Keywords

The purpose of this paper is to address the question: Using our knowledge of infrared planetary spectroscopy, what can we learn about the atmospheres of exoplanets? In a first part, a simplified classification of exoplanets, assuming thermochemical equilibrium, is presented, based on their masses and their equilibrium temperatures, in order to propose some possible estimations about their atmospheric composition. In the second part, infrared spectra of planets are discussed, in order to see what lessons can be drawn for exoplanetary spectroscopy. In the last part, we consider the solar system as it would appear from a star located in the ecliptic plane. It first appears that the solar system (except in a few specific cases) would not be seen as a multiple system, because, contrary to many exoplanetary systems, the planets are too far from the Sun and the inclinations of their orbits with respect to the ecliptic plane are too high. Primary transit synthetic spectra of solar system planets are used to discuss the relative merits of transmission and direct emission spectroscopy for probing exoplanetary atmospheres.

The Mare Moscoviense is an astonishing rare flatland multi-ring basin and one of the recognizable mare regions on the Moon's farside. The mineralogical, chronological, topographical and morphological studies of the maria surface of the Moon provide a primary understanding of the origin and evolution of the mare provinces. In this study, the Chandrayaan-1 M³ data have been employed to prepare optical maturity index, FeO and TiO² concentration, and standard band ratio map to detect the mafic indexes like olivine and pyroxene minerals. The crater size frequency distribution method has been applied to LROC WAC data to obtain the absolute model ages of the Moscoviense basin. The four geological unit ages were observed as 3.57 Ga (U-2), 3.65 Ga (U-1), 3.8 Ga (U-3) and 3.92 Ga (U-4), which could have been formed between the Imbrian and Nectarian epochs. The M³ imaging and reflectance spectral parameters were used to reveal the minerals like pyroxene, olivine, ilmenite, plagioclase, orthopyroxene-olivine-spinel lithology, and olivine-pyroxene mixtures present in the gabbroic basalt, anorthositic and massive ilmenite rocks, and validated with the existing database. The results show that the Moscoviense basin is dominated by intermediate TiO² basalts that derived from olivine-ilmenite-pyroxene cumulate depths ranging from 200 to 500 km between 3.5 Ga and 3.6 Ga.

Transmission spectrum surveys have suggested the ubiquity of high-altitude clouds in exoplanetary atmospheres. Theoretical studies have investigated the formation processes of the high-altitude clouds; however, cloud particles have been commonly approximated as compact spheres, which is not always true for solid mineral particles that likely constitute exoplanetary clouds. Here, we investigate how the porosity of cloud particles evolves in exoplanetary atmospheres and influences the clouds' vertical profiles. We first construct a porosity evolution model that takes into account the fractal aggregation and the compression of cloud particle aggregates. Using a cloud microphysical model coupled with the porosity model, we demonstrate that the particle internal density can significantly decrease during the cloud formation. As a result, fluffy-aggregate clouds ascend to an altitude much higher than that assumed for compact-sphere clouds thus far. We also examine how the fluffy-aggregate clouds affect transmission spectra. We find that the clouds largely obscure the molecular features and produce a spectral slope originated by the scattering properties of aggregates. Finally, we compare the synthetic spectra with the observations of GJ1214 b and find that its flat spectrum could be explained if the atmospheric metallicity is sufficiently high (>100× solar) and the monomer size is sufficiently small (r_mon < 1 μm). The high-metallicity atmosphere may offer the clues to explore the past formation process of GJ1214b.

Transit spectroscopy of terrestrial planets around nearby M dwarfs will be a primary goal of space missions in coming decades. Three-dimensional climate modeling has shown that slow-synchronous rotating terrestrial planets may develop thick clouds at the substellar point, increasing the albedo. For M dwarfs with T_eff > 3000 K, such planets at the inner habitable zone (IHZ) have been shown to retain moist greenhouse conditions, with enhanced stratospheric water vapor (fH₂O > 10⁻³) and low Earth-like surface temperatures. However, M dwarfs also possess strong UV activity, which may effectively photolyze stratospheric H₂O. Prior modeling efforts have not included the impact of high stellar UV activity on the H₂O. Here, we employ a 1D photochemical model with varied stellar UV, to assess whether H₂O destruction driven by high stellar UV would affect its detectability in transmission spectroscopy. Temperature and water vapor profiles are taken from published 3D climate model simulations for an IHZ Earth-sized planet around a 3300 K M dwarf with an N₂–H₂O atmosphere; they serve as self-consistent input profiles for the 1D model. We explore additional chemical complexity within the 1D model by introducing other species into the atmosphere. We find that as long as the atmosphere is well-mixed up to 1 mbar, UV activity appears to not impact detectability of H₂O in the transmission spectrum. The strongest H₂O features occur in the James Webb Space Telescope MIRI instrument wavelength range and are comparable to the estimated systematic noise floor of ∼50 ppm.

Extrasolar satellites are generally too small to be detected by nominal searches. By analogy to the most active body in the solar system, Io, we describe how sodium (Na i) and potassium (K i) gas could be a signature of the geological activity venting from an otherwise hidden exo-Io. Analyzing ∼a dozen close-in gas giants hosting robust alkaline detections, we show that an Io-sized satellite can be stable against orbital decay below a planetary tidal ${{ \mathcal Q }}_{p}\lesssim {10}^{11}$. This tidal energy is also focused into the satellite driving an ∼10^5±2 higher mass-loss rate than Io's supply to Jupiter's Na exosphere based on simple atmospheric loss estimates. The remarkable consequence is that several exo-Io column densities are, on average, more than sufficient to provide the ∼10^10±1 Na cm⁻² required by the equivalent width of exoplanet transmission spectra. Furthermore, the benchmark observations of both Jupiter's extended (∼1000 R_J) Na exosphere and Jupiter's atmosphere in transmission spectroscopy yield similar Na column densities that are purely exogenic in nature. As a proof of concept, we fit the "high-altitude" Na at WASP-49b with an ionization-limited cloud similar to the observed Na profile about Io. Moving forward, we strongly encourage time-dependent ingress and egress monitoring along with spectroscopic searches for other volcanic volatiles.

We use a thermodynamic statistical model to evaluate how the composition of Europa's internal ocean may have been affected by clathrate hydrate formation. Assuming an input of the observed O₂ and CO₂ from the surface into a mildly acidic ocean (pH < 6), and considering the possibility of contributions by reduced (with CH₄ and H₂S) or oxidized (CO₂-bearing) hydrothermal fluids, we calculate the fractional occupancies in clathrate and deduce the effect on the ocean's composition. The structure of the clathrate formed, and therefore its density and composition is influenced by the relative amount of O₂ compared to the other compounds present. We also include a mixture of noble gases—argon, krypton, and xenon—based on cometary abundances measured at comet 67P and find that the Ar/Kr ratio can be affected by almost two orders of magnitude. In most cases, the formed clathrate is likely to become part of the icy crust, with guest molecules possibly accessible to future in situ measurements by the Europa Clipper and JUICE missions.

Almost all planetary atmospheres are affected by disequilibrium chemical processes. In this paper, we introduce our recently developed chemical kinetic model (ChemKM). We show that the results of our HD 189733b model are in good agreement with previously published results, except at the μbar regime, where molecular diffusion and photochemistry are the dominant processes. We thus recommend careful consideration of these processes when abundances at the top of the atmosphere are desired. We also propose a new metric for a quantitative measure of quenching levels. By applying this metric, we find that quenching pressure decreases with the effective temperature of planets, but it also varies significantly with other atmospheric parameters such as [Fe/H], log(g), and C/O. In addition, we find that the "methane valley," a region between 800 and 1500 K where above a certain C/O threshold value a greater chance of CH₄ detection is expected, still exists after including the vertical mixing. The first robust CH₄ detection on an irradiated planet (HD 102195b) places this object within this region, supporting our prediction. We also investigate the detectability of disequilibrium spectral fingerprints by the James Webb Space Telescope and suggest focusing on the targets with T_eff between 1000 and 1800 K, orbiting around M dwarfs, and having low surface gravity but high metallicity and a C/O ratio value around unity. Finally, constructing Spitzer color maps suggests that the main two color populations are largely insensitive to the vertical mixing. Therefore, any deviation of observational points from these populations is likely due to the presence of clouds and not disequilibrium processes. However, some cold planets (T_eff < 900 K) with very low C/O ratios (<0.25) show significant deviations, making these planets interesting cases for further investigation.

Although several thousands of exoplanets have now been detected and characterized, observational biases have led to a paucity of long-period, low-mass exoplanets with measured masses and a corresponding lag in our understanding of such planets. In this paper we report the mass estimation and characterization of the long-period exoplanet Kepler-538b. This planet orbits a Sun-like star (V = 11.27) with ${M}_{* }={0.892}_{-0.035}^{+0.051}$ M_⊙ and ${R}_{* }\,={0.8717}_{-0.0061}^{+0.0064}$ R_⊙. Kepler-538b is a ${2.215}_{-0.034}^{+0.040}$ R_⊕ sub-Neptune with a period of P = 81.73778 ± 0.00013 days. It is the only known planet in the system. We collected radial velocity (RV) observations with the High Resolution Echelle Spectrometer (HIRES) on Keck I and High Accuracy Radial velocity Planet Searcher in North hemisphere (HARPS-N) on the Telescopio Nazionale Galileo (TNG). We characterized stellar activity by a Gaussian process with a quasi-periodic kernel applied to our RV and cross-correlation function FWHM observations. By simultaneously modeling Kepler photometry, RV, and FWHM observations, we found a semi-amplitude of $K={1.68}_{-0.38}^{+0.39}$ m s⁻¹ and a planet mass of ${M}_{p}={10.6}_{-2.4}^{+2.5}$ M_⊕. Kepler-538b is the smallest planet beyond P = 50 days with an RV mass measurement. The planet likely consists of a significant fraction of ices (dominated by water ice), in addition to rocks/metals, and a small amount of gas. Sophisticated modeling techniques such as those used in this paper, combined with future spectrographs with ultra high-precision and stability will be vital for yielding more mass measurements in this poorly understood exoplanet regime. This in turn will improve our understanding of the relationship between planet composition and insolation flux and how the rocky to gaseous transition depends on planetary equilibrium temperature.

We report results from a new technique for mapping Io's SO₂ vapor distribution. The Space Telescope Imaging Spectrograph (STIS) instrument on the Hubble Space Telescope observed Io during four Jupiter transit events to obtain medium resolution far-UV spectral images near the Lyα wavelength of 121.6 nm. Jupiter's bright Lyα dayglow provides a bright, mostly uniform background light source for opacity measurements, much like during a stellar occultation or transiting exoplanet event. Peaks in the photoabsorption cross-sections for sulfur dioxide occur near 122 nm, with resulting absorptions raising the altitude where a tangential line-of-sight opacity of ∼1 occurs. This method of measuring column densities along lines of sight above the limb uses detailed image simulations and complements Lyα reflectance imaging and other methods for measuring Io's SO₂ gas on the disk. Our reported near-terminator limb observations with STIS confirm the findings from previous Lyα disk reflectance imaging that Io's polar SO₂ density is an order of magnitude lower than found at the equator. We provide constraints for additional attenuation by atmospheric hydrogen atoms produced by charge exchange reactions between magnetospheric protons and Io's atmosphere. Searches for plume-related features provided no definitive enhancements within the signal quality, ruling out unusually high levels of activity for Pele and Tvashtar.

Terrestrial planets have been found orbiting Sun-like stars with extremely short periods—some as short as 4 hr. These "ultra-short-period planets" or "hot Earths" are so strongly irradiated that any initial H/He atmosphere has probably been lost to photoevaporation. As such, the sample of hot Earths may give us a glimpse at the rocky cores that are often enshrouded by thick H/He envelopes on wider-orbiting planets. However, the mass and radius measurements of hot Earths have been derived from a hodgepodge of different modeling approaches, and include several cases of contradictory results. Here, we perform a homogeneous analysis of the complete sample of 11 known hot Earths with an insolation exceeding 650 times that of the Earth. We combine all available data for each planet, incorporate parallax information from Gaia to improve the stellar and planetary parameters, and use Gaussian process regression to account for correlated noise in the radial-velocity data. The homogeneous analysis leads to a smaller dispersion in the apparent composition of hot Earths, although there does still appear to be some intrinsic dispersion. Most of the planets are consistent with an Earth-like composition (35% iron and 65% rock), but two planets (K2-141b and K2-229b) show evidence for a higher iron fraction, and one planet (55 Cnc e) has either a very low iron fraction or an envelope of low-density volatiles. All of the planets are less massive than 8 M_⊕, despite the selection bias toward more massive planets, suggesting that 8 M_⊕ is the critical mass for runaway accretion.