard JWST and Keck II, we imaged northern hemisphere tropospheric clouds evolving in altitude, which provided new insights and constraints on seasonal convection patterns. These observations pave the way for new observations and modelling of Titan's climate and meteorology as it progresses through the northern fall equinox, when its atmosphere is expected to show notable seasonal changes.

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arXiv2024

A dark, bare rock for TOI-1685 b from a JWST NIRSpec G395H phase curve

We report JWST NIRSpec/G395H observations of TOI-1685 b, a hot rocky super-Earth orbiting an M2.5V star, during a full orbit. We obtain transmission and emission spectra of the planet and characterize the properties of the phase curve, including its amplitude and offset. The transmission spectrum rules out clear H$_2$-dominated atmospheres, while secondary atmospheres (made of water, methane, or carbon dioxide) cannot be statistically distinguished from a flat line. The emission spectrum is featureless and consistent with a blackbody-like brightness temperature, helping rule out thick atmospheres with high mean molecular weight. Collecting all evidence, the properties of TOI-1685 b are consistent with a blackbody with no heat redistribution and a low albedo, with a dayside brightness temperature 0.98$\pm$0.07 times that of a perfect blackbody in the NIRSpec NRS2 wavelength range (3.823-5.172 um). Our results add to the growing number of seemingly airless M-star rocky planets, thus constraining the location of the "Cosmic Shoreline". Three independent data reductions have been carried out, all showing a high-amplitude correlated noise component in the white and spectroscopic light curves. The correlated noise properties are different between the NRS1 and NRS2 detectors - importantly the timescales of the strongest components (4.5 hours and 2.5 hours, respectively) - suggesting the noise is from instrumental rather than astrophysical origins. We encourage the community to look into the systematics of NIRSpec for long time-series observations.

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arXiv2024

No Thick Atmosphere on the Terrestrial Exoplanet Gl 486b

A primary science goal for JWST is to detect and characterize the atmospheres of terrestrial planets orbiting M dwarfs (M-Earths). The existence of atmospheres on M-Earths is highly uncertain because their host stars' extended history of high XUV irradiation may act to completely remove their atmospheres. We present two JWST secondary eclipse observations of the M-Earth Gl 486b (also known as GJ 486b) between 5-12 $μ$m. We combined these observations with a precise analysis of the host star parameters to derive a planetary dayside temperature of $T_{p}=865 \pm 14$ K. We compared this temperature to the maximum expected temperature for a zero albedo, zero heat redistribution bare rock and derived a temperature ratio of $R=\frac{T_{p,dayside}}{T_{p,max}}=0.97 \pm 0.01$. This value is consistent with an airless body with a slight non-zero albedo or a thin atmosphere with $<1$% H$_{2}$O or $<1$ ppm CO$_{2}$. However, it is inconsistent with an Earth- or Venus-like atmosphere, and the spectrum shows no clear emission or absorption features. Additionally, our observations are inconsistent with the water-rich atmospheric scenario allowed by previous transit observations and suggest the transmission spectrum was instead shaped by stellar contamination (Moran et al. 2023). Given the potential for atmospheric escape throughout the system's $\geq6.6$-Gyr lifetime (Diamond-Lowe et al. 2024), we conclude that the observations are likely best explained by an airless planet. This result is the most precise measurement yet of terrestrial exoplanet thermal emission with JWST, which places a strong constraint on the position of the "Cosmic Shoreline" between airless bodies and those with atmospheres.

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arXiv2023

Spitzer IRS Observations of Titan as a Precursor to JWST MIRI Observations

In this work we present, for the first time, infrared spectra of Titan from the Spitzer Space Telescope ($2004-2009$). The data are from both the short wavelength-low resolution (SL, $5.13-14.29\mathrm{μm}, R\sim60-127$) and short wavelength-high resolution channels (SH, $9.89 - 19.51\mathrm{μm}, R\sim600$) showing the emissions of CH$_{4}$, C$_{2}$H$_{2}$, C$_{2}$H$_{4}$, C$_{2}$H$_{6}$, C$_{3}$H$_{4}$, C$_{3}$H$_{6}$, C$_{3}$H$_{8}$, C$_{4}$H$_{2}$, HCN, HC$_{3}$N, and CO$_{2}$. We compare the results obtained for Titan from Spitzer to those of the Cassini Composite Infrared Spectrometer (CIRS) for the same time period, focusing on the $16.35-19.35\mathrm{μm}$ wavelength range observed by the SH channel but impacted by higher noise levels in CIRS observations. We use the SH data to provide estimated haze extinction cross-sections for the $16.67-17.54\mathrm{μm}$ range that are missing in previous studies. We conclude by identifying spectral features in the $16.35-19.35\mathrm{μm}$ wavelength range, including two prominent emission features at 16.39 and $17.35\mathrm{μm}$, that could be analyzed further through upcoming James Webb Space Telescope Cycle 1 observations with the Mid-Infrared Instrument ($5.0-28.3\mathrm{μm}, R\sim1500-3500$). We also highlight gaps in current spectroscopic knowledge of molecular bands, including candidate trace species such as C$_{60}$ and detected trace species such as C$_{3}$H$_{6}$, that could be addressed by theoretical and laboratory study.

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arXiv2018

Analysis of Neptune's 2017 Bright Equatorial Storm

We report the discovery of a large ($\sim$8500 km diameter) infrared-bright storm at Neptune's equator in June 2017. We tracked the storm over a period of 7 months with high-cadence infrared snapshot imaging, carried out on 14 nights at the 10 meter Keck II telescope and 17 nights at the Shane 120 inch reflector at Lick Observatory. The cloud feature was larger and more persistent than any equatorial clouds seen before on Neptune, remaining intermittently active from at least 10 June to 31 December 2017. Our Keck and Lick observations were augmented by very high-cadence images from the amateur community, which permitted the determination of accurate drift rates for the cloud feature. Its zonal drift speed was variable from 10 June to at least 25 July, but remained a constant $237.4 \pm 0.2$ m s$^{-1}$ from 30 September until at least 15 November. The pressure of the cloud top was determined from radiative transfer calculations to be 0.3-0.6 bar; this value remained constant over the course of the observations. Multiple cloud break-up events, in which a bright cloud band wrapped around Neptune's equator, were observed over the course of our observations. No "dark spot" vortices were seen near the equator in HST imaging on 6 and 7 October. The size and pressure of the storm are consistent with moist convection or a planetary-scale wave as the energy source of convective upwelling, but more modeling is required to determine the driver of this equatorial disturbance as well as the triggers for and dynamics of the observed cloud break-up events.

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