Photoionization heating from UV radiation incident on the atmospheres of hot Jupiters may drive planetary mass loss. Observations of stellar Lyman-α absorption have suggested that the hot Jupiter HD 209458b is losing atomic hydrogen. We construct a model of escape that includes realistic heating and cooling, ionization balance, tidal gravity, and pressure confinement by the host star wind. We show that mass loss takes the form of a hydrodynamic ("Parker") wind, emitted from the planet's dayside during lulls in the stellar wind. When dayside winds are suppressed by the confining action of the stellar wind, nightside winds might pick up if there is sufficient horizontal transport of heat. A hot Jupiter loses mass at maximum rates of ∼2 × 10 12 g s −1 during its host star's pre-main-sequence phase and ∼2 × 10 10 g s −1 during the star's main sequence lifetime, for total maximum losses of ∼0.06% and ∼0.6% of the planet's mass, respectively. For UV fluxes F UV 10 4 erg cm −2 s −1 , the mass loss rate is approximately energy-limited and scales asṀ ∝ F 0.9 UV . For larger UV fluxes, such as those typical of T Tauri stars, radiative losses and plasma recombination forceṀ to increase more slowly as F 0.6 UV . Dayside winds are quenched during the T Tauri phase because of confinement by overwhelming stellar wind pressure. During this early stage, nightside winds can still blow if the planet resides outside the stellar Alfvén radius; otherwise, even nightside winds are stifled by stellar magnetic pressure, and mass loss is restricted to polar regions. We conclude that while UV radiation can indeed drive winds from hot Jupiters, such winds cannot significantly alter planetary masses during any evolutionary stage. They can, however, produce observable signatures. Candidates for explaining why the Lyman-α photons of HD 209458 are absorbed at Doppler-shifted velocities of ±100 km/s include charge-exchange in the shock between the planetary and stellar winds.

times Earth's radius (R ⊕ ), indicating that it is intermediate in stature betweenEarth and the ice giants of the Solar System. We find that the planetary mass and radius are consistent with a composition of primarily water enshrouded by a hydrogen-helium envelope that is only 0.05% of the mass of the planet. The atmosphere is probably escaping hydrodynamically, indicating that it has undergone significant evolution during its history.As the star is small and only 13 parsecs away, the planetary atmosphere is amenable to study with current observatories.The recently commissioned MEarth Project 10,11 uses an array of eight identical 40-cm automated telescopes to photometrically monitor 2,000 nearby M dwarfs with masses between

The C/O ratio is predicted to regulate the atmospheric chemistry in hot Jupiters. Recent observations suggest that some exo-planets, e.g. Wasp 12b, have atmospheric C/O ratios substantially different from the solar value of 0.54. In this paper we present a mechanism that can produce such atmospheric deviations from the stellar C/O ratio. In protoplanetary disks, different snowlines of oxygen-and carbon-rich ices, especially water and carbon monoxide, will result in systematic variations in the C/O ratio both in the gas and in the condensed phase. In particular, between the H 2 O and CO snowlines most oxygen is present in icy grains -the building blocks of planetary cores in the core accretion modelwhile most carbon remains in the gas-phase. This region is coincidental with the giant-planet forming zone for a range of observed protoplanetary disks. Based on standard core accretion models of planet formation, gas giants that sweep up most of their atmospheres from disk gas outside of the water snowline will have C/O∼1, while atmospheres significantly contaminated by evaporating planetesimals will have stellar or sub-stellar C/O when formed at the same disk radius. The overall metallicity will also depend on the atmosphere formation mechanism, and exoplanetary atmospheric compositions may therefore provide constraints on where and how a specific planet formed.Subject headings: astrochemistry -circumstellar matter -planetary systems -molecular processes -planets and satellites: atmospheres -planet-disk interactions 1 Hubble Fellow

Exoplanet discoveries of recent years have provided a great deal of new data for studying the bulk compositions of giant planets. Here we identify 47 transiting giant planets (20M ⊕ < M < 20M J ) whose stellar insolation is low enough (F * < 2 × 10 8 erg s −1 cm −2 , or roughly T eff < 1000) that they are not affected by the hot Jupiter radius inflation mechanism(s). We compute a set of new thermal and structural evolution models and use these models in comparison with properties of the 47 transiting planets (mass, radius, age) to determine their heavy element masses. A clear correlation emerges between the planetary heavy element mass M z and the total planet mass, approximately of the form M z ∝ √ M . This finding is consistent with the core accretion model of planet formation. We also study how stellar metallicity [Fe/H] affects planetary metal-enrichment and find a weaker correlation than has been previously reported from studies with smaller sample sizes. We confirm a strong relationship between the planetary metal-enrichment relative to the parent star Z planet /Z star and the planetary mass, but see no relation in Z planet /Z star with planet orbital properties or stellar mass. The large heavy element masses of many planets (> 50 M ⊕ ) suggest significant amounts of heavy elements in H/He envelopes, rather than cores, such that metal-enriched giant planet atmospheres should be the rule. We also discuss a model of coreaccretion planet formation in a one-dimensional disk and show that it agrees well with our derived relation between mass and Z planet /Z star .