The formation of clouds affects brown dwarf and planetary atmospheres of nearly all effective temperatures. Iron and silicate condense in L dwarf atmospheres and dissipate at the L/T transition. Minor species such as sulfides and salts condense in mid-late T dwarfs. For brown dwarfs below T eff ∼450 K, water condenses in the upper atmosphere to form ice clouds. Currently over a dozen objects in this temperature range have been discovered, and few previous theoretical studies have addressed the effect of water clouds on brown dwarf or exoplanetary spectra. Here we present a new grid of models that include the effect of water cloud opacity. We find that they become optically thick in objects below T eff ∼350-375 K. Unlike refractory cloud materials, water ice particles are significantly non-gray absorbers; they predominantly scatter at optical wavelengths through J band and absorb in the infrared with prominent features, the strongest of which is at 2.8 µm. H 2 O, NH 3 , CH 4 , and H 2 CIA are dominant opacity sources; less abundant species such as may also be detectable, including the alkalis, H 2 S, and PH 3 . PH 3 , which has been detected in Jupiter, is expected to have a strong signature in the mid-infrared at 4.3 µm in Y dwarfs around T eff =450 K; if disequilibrium chemistry increases the abundance of PH 3 , it may be detectable over a wider effective temperature range than models predict. We show results incorporating disequilibrium nitrogen and carbon chemistry and predict signatures of low gravity in planetarymass objects. Lastly, we make predictions for the observability of Y dwarfs and planets with existing and future instruments including the James Webb Space Telescope and Gemini Planet Imager.

We report Warm Spitzer full-orbit phase observations of WASP-12b at 3.6 and 4.5 µm. This extremely inflated hot Jupiter is thought to be overflowing its Roche lobe, undergoing mass loss, accretion onto its host star, and has been claimed to have a C/O ratio in excess of unity. We are able to measure the transit depths, eclipse depths, thermal and ellipsoidal phase variations at both wavelengths. The large amplitude phase variations, combined with the planet's previously-measured day-side spectral energy distribution, is indicative of non-zero Bond albedo and very poor day-night heat redistribution. The transit depths in the mid-infrared -(R p /R * ) 2 = 0.0123(3) and 0.0111(3) at 3.6 and 4.5 µm, respectively-indicate that the atmospheric opacity is greater at 3.6 than at 4.5 µm, in disagreement with model predictions, irrespective of C/O ratio. The secondary eclipse depths are consistent with previous studies: F day /F * = 0.0038(4) and 0.0039(3) at 3.6 and 4.5 µm, respectively. We do not detect ellipsoidal variations at 3.6 µm, but our parameter uncertainties -estimated via prayer-bead Monte Carlo-keep this non-detection consistent with model predictions. At 4.5 µm, on the other hand, we detect ellipsoidal variations that are much stronger than predicted. If interpreted as a geometric effect due to the planet's elongated shape, these variations imply a 3:2 ratio for the planet's longest:shortest axes and a relatively bright day-night terminator. If we instead presume that the 4.5 µm ellipsoidal variations are due to uncorrected systematic noise and we fix the amplitude of the variations to zero, the best fit 4.5 µm transit depth becomes commensurate with the 3.6 µm depth, within the uncertainties. The relative transit depths are then consistent with a Solar composition and short scale height at the terminator. Assuming zero ellipsoidal variations also yields a much deeper 4.5 µm eclipse depth, consistent with a Solar composition and modest temperature inversion. We suggest future observations that could distinguish between these two scenarios. 11 We follow Agol et al. (2010), who compared many centroiding algorithms and found this one to be optimal. Using fluxweighted centroiding instead of PSF-fitting results in slightly worse χ 2 , commensurate correlated noise as measured using β (see first Section 4.1), and consistent astrophysical parameters.

Doppler and transit surveys are finding extrasolar planets of ever smaller mass and radius, and are now sampling the domain of superEarths (1 − 3R ⊕ ).Recent results from the Doppler surveys suggest that discovery of a transiting superEarth in the habitable zone of a lower main sequence star may be possible.We evaluate the prospects for an all-sky transit survey targeted to the brightest stars, that would find the most favorable cases for photometric and spectroscopic characterization using the James Webb Space Telescope (JWST). We use the proposed Transiting Exoplanet Survey Satellite (TESS) as representative of an all-sky survey. We couple the simulated TESS yield to a sensitivity model for the MIRI and NIRSpec instruments on JWST. Our sensitivity model includes all currently known and anticipated sources of random and systematic error for these instruments. We focus on the TESS planets with radii between Earth and Neptune. Our simulations consider secondary eclipse filter photometry using JWST/MIRI, comparing the 11− and 15 µm bands to measure CO 2 absorption in superEarths, as well as JWST/NIRSpec spectroscopy of water absorption from 1.7− to 3.0 µm, and CO 2 absorption at 4.3 µm. We find that JWST will be capable of characterizing dozens of TESS superEarths with temperatures above the habitable range, using both MIRI and NIRspec. We project that TESS will discover about eight nearby habitable transiting superEarths, all orbiting lower main sequence stars. The principal sources of uncertainty in the prospects for JWST characterization of habitable superEarths are superEarth frequency and the nature of superEarth atmospheres. Based on our estimates of these uncertainties, we project that JWST will be able to measure the temperature, and identify molecular absorptions (water, CO 2 ) in 1 to 4 nearby habitable TESS superEarths orbiting lower main sequence stars.

Small, cool planets represent the typical end-products of planetary formation. Studying the architectures of these systems, measuring planet masses and radii, and observing these planets' atmospheres during transit directly informs theories of planet assembly, migration, and evolution. Here we report the discovery of three small planets orbiting a bright (K s = 8.6 mag) M0 dwarf using data collected as part of K2, the new ecliptic survey using the re-purposed Kepler spacecraft. Stellar spectroscopy and K2 photometry indicate that the system hosts three transiting planets with radii 1.5 -2.1 R ⊕ , straddling the transition region between rocky and increasingly volatile-dominated compositions. With orbital periods of 10-45 days the planets receive just 1.5-10× the flux incident on Earth, making these some of the coolest small planets known orbiting a nearby star; planet d is located near the inner edge of the system's habitable zone. The bright, low-mass star makes this system an excellent laboratory to determine the planets' masses via Doppler spectroscopy and to constrain their atmospheric compositions via transit spectroscopy. This discovery demonstrates the ability of K2 and future space-based transit searches to find many fascinating objects of interest.

9down to "super-Earths" with diameters less than three times that of the Earth ).JWST will revolutionize our knowledge of the physical properties of dozens to possibly hundreds of exoplanets by making a variety of different types of observations. Here we focus on transits and phase curves; direct detection via coronagraphy was considered in a 2007 white paper (for all JWST white papers see http://www.stsci.edu/jwst/doc-archive/whitepapers) and will be revisited in the near future.JWST 's unique combination of high sensitivity and broad wavelength coverage enables the accurate measurement of transit and orbital parameters with high signal-to-noise (SNR). Most importantly, JWST will investigate planetary atmospheres, determine atomic and molecular compositions, probe vertical and horizontal structure, and follow dynamical evolution (i.e. exoplanet weather). It will do this for a diverse population of planets of varying masses and densities, in a wide variety of environments characterized by a range of host star masses and metallicities, orbital semi-major axes and eccentricities. 3The sensitivity of JWST over its wavelength range of 0.6 to 28 microns compared to other missions and ground-based facilities has been amply documented (http://www.stsci.edu/jwst/science/sensitivity) and JWST 's halo orbit around the Earth-Sun L2 point provides long, highly stable, uninterrupted observing sequences 3 Of particular interest for JWST will be small planets (R< 2−4R ⊕ ) located at a distance from their host stars such that their equilibrium temperatures could be comparable to that of our Earth. The range of the so-called "Habitable Zone" has been argued over by many authors since its original definition (Kasting et al. 1993). We take an agnostic approach to this question, referring loosely to planets whose stellar insolation is comparable to that of our own.-10compared with the ground or HST. JWST 's detectors are capable of much better than 100 parts per million (ppm) precision over time periods from hours to days. Its suite of four instruments and multiple operating modes provides a large range of choices in trading off spectral resolution (R between 4 -3000), photometric sensitivity, and observing time. Taken together, these characteristics will make JWST's transit and eclipse observations the best method for characterizing exoplanet atmospheres in the foreseeable future.