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Planets orbiting M-dwarf stars are prime targets in the search for rocky exoplanet atmospheres. The small size of M dwarfs renders their planets exceptional targets for transmission spectroscopy, facilitating atmospheric characterization. However, it remains unknown whether their host stars' highly variable extreme-UV radiation environments allow atmospheres to persist. With JWST, we have begun to determine whether or not the most favorable rocky worlds orbiting M dwarfs have detectable atmospheres. Here, we present a 2.8-5.2 micron JWST NIRSpec/G395H transmission spectrum of the warm (700 K, 40.3x Earth's insolation) super-Earth GJ 486b (1.3 R$_{\oplus}$ and 3.0 M$_{\oplus}$). The measured spectrum from our two transits of GJ 486b deviates from a flat line at 2.2 - 3.3 $\sigma$, based on three independent reductions. Through a combination of forward and retrieval models, we determine that GJ 486b either has a water-rich atmosphere (with the most stringent constraint on the retrieved water abundance of H2O > 10% to 2$\sigma$) or the transmission spectrum is contaminated by water present in cool unocculted starspots. We also find that the measured stellar spectrum is best fit by a stellar model with cool starspots and hot faculae. While both retrieval scenarios provide equal quality fits ($\chi^2_\nu$ = 1.0) to our NIRSpec/G395H observations, shorter wavelength observations can break this degeneracy and reveal if GJ 486b sustains a water-rich atmosphere., Comment: 18 pages, 7 figures, 5 tables. Accepted in ApJ Letters. Co-First Authors

Brown dwarfs in close-in orbits around white dwarfs offer an excellent opportunity to investigate properties of fast-rotating, tidally-locked, and highly-irradiated atmospheres. We present Hubble Space Telescope Wide Field Camera 3 G141 phase-resolved observations of two brown dwarf-white dwarf binaries: WD 0137-349 and EPIC 212235321. Their 1.1 to 1.7 $\mu$m phase curves demonstrate rotational modulations with semi-amplitudes of $5.27\pm0.02$% and $29.1\pm0.1$%; both can be well fit by multi-order Fourier series models. The high-order Fourier components have the same phase as the first order and are likely caused by hot spots located at the substellar points, suggesting inefficient day/night heat transfer. Both brown dwarfs' phase-resolved spectra can be accurately represented by linear combinations of their day- and night-side spectra. Fitting the irradiated brown dwarf model grids to the day-side spectra require a filling factor of ~50%, further supporting a hot spot dominating the emission of the day-sides. The night-side spectrum of WD 0137-349B is reasonably well fit by non-irradiated substellar models and the one of EPIC 212235321B can be approximated by a Planck function. We find strong spectral variations in the brown dwarfs' day/night flux and brightness temperature contrasts, which highlights the limitations of band-integrated measurements in probing heat transfer in irradiated objects. On the color-magnitude diagram, WD 0137-349B evolves along a cloudless model track connecting the early-L and mid-T spectral types, demonstrating that clouds and disequilibrium chemistry have a negligible effect on this object. A full interpretation of these high-quality phase-resolved spectra calls for new models that couple atmospheric circulation and radiative transfer under high-irradiation conditions., Comment: Accepted for publication in the AAS Journals. Please also see a companion paper led by Ben W. P. Lew, in which we discuss results from another set of observations of the same program. A video abstract of the paper can be viewed at https://youtu.be/WNz4iBKO99M

To increase the sample size of future atmospheric characterization efforts, we build on the planetary infrared excess (PIE) technique that has been proposed as a means to detect and characterize the thermal spectra of transiting and non-transiting exoplanets using sufficiently broad wavelength coverage to uniquely constrain the stellar and planetary spectral components from spatially unresolved observations. We performed simultaneous retrievals of stellar and planetary spectra for the archetypal planet WASP-43b in its original configuration and a non-transiting configuration to determine the efficacy of the PIE technique for characterizing the planet's nightside atmospheric thermal structure and composition using typical out-of-transit JWST observations. We found that using PIE with JWST should enable the stellar and planetary spectra to be disentangled with no degeneracies seen between the two flux sources, thus allowing robust constraints on the planet's nightside thermal structure and water abundance to be retrieved. The broad wavelength coverage achieved by combining spectra from NIRISS, NIRSpec, and MIRI enables PIE retrievals that are within 10% of the precision attained using traditional secondary eclipse measurements, although mid-IR observations with MIRI alone may face up to 3.5 times lower precision on the planet's irradiation temperature. For non-transiting planets with unconstrained radius priors, we were able to identify and break the degeneracy between planet radius and irradiation temperature using data that resolved the peak of both the stellar and planetary spectra, thus potentially increasing the number of planets amenable to atmospheric characterization with JWST and other future mission concepts., 13 pages, 5 figures, accepted to ApJ Letters

Optical, reflected light eclipse observations provide a direct probe of the exoplanet scattering properties, such as from aerosols. We present here the photometric, reflected light observations of WASP-43b using the HST WFC3/UVIS instrument with the F350LP filter (346-822nm) encompassing the entire optical band. This is the first reflected light, photometric eclipse using UVIS in scanning mode; as such we further detail our scanning extraction and analysis pipeline Arctor. Our HST WFC3/UVIS eclipse light curve for WASP-43 b derived a 3-{\sigma} upper limit of 67 ppm on the eclipse depth, which implies that WASP-43b has a very dark dayside atmosphere. With our atmospheric modeling campaign, we compared our reflected light constraints with predictions from global circulation and cloud models, benchmarked with HST and Spitzer observations of WASP-43b. We infer that we do not detect clouds on the dayside within the pressure levels probed by HST WFC3/UVIS with the F350LP filter (P > 1 bar). This is consistent with the GCM predictions based on previous WASP-43b observations. Dayside emission spectroscopy results from WASP-43b with HST and Spitzer observations are likely to not be significantly affected by contributions from cloud particles., Comment: 29 pages, 22 figures, accepted to AAS/ApJ

We outline specific steps that NASA and the space science community can take to advance collaboration and coordination between the communities represented by the four NASA Science Mission Directorate Divisions. It is important to note that the only way that this effort can succeed is if NASA initiates and supports it through directed resources.

There is an opportunity to advance solar system and extrasolar planetary studies that does not require new telescopes or new missions but better use and access to data sets. This approach leverages significant investment from space agencies in exploring the solar system and using those discoveries for the study of extrasolar planets.

With the growing number of spectroscopic observations and observational platforms capable of exoplanet atmospheric characterization, there is a growing need for analysis techniques that can distill information about a large population of exoplanets into a coherent picture of atmospheric trends expressed within the statistical sample. In this work, we develop a Hierarchical Bayesian Atmospheric Retrieval (HBAR) model to infer population-level trends in exoplanet atmospheric characteristics. We demonstrate HBAR on the case of inferring a trend in atmospheric CO2 with incident stellar flux, predicted by the presence of a functioning carbonate-silicate weathering negative feedback cycle, an assumption upon which all calculations of the habitable zone (HZ) rest. Using simulated transmission spectra and JWST-quality observations of rocky planets with H2O, CO2, and N2 bearing atmospheres, we find that the predicted trend in CO2 causes subtle differences in the spectra of order 10 ppm in the 1-5 um range, underscoring the challenge inherent to testing this hypothesis. In the limit of highly precise data (100 stacked transits per planet), we show that our HBAR model is capable of inferring the population-level parameters that characterize the trend in CO2, and we demonstrate that the null hypothesis and other simpler trends can be rejected at high confidence. Although we find that this specific empirical test of the HZ may be prohibitively challenging in the JWST era, the HBAR framework developed in this work may find a more immediate usage for the analysis of gas giant spectra observed with JWST, Ariel, and other upcoming missions., Comment: 15 pages, 5 figures, accepted for publication in AJ

Upcoming NASA astrophysics missions such as the James Webb Space Telescope will search for signs of life on planets transiting nearby stars. Doing so will require coadding dozens of transmission spectra to build up sufficient signal to noise while simultaneously accounting for challenging systematic effects such as surface/weather variability, atmospheric refraction, and stellar activity. To determine the magnitude and impacts of both stellar and planet variability on measured transmission spectra, we must assess the feasibility of stacking multiple transmission spectra of exo-Earths around their host stars. Using our own solar system, we can determine if current methodologies are sufficient to detect signs of life in Earth’s atmosphere and measure the abundance of habitability indicators, such as H2O and CO2, and biosignature pairs, such as O2 and CH4. We assess the impact on transmission spectra of Earth transiting across the Sun from solar and planetary variability and identify remaining unknowns for understanding exoplanet transmission spectra. We conclude that a satellite observing Earth transits across the Sun from beyond L2 is necessary to address these long-standing concerns about the reliability of coadding planet spectra at UV, optical, and infrared wavelengths from multiple transits in the face of relatively large astrophysical systematics.

The reflected light signal from a planet throughout its orbit is a powerful probe of a planet's atmospheric properties. There are a number of planets that are amenable to reflected light phase curve studies with present and future space-based instrumentation and here we assess our ability to characterize these worlds. Using simulated TESS populations we identify the Nine, a set of archetypal exoplanets with the potential to be bright in reflected light, because of their radii and proximity to their star, while still being cool enough to have minimal thermal contamination at optical wavelengths. For each planet we compute albedo spectra for several cloud and atmosphere assumptions (e.g. thermochemical equilibrium, solar composition). We find that in the TESS bandpass the estimated contrast at optical wavelengths is typically, 28 pages, 9 Figures, 3 Tables, Accepted to AJ

For terrestrial exoplanets with thin atmospheres or no atmospheres, the surface contributes light to the reflected light signal of the planet. Measurement of the variety of disk-integrated brightnesses of bodies in the Solar System and the variation with illumination and wavelength is essential for both planning imaging observations of directly imaged exoplanets and interpreting the eventual datasets. Here we measure the change in brightness of the Galilean satellites as a function of planetocentric longitude, illumination phase angle, and wavelength. The data span a range of wavelengths from 400-950nm and predominantly phase angles from 0-25 degrees, with some constraining observations near 60-140 degrees. Despite the similarity in size and density between the moons, surface inhomogeneities result in significant changes in the disk-integrated reflectivity with planetocentric longitude and phase angle. We find that these changes are sufficient to determine the rotational periods of the moon. We also find that at low phase angles the surface can produce reflectivity variations of 8-36% and the limited high phase angle observations suggest variations will have proportionally larger amplitudes at higher phase angles. Additionally, all the Galilean satellites are darker than predicted by an idealized Lambertian model at the phases most likely to be observed by direct-imaging missions. If Earth-size exoplanets have surfaces similar to that of the Galilean moons, we find that future direct imaging missions will need to achieve precisions of less than 0.1\,ppb. Should the necessary precision be achieved, future exoplanet observations could exploit similar observation schemes to deduce surface variations, determine rotation periods, and potentially infer surface composition., 21 pages, 7 tables, 15 figures, including machine-readable table, accepted to AJ

The recently launched Transiting Exoplanet Survey Satellite (TESS) is expected to produce many new exoplanet discoveries which will be especially amenable to follow-up study. Assessments of the planet discovery yield of TESS, such as Sullivan et al. (2015) and Barclay et al. (2018), will be important for planning follow-up work. Analyzing these predicted planet samples, however, we find that giant planet radii derived from the current bulk transiting planet sample have been used at all potential orbits without accounting for the temperature dependence of radius inflation. These phantom inflated planets (PIPs) make up just over 1.1% of the Sullivan et al. (2015) predicted population and about 8% of the Barclay et al. (2018) sample. Similar population predictions for direct imaging studies should likewise take care not to include inflated planets at large separations as such planets will appear larger and, depending on assumed albedo, brighter and more easily detectable than physically possible. Despite their high false positive rate, giant planets are the testbeds for atmospheric characterization techniques and care should be taken to understand and account for potential contaminating factors in this population.

We present phase curves of Jupiter from 0-140 degrees as measured in multiple optical bandpasses by Cassini/ISS during the Millennium flyby of Jupiter in late 2000 to early 2001. Phase curves are of interest for studying the energy balance of Jupiter and understanding the scattering behavior of Jupiter as an exoplanet analog. We find that Jupiter is significantly darker at partial phases than an idealized Lambertian planet by roughly 25% and is not well fit by Jupiter-like exoplanet atmospheric models across all wavelengths. We provide analytic fits to Jupiter's phase function in several Cassini/ISS imaging filter bandpasses. In addition, these observations show that Jupiter's color is more variable with phase angle than predicted by models. Therefore, the color of even a near Jupiter-twin planet observed at a partial phase cannot be assumed to be comparable to that of Jupiter at full phase. We discuss how WFIRST and other future direct-imaging missions can enhance the study of cool giants., Comment: 29 pages, 9 figures, accepted to AJ

Many brown dwarfs are on ultra-short period and tidally-locked orbits around white dwarf hosts. Because of these small orbital separations, the brown dwarfs are irradiated at levels similar to hot Jupiters. Yet, they are easier to observe than hot Jupiters because white dwarfs are fainter than main-sequence stars at near-infrared wavelengths. Irradiated brown dwarfs are, therefore, ideal hot Jupiter analogs for studying the atmospheric response under strong irradiation and fast rotation. We present the 1.1--1.67$\rm \mu m$ spectroscopic phase curve of the irradiated brown dwarf (SDSS1411-B) in the SDSS J141126.20+200911.1 brown-dwarf white-dwarf binary with the near-infrared G141 grism of Hubble Space Telescope Wide Field Camera 3. SDSS1411-B is a $50~\rm M_{\rm Jup}$ brown dwarf with an irradiation temperature of 1300K and has an orbital period of 2.02864 hours. Our best-fit model suggests a phase-curve amplitude of 1.4$\%$ and places an upper limit of 11 degrees for the phase offset from the secondary eclipse. After fitting the white-dwarf spectrum, we extract the phase-resolved brown-dwarf emission spectra. We report a highly wavelength-dependent day-night spectral variation, with the water-band flux variation of about $360\pm70\% $ and a comparatively small J-band flux variation of $37\pm2\%$. By combining the atmospheric modeling results and the day-night brightness-temperature variations, we derive a pressure-dependent temperature contrast. We discuss the difference in the spectral features of SDSS1411-B and hot Jupiter WASP-43b, and the lower-than-predicted day-night temperature contrast of J4111-BD. Our study provides the high-precision observational constraints on the atmospheric structures of an irradiated brown dwarf at different orbital phases., Comment: Accepted for publication in The Astronomical Journal