Direct imaging measures how bright a planet appears in reflected starlight, but brightness alone cannot tell whether the planet is large and dark or small and bright. This radius-albedo degeneracy limits the characterization of non-transiting exoplanets, including those targeted by the Habitable Worlds Observatory (HWO). From multi-epoch observations we construct ultraviolet observables that are independent of planetary radius: the normalized 400 nm lightcurve shape, the ultraviolet intensity and polarization colors, and the degree of linear polarization. In a clear 360-400 nm spectral window where Rayleigh scattering is strong and bright ultraviolet surfaces are nearly colorless, these observables constrain the atmospheric column, the surface reflectivity, and, when relevant, the observed phases before radius is inferred. Using the GPU accelerated vector radiative transfer model vSmartMOM, we test how uniquely these radius-free observables determine the scattering state at signal-to-noise ratios (SNRs) of 5, 20, and 100. For the chosen six phase sequence, spectropolarimetry gives median radius consequences of 9.5

With its extreme axial tilt, radiant energy budget and internal heat of Uranus remain among the most intriguing mysteries of our Solar System. Here, we present the global radiant energy budget spanning a complete orbital period, revealing significant seasonal variations driven primarily by the highly variable solar flux. Despite these fluctuations, emitted thermal power consistently exceeds absorbed solar power, indicating a net energy loss and ongoing global cooling. Based on the seasonal variations of radiant energy budget, we determine a statistically significant internal heat flux. This finding resolves a long-standing debate over whether Uranus possesses internal heat. We also examine the energy budget of the weather layer by combining the internal heat with the radiant energies, revealing significant energy imbalances at both global and hemispheric scales. These global and hemispheric imbalances should be considered in theoretical and numerical models. The Uranus flagship mission, as recommended by the recent survey, will provide crucial observations to address more unresolved questions and advance our understanding of this enigmatic ice giant.

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.

With its extreme axial tilt, Uranus' radiant energy budget (REB) and internal heat flux remain among the most intriguing mysteries in our solar system. By combining observations with modeling, we present the global REB over a complete orbital period (1946–2030), revealing significant seasonal variations. Despite these fluctuations, the global average emitted thermal power consistently exceeds absorbed solar power, indicating a net energy loss. Assuming no significant seasonal variation in emitted power, we estimate an internal heat flux of 0.078 ± 0.018 W/m 2 by analyzing the energy budget over one orbital period. The combination of internal heat and radiant energies indicates substantial global and hemispheric imbalances, with excesses or deficits exceeding 85% of emitted power at the hemispheric scale. These findings are crucial for understanding Uranus' interior and atmosphere. A future flagship mission to Uranus would provide critical observations to address more unresolved questions of this enigmatic ice giant.

The Imaging Science Subsystem onboard the Cassini spacecraft recorded numerous high-quality images of Jupiter and Saturn at various wavelengths, from ultraviolet to near-infrared, during its 20-year mission from 1997 to 2017. Using these images, we have developed global maps of Jupiter and Saturn across multiple wavelengths. These maps reveal the global atmospheric structures of Jupiter and Saturn, offering a comprehensive tool to study the physical and dynamic processes of these atmospheric systems on a global scale. Additionally, these multi-wavelength maps, which probe different pressure levels within the atmospheres, help to explore the vertical structure of these processes. Moreover, global maps at different times enable tracking the movement of dynamic phenomena (e.g., clouds, storms, vortices, eddies, waves, and turbulence), thereby enhancing our understanding of atmospheric dynamics on the giant planets.

The photochemical haze layer in the stratosphere above about 80 km and the condensation haze (hereafter, mist) in the lower stratosphere and troposphere completely cover Titan and play a dominant role in its climate. These hazes determine the atmospheric radiative balance and are also good tracers of the atmospheric circulation. Spatial and temporal characterization of the haze is therefore essential for understanding Titan's climate. It is also an important step towards accessing surface properties and cloud characteristics. Recently, the James Webb Space Telescope (JWST) has provided observations of very high spectral quality. These images and spectra provide information on the latitudinal distribution of haze, from the lower stratosphere (100-150 km) down into the troposphere with good vertical resolution. With this dataset, we are able to describe the mist layer in five distinct sublayers : the deepest layer between 0 and 40 km and four layers of 10 km thickness between 40 and 80 km. The retrievals performed allow us to reconstruct a map of the haze and mist layer on the date of observation (5 November 2022) and thus highlight haze distributions related to circulation. For this analysis, we used a plane-parallel model and thus we are restricted to latitudes between -30°S and 60°N. In details, we found a photochemical haze layer increasing from the north to the south, as expected. Below 80 km, the mist layers have a more complex distribution. The deepest layer (below 40 km) has an opacity minimum near the equator and increasing toward both poles. The opacity of the mist layer between 40 and 80 km gradually changes, from bottom to top, to reach a monotonously increasing opacity from the south to the north, as for the stratospheric haze. In all bands, we find a marked opacity feature above 50 km, south to 10°S, that form a detached mist layer slightly emerging from the background opacity. We could also retrieve the surface reflectivity, with a quite low spatial resolution, that will also be discussed. With the same model, we also analyzed a selected part of the huge VIMS/Cassini data set. We could monitored, albeit with a lower vertical resolution, how these hazes evolve over the season. Here the mist layer is described in only three distinct layers but the observation set spans from 2004 to 2017. During Cassini era, we probe the seasonal change of the haze and the mist layer and have a view on the seasonal modulation of all the layers. These results allow for putting in context the results obtain with the JWST. Moreover, it also offers an interesting comparison for the results obtained with JWST since Cassini observed the opposite season, in 2007. Finally, we can fully understand our results with the help of the Titan Planetary Climate Model (PCM), now coupled with haze and cloud microphysics. This model predicts the evolution of the haze and the mist within the seasonal cycle. The haze layer is transported by dynamics and its distribution is fairly straightforward to understand. The mist, in addition, is also controlled by thermodynamical conditions which must be modeled in detail to be predicted. Then, the mist layers, less opaque in the equatorial region and increasing toward the mid-latitudes and the detached mist layer found with the JSWT are described by the PCM and correlated to the hadley-type cell below 80 km and to the temperature latitude profile. Figure 1 shows the haze and mist layers retrieved in our work. Over the JWST lifetime, we hope to complete the seasonal survey of atmospheric properties for the upcoming period from the northern autumn equinox to the winter solstice. This is a poorly observed period but corresponds to a complete reversal of atmospheric circulation. Significant changes are expected to be observed during this period. With the upcoming very large telescopes (EELT, TMT, etc.) with high sensitivity and spectral resolution, this work shows that an increase in spectral resolution translates - up to a certain point - into an increase in vertical resolution. Finally, the complete characterization of haze and cloud cycles, with models and constraints from the most precise observations possible, is a major objective to prepare for future missions to Titan; Dragonfly but also those which may be decided upon in the near or distant future.Figure 1 : Haze and mist extinction retrieved with the NIRSpec/JWST (5 Nov 2022) (top) and with VIMS/Cassini (2007) (bottom) at 2 μm. The sub-solar latitude was in the northern hemisphere (12.62°N) at the JWST observation date and in the southern hemisphere (between 14.25°S and 8.90°S) during the year 2007.

A planet’s Ly α emission is sensitive to its thermospheric structure. Here we report joint Hubble Space Telescope and Cassini cross-calibration observations of the Saturn Ly α emission made 2 weeks before the Cassini grand finale. To investigate the long-term Saturn Ly α airglow observed by different ultraviolet instruments, we cross-correlate their calibration, finding that while the official Cassini/UVIS sensitivity should be lowered by ∼75%, the Voyager 1/UVS sensitivities should be enhanced by ∼20% at the Ly α channels. This comparison also allowed us to discover a permanent feature of the Saturn disk Ly α brightness that appears at all longitudes as a brightness excess (Ly α bulge) of ∼30% (∼12 σ ) extending over the latitude range ∼5°–35° N compared to the regions at equator and ∼60° N. This feature is confirmed by three distinct instruments between 1980 and 2017 in the Saturn north hemisphere. To analyze the Ly α observations, we use a radiation transfer model of resonant scattering of solar and interplanetary Ly α photons and a latitude-dependent photochemistry model of the upper atmosphere constrained by occultation and remote-sensing observations. For each latitude, we show that the Ly α observations are sensitive to the temperature profile in the upper stratosphere and lower thermosphere, thus providing useful information in a region of the atmosphere that is difficult to probe by other means. In the Saturn Ly α bulge region, at latitudes between ∼5° and ∼35°, the observed brightening and line broadening support seasonal effects, variation of the temperature vertical profile, and potential superthermal atoms that require confirmation.

We present a study of the long term variability of Jupiter’s mid-infrared CH4 auroral emissions. 7.7–7.9 μm images of Jupiter recorded by NASA’s Infrared Telescope Facility, Subaru and Gemini-South over the last three decades were collated in order to quantify the magnitude and timescales over which the northern auroral hotspot’s CH4 emission varies. These emissions predominantly sound the 10- to 1-mbar pressure range and therefore highlight the temporal variability of lower-stratospheric auroral-related heating. We find that the ratio of the radiance of the poleward northern auroral emissions to a lower-latitude zonal-mean, henceforth ‘Relative Poleward Radiance’ or RPR, exhibits variability over a 37% range and over a range of apparent timescales. We searched for patterns of variability in order to test whether seasonally varying solar insolation, the 11-year solar cycle, or short-term solar wind variability at Jupiter’s magnetopause could explain the observed evolution. The variability of the RPR exhibits a weak (r < 0.2) correlation with both the instantaneous and phase-lagged solar insolation received at Jupiter’s high-northern latitudes. This rules out the hypothesis suggested in previous work (e.g. Sinclair et al. 2017a, 2018) that shortwave solar heating of aurorally produced haze particles is the dominant auroral-related heating mechanism in the lower stratosphere. We also find the variability exhibits negligible (r < 0.18) correlation with both the instantaneous and phase-lagged monthly-mean sunspot number, which therefore rules out a long-term variability associated with the solar cycle. On shorter timescales, we find moderate correlations of the RPR with solar wind conditions at Jupiter in the preceding days before images were recorded. For example, we find correlations of r = 0.45 and r = 0.51 of the RPR with the mean and standard deviation solar wind dynamical pressure in the preceding 7 days. The moderate correlation suggests that either: (1) only a subset of solar wind compressions lead to brighter, poleward CH4 emissions and/or (2) a subset of CH4 emission brightening events are driven by internal magnetospheric processes (e.g. Io activity) and independent of solar wind enhancements.

In our solar system, the densely cloud-covered atmosphere of Venus stands out as an example of how polarimetry can be used to gain information on cloud composition and particle mean radius. With current interest running high on discovering and characterizing extrasolar planets in the habitable zone where water exists in the liquid state, making use of spectropolarimetric measurements of directly imaged exoplanets could provide key information unobtainable through other means. In principle, spectropolarimetric measurements can determine if acidity causes water activities in the clouds to be too low for life. To this end, we show that a spectropolarimeter measurement over the range 400–1000 nm would need to resolve linear polarization to a precision of about 1% or better for reflected starlight from an optically thick cloud-enshrouded exoplanet. We assess the likelihood of achieving this goal by simulating measurements from a notional spectropolarimeter as part of a starshade configuration for a large space telescope (a HabEx design, but for a 6 m diameter primary mirror). Our simulations include consideration of noise from a variety of sources. We provide guidance on limits that would need to be levied on instrumental polarization to address the science issues we discuss. For photon-limited noise, integration times would need to be of order 1 hr for a large radius (10 Earth radii) planet to more than 100 hr for smaller exoplanets depending on the star–planet separation, planet radius, phase angle, and desired uncertainty. We discuss implications for surface chemistry and habitability.

In response to ESA’s “Voyage 2050” announcement of opportunity, we propose an ambitious L-class mission to explore one of the most exciting bodies in the Solar System, Saturn’s largest moon Titan. Titan, a “world with two oceans”, is an organic-rich body with interior-surface-atmosphere interactions that are comparable in complexity to the Earth. Titan is also one of the few places in the Solar System with habitability potential. Titan’s remarkable nature was only partly revealed by the Cassini-Huygens mission and still holds mysteries requiring a complete exploration using a variety of vehicles and instruments. The proposed mission concept POSEIDON (Titan POlar Scout/orbitEr and In situ lake lander DrONe explorer) would perform joint orbital and in situ investigations of Titan. It is designed to build on and exceed the scope and scientific/technological accomplishments of Cassini-Huygens, exploring Titan in ways that were not previously possible, in particular through full close-up and in situ coverage over long periods of time. In the proposed mission architecture, POSEIDON consists of two major elements: a spacecraft with a large set of instruments that would orbit Titan, preferably in a low-eccentricity polar orbit, and a suite of in situ investigation components, i.e. a lake lander, a “heavy” drone (possibly amphibious) and/or a fleet of mini-drones, dedicated to the exploration of the polar regions. The ideal arrival time at Titan would be slightly before the next northern Spring equinox (2039), as equinoxes are the most active periods to monitor still largely unknown atmospheric and surface seasonal changes. The exploration of Titan’s northern latitudes with an orbiter and in situ element(s) would be highly complementary in terms of timing (with possible mission timing overlap), locations, and science goals with the upcoming NASA New Frontiers Dragonfly mission that will provide in situ exploration of Titan’s equatorial regions, in the mid-2030s.

The formation of hazes at microbar pressures has been explored by theoretical models of exoplanet atmospheres to explain Rayleigh scattering and/or featureless transmission spectra; however observational evidence of aerosols in the low-pressure formation environments has proved elusive. Here, we show direct evidence of aerosols existing at ∼1 microbar pressures in the atmosphere of the warm sub-Saturn WASP-69b using observations taken with the Space Telescope Imaging Spectrograph and Wide Field Camera 3 instruments on the Hubble Space Telescope. The transmission spectrum shows a wavelength-dependent slope induced by aerosol scattering that covers 11 scale heights of spectral modulation. Drawing on the extensive studies of haze in our solar system, we model the transmission spectrum based on a scaled version of Jupiter’s haze-density profile to show that the WASP-69b transmission spectrum can be produced by scattering from an approximately constant density of particles extending throughout the atmospheric column from 40 millibar to microbar pressures. These results are consistent with theoretical expectations based on microphysics of the aerosol particles that have suggested haze can exist at microbar pressures in exoplanet atmospheres.

Radiant energies of planets and moons are of wide interest in the fields of geoscience and planetary science. Based on long‐term multiinstrument observations from the Cassini spacecraft, we provide here the first observational study of Titan's global radiant energy budget and its seasonal variations. Our results show that Titan's radiant energy budget is not balanced over the Cassini era (2004–2017) with the absorbed solar energy (1.208 ± 0.008) × 10 23 J larger than the emitted thermal energy (1.174 ± 0.005) × 10 23 J. The energy imbalance is 2.9 ± 0.8% of the emitted thermal energy. Titan's global radiant energy budget is not balanced either at the timescales of Earth's years and Titan's seasons. In particular, the energy imbalance can be beyond 10% of the emitted thermal energy at the timescale of an Earth year. The energy imbalance revealed in this study has important impacts on Titan, which should be examined further by theories and models.

Missions like the upcoming Roman Space Telescope and its follow-on missions, Habitable Exoplanet Observatory (HabEx) and the Large UV/Optical/IR Surveyor (LUVOIR), will provide direct imaging observations of stellar light reflected by exoplanets with successively closer orbits. The synergistic use of ground-based polarimeters like Gemini Planet Imager and Very Large Telescope/Spectro-Polarimetric High-contrast Exoplanet Research instrument (SPHERE) would allow us to characterize cloudy exoplanet atmospheres using spectropolarimetric direct imaging. We present an extension of our semianalytic 3D radiative transfer modeling framework for brown dwarfs to include stellar light reflected by exoplanets with cloudy atmospheres. Using Mie theory to compute scattering by cloud and haze consisting of spherical particles, we show that the currently widespread use of approximations like the scalar Two-Term Henyey–Greenstein or the vector Henyey–Greenstein Rayleigh (HGR) composite result in a blurring of the phase-dependent features of exoplanet lightcurves, causing a 10%–39% loss of sensitivity to atmospheric parameters in an average measurement for signal-to-noise ratios (S/Ns) between 5 and 500. The HGR approximation creates the misleading impression that clouds are as polarizing as Rayleigh scatterers, regardless of their droplet size. This not only causes significant errors in the scientific interpretation of polarimetric measurements, but also results in a negligible sensitivity of HGR simulations to polarization measurements at the S/Ns considered, whereas Mie simulations show a 10%–30% gain in parametric sensitivity through the addition of polarimetry.

This study presents a 13 yr survey of haze UV extinction profiles, monitoring the temporal evolution of the detached haze layer (DHL) in Titan's upper atmosphere (350-600 km). As reported by West et al. (GRL vol.38, L06204) at the equator, we show that the DHL was present at all latitudes below 55 degrees N during the northern winter (2004-2009). Subsequently, it globally sunk and disappeared in 2012. No permanent DHL was observed between 2012 and 2015. Only in late 2015 did a new structure emerge from the Northern hemisphere, and propagate to the equator. This new DHL is not as pronounced as that observed in 2004, and is much more complex. In one specific sequence, in 2005, we were able to investigate the short timescale variability of the DHL, and no major changes were observed. When both sides of the limb were visible (dawn/dusk), we notice that the extinction of the DHL is slightly higher on the dawn side. Moreover, during a polar flyby in 2009, we observed the longitudinal variability of the DHL and spotted some local inhomogeneities. Finally, comparisons with UVIS stellar occultations and General Climate Models are both consistent with our findings. However, we noticed that the timing of the DHL main pattern predicted by the GMCs can be off by up to 30 degrees in solar longitude. All these observations offer new perspectives on the seasonal cycle of Titan's upper atmosphere, the evolution of the DHL and its interaction with the dynamics.

We report the detection of an atmosphere on a rocky exoplanet, GJ 1132 b, which is similar to Earth in terms of size and density. The atmospheric transmission spectrum was detected using Hubble WFC3 measurements and shows spectral signatures of aerosol scattering, HCN, and CH$_{4}$ in a low mean molecular weight atmosphere. We model the atmospheric loss process and conclude that GJ 1132 b likely lost the original H/He envelope, suggesting that the atmosphere that we detect has been reestablished. We explore the possibility of H$_{2}$ mantle degassing, previously identified as a possibility for this planet by theoretical studies, and find that outgassing from ultrareduced magma could produce the observed atmosphere. In this way we use the observed exoplanet transmission spectrum to gain insights into magma composition for a terrestrial planet. The detection of an atmosphere on this rocky planet raises the possibility that the numerous powerfully irradiated Super-Earth planets, believed to be the evaporated cores of Sub-Neptunes, may, under favorable circumstances, host detectable atmospheres.

Knowledge of the radiant energy budgets and internal heat of planets and moons is of wide interest in the planetary science community. Some progress has been achieved with recent studies, but there are still significant limitations in current observations and studies. We recommend future exploration to better understand the radiant energy budgets and internal heat of planets and moons in our solar system. 1. Big picture and significance As a fundamental parameter of planets and moons, the radiant energy budget is determined by the absorbed solar energy and the emitted thermal energy (1, 2). Such an energy budget plays an important role in determining the thermal structures of planets and moons (3-6). It can help us understand the geology (e.g., polar ices of Mars) (7), internal heat related to the formation and evolution of giant planets [8-10], and sub-surface internal heat driving the jet plumes on some moons (11-15). For bodies with atmospheres, the radiant energy budgets at the top of atmospheres also set critical boundary conditions for the atmospheric systems (3). The transfer and distribution of radiant energies within the atmospheric systems modify the thermal structure to generate available potential energy. The available potential energy can be converted into kinetic energy to drive atmospheric circulation and the related weather and climate (3-6, 16, 17). Unfortunately, the global radiant energy budget has not been well determined for most of planets and moons (4-6) in our solar system, mainly because the observations of the radiant energies are limited (1, 2, 18, 19). With the advance of space exploration, we expect to get a much better picture of the global radiant energy budget and internal heat for the planets and moons in our solar system. 2. Methodology and observations To determine the radiant energy budgets and hence the internal heat of planets and moons, we have to measure two radiant energies – the absorbed solar energy and the emitted thermal energy. The emitted thermal energy of planets and moons in our solar system is concentrated in the infrared wavelengths, which can be measured by an instrument in the wavelength range 5-400 microns. On the other hand, the solar energy from the Sun is mainly concentrated in the ultraviolet, visible and near-infrared wavelengths (0-5 microns). Generally, we measure the reflected solar energy and then compute the absorbed solar energy. The precise measurements of the emitted thermal energy and the reflected solar energy require accurate observations with complete coverage of wavelength and viewing angles (e.g., emission angle and phase angle). The basic methodology of computing the radiant energies is to integrate the radiance over wavelength and viewing angle, which is described in detail in our previous studies (18, 19). The difference between the emitted thermal energy and the absorbed solar energy is generally used to estimate the internal heat of planets and moons

There is an opportunity to advance both solar system and extrasolar planetary studies that does not require the construction of new telescopes or new missions but better use and access to inter-disciplinary data sets. This approach leverages significant investment from NASA and international space agencies in exploring this solar system and using those discoveries as ground truth for the study of extrasolar planets. This white paper illustrates the potential, using phase curves and atmospheric modeling as specific examples. A key advance required to realize this potential is to enable seamless discovery and access within and between planetary science and astronomical data sets. Further, seamless data discovery and access also expands the availability of science, allowing researchers and students at a variety of institutions, equipped only with Internet access and a decent computer to conduct cutting-edge research.

Literature on the theory of exoplanet atmospheric disequilibrium chemistry is rich, although its observational counterpart has yet to emerge beyond the hints provided by a few targets in dedicated studies. We report results from an uniform data reduction and analysis for a catalog of 62 Hubble Space Telescope exoplanet transit spectra where we assess the atmospheric model preference for disequilibrium chemistry (i.e., water vapor is not the dominant absorption spectral signature) over thermal equilibrium chemistry in a comparative planetology context. Where model preference assessment is possible, we find that disequilibrium occurs in about half of the atmospheres, indicating that disequilibrium processes play an important role in the composition of exoplanet atmospheres. While very hot atmospheres, over 1800 K, prefer equilibrium chemistry, we find a clustering of preference for disequilibrium in the 1200–1800 K temperature range. We suggest that UV-augmented thermochemistry may play a significant role for those atmospheres.