Some close-in gaseous exoplanets are nearly in Roche lobe contact, and previous studies show that tidal decay can drive hot Jupiters into contact during the main sequence of their host stars. Improving on a previous model, we present a revised model for mass transfer in a semidetached binary system that incorporates an extended atmosphere around the donor and allows for an arbitrary mass ratio. We apply this new formalism to hypothetical, confirmed, and candidate planetary systems to estimate mass-loss rates and compare with models of evaporative mass loss. Overflow may be significant for hot Neptunes out to periods of similar to 2 days, while for hot Jupiters, it may only be important inward of 0.5 days. We find that CoRoT-24 b may be losing mass at a rate of more than an Earth mass in a gigayear. The hot Jupiter WASP-12 b may lose an Earth mass in a megayear, while the putative planet PTFO8-8695 orbiting a T Tauri star might shed its atmosphere in a few megayears. We point out that the orbital expansion that can accompany mass transfer may be less effective than previously considered because the gas accreted by the host star removes some of the angular momentum from the orbit, but simple scaling arguments suggest that the Roche lobe overflow might remain stable. Consequently, the recently discovered small planets in ultrashort periods (< 1 day) may not be the remnants of hot Jupiters/Neptunes. The new model presented here has been incorporated into Modules for Experiments in Stellar Astrophysics (MESA).

Context. Radial velocity and transit methods are effective for the study of short orbital period exoplanets but they hardly probe objects at large separations for which direct imaging can be used. Aims. We carried out the international deep planet survey of 292 young nearby stars to search for giant exoplanets and determine their frequency. Methods. We developed a pipeline for a uniform processing of all the data that we have recorded with NIRC2/Keck II, NIRI/Gemini North, NICI/Gemini South, and NACO/VLT for 14 yr. The pipeline first applies cosmetic corrections and then reduces the speckle intensity to enhance the contrast in the images. Results. The main result of the international deep planet survey is the discovery of the HR8799 exoplanets. We also detected 59 visual multiple systems including 16 new binary stars and 2 new triple stellar systems, as well as 2279 point-like sources. We used Monte Carlo simulations and the Bayesian theorem to determine that 1.05(-0.70)(+2.80)% of stars harbor at least one giant planet between 0.5 and 14 MJ and between 20 and 300AU. This result is obtained assuming uniform distributions of planet masses and semi-major axes. If we consider power law distributions as measured for close-in planets instead, the derived frequency is 2.30(-1.55)(+5.95)%, recalling the strong impact of assumptions on Monte Carlo output distributions. We also find no evidence that the derived frequency depends on the mass of the hosting star, whereas it does for close-in planets. Conclusions. The international deep planet survey provides a database of confirmed background sources that may be useful for other exoplanet direct imaging surveys. It also puts new constraints on the number of stars with at least one giant planet reducing by a factor of two the frequencies derived by almost all previous works.

Hot Jupiters receive strong stellar irradiation, producing equilibrium temperatures of 1000-2500 K. Incoming irradiation directly heats just their thin outer layer, down to pressures of similar to 0.1 bars. In standard irradiated evolution models of hot Jupiters, predicted transit radii are too small. Previous studies have shown that deeper heating-at a small fraction of the heating rate from irradiation-can explain observed radii. Here we present a suite of evolution models for HD 209458b, where we systematically vary both the depth and intensity of internal heating, without specifying the uncertain heating mechanism(s). Our models start with a hot, high-entropy planet whose radius decreases as the convective interior cools. The applied heating suppresses this cooling. We find that very shallow heating-at pressures of 1-10 bars-does not significantly suppress cooling, unless the total heating rate is greater than or similar to 10% of the incident stellar power. Deeper heating, at 100 bars, requires heating at only 1% of the stellar irradiation to explain the observed transit radius of 1.4R(Jup) after 5 Gyr of cooling. In general, more intense and deeper heating results in larger hot-Jupiter radii. Surprisingly, we find that heat deposited at 10(4) bars-which is exterior to approximate to 99% of the planet's mass-suppresses planetary cooling as effectively as heating at the center. In summary, we find that relatively shallow heating is required to explain the radii of most hot Jupiters, provided that this heat is applied early and persists throughout their evolution.

We present spectrophotometry spanning 1-5 mu m of 51 Eridani b, a 2-10 M-Jup planet discovered by the Gemini Planet Imager Exoplanet Survey. In this study, we present new K1 (1.90-2.19 mu m) and K2 (2.10-2.40 mu m) spectra taken with the Gemini Planet Imager as well as an updated L-P (3.76 mu m) and new M-S (4.67 mu m) photometry from the NIRC2 Narrow camera. The new data were combined with J (1.13-1.35 mu m) and H (1.50-1.80 mu m) spectra from the discovery epoch with the goal of better characterizing the planet properties. The 51 Eri b photometry is redder than field brown dwarfs as well as known young T-dwarfs with similar spectral type (between T4 and T8), and we propose that 51 Eri b might be in the process of undergoing the transition from L-type to T-type. We used two complementary atmosphere model grids including either deep iron/silicate clouds or sulfide/salt clouds in the photosphere, spanning a range of cloud properties, including fully cloudy, cloud-free, and patchy/intermediate-opacity clouds. The model fits suggest that 51 Eri. b has an effective temperature ranging between 605 and 737 K, a solar metallicity, and a surface gravity of log(g) = 3.5-4.0 dex, and the atmosphere requires a patchy cloud atmosphere to model the spectral energy distribution (SED). From the model atmospheres, we infer a luminosity for the planet of -5.83 to -5.93 (logL/L circle dot),leaving 51 Eri b in the unique position of being one of the only directly imaged planets consistent with having formed via a cold-start scenario. Comparisons of the planet SED against warm-start models indicate that the planet luminosity is best reproduced by a planet formed via core accretion with a core mass between 15 and 127 M-circle plus.

We use a model of aerosol microphysics to investigate the impact of high-altitude photochemical aerosols on the transmission spectra and atmospheric properties of close-in exoplanets, such as HD 209458 b and HD 189733 b. The results depend strongly on the temperature profiles in the middle and upper atmospheres, which are poorly understood. Nevertheless, our model of HD 189733 b, based on the most recently inferred temperature profiles, produces an aerosol distribution that matches the observed transmission spectrum. We argue that the hotter temperature of HD 209458 b inhibits the production of high-altitude aerosols and leads to the appearance of a clearer atmosphere than on HD 189733 b. The aerosol distribution also depends on the particle composition, photochemical production, and atmospheric mixing. Due to degeneracies among these inputs, current data cannot constrain the aerosol properties in detail. Instead, our work highlights the role of different factors in controlling the aerosol distribution that will prove useful in understanding different observations, including those from future missions. For the atmospheric mixing efficiency suggested by general circulation models, we find that the aerosol particles are small (similar to nm) and probably spherical. We further conclude that a composition based on complex hydrocarbons (soots) is the most likely candidate to survive the high temperatures in hot-Jupiter atmospheres. Such particles would have a significant impact on the energy balance of HD 189733 b's atmosphere and should be incorporated in future studies of atmospheric structure. We also evaluate the contribution of external sources to photochemical aerosol formation and find that their spectral signature is not consistent with observations.

The past decade has seen significant progress on the direct detection and characterization of young, self-luminous giant planets at wide orbital separations from their host stars. Some of these planets show evidence for disequilibrium processes like transport-induced quenching in their atmospheres; photochemistry may also be important, despite the large orbital distances. These disequilibrium chemical processes can alter the expected composition, spectral behavior, thermal structure, and cooling history of the planets, and can potentially confuse determinations of bulk elemental ratios, which provide important insights into planet-formation mechanisms. Using a thermo/photochemical kinetics and transport model, we investigate the extent to which disequilibrium chemistry affects the composition and spectra of directly imaged giant exoplanets. Results for specific "young Jupiters" such as HR 8799 b and 51 Eri b are presented, as are general trends as a function of planetary effective temperature, surface gravity, incident ultraviolet flux, and strength of deep atmospheric convection. We find that quenching is very important on young Jupiters, leading to CO/CH4 and N-2/NH3 ratios much greater than, and H2O mixing ratios a factor of a few less than, chemical-equilibrium predictions. Photochemistry can also be important on such planets, with CO2 and HCN being key photochemical products. Carbon dioxide becomes a major constituent when stratospheric temperatures are low and recycling of water via the H-2 + OH reaction becomes kinetically stifled. Young Jupiters with effective temperatures less than or similar to 700 K are in a particularly interesting photochemical regime that differs from both transiting hot Jupiters and our own solar-system giant planets.

Avec la découverte de milliers de nouvelles planètes au cours des vingt dernières années, une nouvelle population complexe de planètes plus petites que Neptune et plus grandes que la Terre a été découverte. Ces planètes se divisent en deux groupes : les plus grandes sub-Neptunes avec des atmosphères étendues dominées par H, et les plus petites super-Terres qui ont tout au plus des atmosphères minces. Cette division peut être expliquée par une variété de mécanismes, y compris la photoévaporation, la perte de masse alimentée par le noyau, et la formation de gaz pauvres et vides : la population de petites planètes est probablement façonnée par une combinaison de ces mécanismes qui peut dépendre du type stellaire. Dans ce travail, nous décrivons la découverte de deux nouvelles planètes qui sont bien adaptées à l'étude de la nature de la population des petites planètes : Wolf 503 b et LP 791-18 d. Wolf 503 b est une planète de \(2.03^{+0.08}_{-0.07} R_{\oplus}\) orbitant autour de l'étoile brillante (\(J=8.32\) mag), proche (\(D=44.5\) pc) à mouvement propre élevé K3.5V Wolf 503 (EPIC 212779563). Nous confirmons que la signature du transit K2 est planétaire en utilisant à la fois des images d'archives et des images d'optique adaptative à haut contraste de l'observatoire Palomar. Son rayon place Wolf 503b directement entre les populations de super-Terre et de sub-Neptune, un rayon auquel les planètes sont rarement trouvées et la composition de masse attendue est ambiguë, et la luminosité de l'étoile hôte fait de Wolf 503b une cible de choix pour le suivi des vitesses radiales et la spectroscopie de transit. La deuxième planète que nous présentons est une planète de taille terrestre orbitant autour de la naine froide M6 LP 791-18. La nouvelle planète d rejoint un système bien aligné avec au moins deux autres planètes, la plus externe étant une sous-Neptune, offrant une occasion unique à ce jour d'étudier un système avec une planète de taille terrestre tempérée et une sous-Neptune qui a conservé son enveloppe gazeuse ou volatile. La découverte de LP 791-18d permet de mesurer la masse du système grâce aux variations du temps de transit, et nous trouvons une masse de \( {9.3_{-1.4}^{+1.5}\,M_\oplus}\) pour la sub-Neptune LP 791-18c et une masse de \( {0.8_{-0.4}^{+0.5}\,M_\oplus}\) pour l'exo-Terre LP 791-18d (\({<2.3 M_{\oplus}}\) à 3\( {\sigma}\)). La planète est également soumise à un fort réchauffement continu par les marées, ce qui peut entraîner une activité géologique et un dégazage volcanique. Pour l'avenir, LP 791-18d et Wolf 503b offrent des opportunités uniques d'étudier les origines et la conservation des atmosphères des petites planètes. / With the discovery of thousands of new planets in the past twenty years, a new and complex population of planets has been discovered which are smaller than Neptune and larger than the Earth. These planets are split into two groups: the larger sub-Neptunes with extended H-dominated atmospheres, and the smaller super-Earths which have at most thin atmospheres. This division can be explained by a variety of mechanisms, including photoevaporation, core-powered mass-loss, and gas-poor and gas-empty formation: the small-planet population is likely shaped by a combination of these which may depend on stellar type. In this work we describe the discovery of two new planets which are well-suited to investigating the nature of the small planet population: Wolf 503b and LP 791-18d. Wolf 503 b is a \(2.03^{+0.08}_{-0.07} R_{\oplus}\) planet orbiting the bright (\(J=8.32\) mag), nearby (\(D=44.5\) pc) high proper motion K3.5V star Wolf 503 (EPIC 212779563). We confirm that the K2 transit signature is planetary using both archival images and high-contrast adaptive optics images from the Palomar observatory. Its radius places Wolf 503 b directly between the populations of super-Earths and sub-Neptunes, a radius at which planets are rarely found and the expected bulk composition is ambiguous, and the brightness of the host star makes Wolf 503b a prime target for radial velocity follow-up and transit spectroscopy. The second planet we introduce is an Earth-sized planet orbiting the cool M6 dwarf LP 791-18. The new planet d joins a well-aligned system with at least two more planets, the outermost being a sub-Neptune, providing a to-date unique opportunity to investigate a system with a temperate Earth-sized planet and a sub-Neptune that retained its gas or volatile envelope. The discovery of LP 791-18d makes the system amenable to mass measurements via transit timing variations, and we find a mass of \( {9.3_{-1.4}^{+1.5}\,M_\oplus}\) for the sub-Neptune LP 791-18c and a mass of \( {0.8_{-0.4}^{+0.5}\,M_\oplus}\) for the exo-Earth LP 791-18d (\( {<2.3 M_{\oplus}}\) at 3\( {\sigma}\)). The planet is also subject to strong continued tidal heating, which may result in geological activity and volcanic outgassing. Looking forward, LP 791-18d and Wolf 503b offer unique opportunities to study the origins and retention of small-planet atmospheres.