Jean-Michel Desert

A closely packed system of low-mass, low-density planets transiting Kepler-11

When an extrasolar planet passes in front of (transits) its star, its radius can be measured from the decrease in starlight and its orbital period from the time between transits. Multiple planets transiting the same star reveal much more: period ratios determine stability and dynamics, mutual gravitational interactions reflect planet masses and orbital shapes, and the fraction of transiting planets observed as multiples has implications for the planarity of planetary systems. But few stars have more than one known transiting planet, and none has more than three. Here we report Kepler spacecraft observations of a single Sun-like star, which we call Kepler-11, that reveal six transiting planets, five with orbital periods between 10 and 47u2009days and a sixth planet with a longer period. The five inner planets are among the smallest for which mass and size have both been measured, and these measurements imply substantial envelopes of light gases. The degree of coplanarity and proximity of the planetary orbits imply energy dissipation near the end of planet formation.

A continuum from clear to cloudy hot-Jupiter exoplanets without primordial water depletion

Thousands of transiting exoplanets have been discovered, but spectral analysis of their atmospheres has so far been dominated by a small number of exoplanets and data spanning relatively narrow wavelength ranges (such as 1.1–1.7 micrometres). Recent studies show that some hot-Jupiter exoplanets have much weaker water absorption features in their near-infrared spectra than predicted. The low amplitude of water signatures could be explained by very low water abundances, which may be a sign that water was depleted in the protoplanetary disk at the planet’s formation location, but it is unclear whether this level of depletion can actually occur. Alternatively, these weak signals could be the result of obscuration by clouds or hazes, as found in some optical spectra. Here we report results from a comparative study of ten hot Jupiters covering the wavelength range 0.3–5 micrometres, which allows us to resolve both the optical scattering and infrared molecular absorption spectroscopically. Our results reveal a diverse group of hot Jupiters that exhibit a continuum from clear to cloudy atmospheres. We find that the difference between the planetary radius measured at optical and infrared wavelengths is an effective metric for distinguishing different atmosphere types. The difference correlates with the spectral strength of water, so that strong water absorption lines are seen in clear-atmosphere planets and the weakest features are associated with clouds and hazes. This result strongly suggests that primordial water depletion during formation is unlikely and that clouds and hazes are the cause of weaker spectral signatures.

A precise water abundance measurement for the hot Jupiter WASP-43b

The water abundance in a planetary atmosphere provides a key constraint on the planet’s primordial origins because water ice is expected to play an important role in the core accretion model of planet formation. However, the water content of the Solar System giant planets is not well known because water is sequestered in clouds deep in their atmospheres. By contrast, short-period exoplanets have such high temperatures that their atmospheres have water in the gas phase, making it possible to measure the water abundance for these objects. We present a precise determination of the water abundance in the atmosphere of the 2 MJup short-period exoplanet WASP-43b based on thermal emission and transmission spectroscopy measurements obtained with the Hubble Space Telescope. We nd the water content is consistent with the value expected in a solar composition gas at planetary temperatures (0:4 3:5 solar at 1 condence). The metallicity of WASP-43b’s atmosphere suggested by this result extends the trend observed in the Solar System of lower metal enrichment for higher planet masses. Subject headings: planets and satellites: atmospheres | planets and satellites: composition | planets and satellites: individual: WASP-43b

Thermal structure of an exoplanet atmosphere from phase-resolved emission spectroscopy

Exoplanets that orbit close to their host stars are much more highly irradiated than their solar system counterparts. Understanding the thermal structures and appearances of these planets requires investigating how their atmospheres respond to such extreme stellar forcing. We present spectroscopic thermal emission measurements as a function of orbital phase (“phase-curve observations”) for the highly irradiated exoplanet WASP-43b spanning three full planet rotations using the Hubble Space Telescope. With these data, we construct a map of the planet’s atmospheric thermal structure, from which we find large day-night temperature variations at all measured altitudes and a monotonically decreasing temperature with pressure at all longitudes. We also derive a Bond albedo of 0.18−0.12+0.07 and an altitude dependence in the hot-spot offset relative to the substellar point. Thermal spectra probe atmospheric layers of the day and night sides of a tidally locked planet and show poor heat circulation. Sunny side hot for tidally locked world Most planets in our solar system spread their heat evenly across their surfaces in the course of a day. They accomplish this with relatively fast rotations and a generous distance from the Sun. Some exoplanets, however, arent so well balanced. Stevenson et al. show that planets like the Jupiter-sized WASP-43b whirl closely around their star in less than 24 hours, which leaves them tidally locked with little chance for heat redistribution. Science, this issue p. 838

A DETECTION OF WATER IN THE TRANSMISSION SPECTRUM OF THE HOT JUPITER WASP-12b AND IMPLICATIONS FOR ITS ATMOSPHERIC COMPOSITION

Detailed characterization of exoplanets has begun to yield measurements of their atmospheric properties that constrain the planets’ origins and evolution. For example, past observations of the dayside emission spectrum of the hot Jupiter WASP-12b indicated that its atmosphere has a high carbon-tooxygen ratio (C/O > 1), suggesting it had a dierent formation pathway than is commonly assumed for giant planets. Here we report a precise near-infrared transmission spectrum for WASP-12b based on six transit observations with the Hubble Space Telescope/Wide Field Camera 3. We bin the data in 13 spectrophotometric light curves from 0.84 - 1.67 m and measure the transit depths to a median precision of 51 ppm. We retrieve the atmospheric properties using the transmission spectrum and nd strong evidence for water absorption (7 condence). This detection marks the rst high-condence, spectroscopic identication of a molecule in the atmosphere of WASP-12b. The retrieved 1 water volume mixing ratio is between 10 5 10 2 , which is consistent with C/O > 1 to within 2 . However, we also introduce a new retrieval parameterization that ts for C/O and metallicity under the assumption of chemical equilibrium. With this approach, we constrain C/O to 0:5 +0:2 0:3 at 1 and rule out a carbon-rich atmosphere composition (C/O> 1) at > 3 condence. Further observations and modeling of the planet’s global thermal structure and dynamics would aid in resolving the tension between our inferred C/O and previous constraints. Our ndings highlight the importance of obtaining high-precision data with multiple observing techniques in order to obtain robust constraints on the chemistry and physics of exoplanet atmospheres. Subject headings: planets and satellites: atmospheres | planets and satellites: composition | planets and satellites: individual: WASP-12b

HST hot-Jupiter transmission spectral survey: detection of potassium in WASP-31b along with a cloud deck and Rayleigh scattering

We present Hubble Space Telescope optical and near-IR transmission spectra of the transiting hot-Jupiter WASP-31b. The spectrum covers 0.3–1.7 μm at a resolution R ∼ 70, which we combine with Spitzer photometry to cover the full-optical to IR. The spectrum is dominated by a cloud deck with a flat transmission spectrum which is apparent at wavelengths > 0.52u2009μm. The cloud deck is present at high altitudes and low pressures, as it covers the majority of the expected optical Na line and near-IR H2O features. While Na i absorption is not clearly identified, the resulting spectrum does show a very strong potassium feature detected at the 4.2σ confidence level. Broadened alkali wings are not detected, indicating pressures below ∼10 mbar. The lack of Na and strong K is the first indication of a sub-solar Na/K abundance ratio in a planetary atmosphere (ln[Na/K] = −3.3 ± 2.8), which could potentially be explained by Na condensation on the planets night side, or primordial abundance variations. A strong Rayleigh scattering signature is detected at short wavelengths, with a 4σ significant slope. Two distinct aerosol size populations can explain the spectra, with a smaller sub-micron size grain population reaching high altitudes producing a blue Rayleigh scattering signature on top of a larger, lower lying population responsible for the flat cloud deck at longer wavelengths. We estimate that the atmospheric circulation is sufficiently strong to mix micron size particles upwards to the required 1–10 mbar pressures, necessary to explain the cloud deck. These results further confirm the importance of clouds in hot Jupiters, which can potentially dominate the overall spectra and may alter the abundances of key gaseous species.

Spitzer secondary eclipses of the dense, modestly-irradiated, giant exoplanet hat-P-20b using pixel-level decorrelation

HAT-P-20b is a giant metal-rich exoplanet orbiting a metal-rich star. We analyze two secondary eclipses of the planet in each of the 3.6 and 4.5 μm bands of Warm Spitzer. We have developed a simple, powerful, and radically different method to correct the intra-pixel effect for Warm Spitzer data, which we call pixel-level decorrelation (PLD). PLD corrects the intra-pixel effect very effectively, but without explicitly using—or even measuring—the fluctuations in the apparent position of the stellar image. We illustrate and validate PLD using synthetic and real data and comparing the results to previous analyses. PLD can significantly reduce or eliminate red noise in Spitzer secondary eclipse photometry, even for eclipses that have proven to be intractable using other methods. Our successful PLD analysis of four HAT-P-20b eclipses shows a best-fit blackbody temperature of 1134 ± 29 K, indicating inefficient longitudinal transfer of heat, but lacking evidence for strong molecular absorption. We find sufficient evidence for variability in the 4.5 μm band that the eclipses should be monitored at that wavelength by Spitzer, and this planet should be a high priority for James Webb Space Telescope spectroscopy. All four eclipses occur about 35 minutes after orbital phase 0.5, indicating a slightly eccentric orbit. A joint fit of the eclipse and transit times with extant RV data yields e cos ω = 0.01352^(+0.00054)_(-0.00057) and establishes the small eccentricity of the orbit to high statistical confidence. HAT-P-20b is another excellent candidate for orbital evolution via Kozai migration or other three-body mechanisms.

HST hot-Jupiter transmission spectral survey: haze in the atmosphere of WASP-6b

© 2014 The Authors. We report Hubble Space Telescope optical to near-infrared transmission spectroscopy of the hot-Jupiter WASP-6b, measured with the Space Telescope Imaging Spectrograph and Spitzers InfraRed Array Camera. The resulting spectrum covers the range 0.29-4.5 μm. We find evidence for modest stellar activity of WASP-6 and take it into account in the transmission spectrum. The overall main characteristic of the spectrum is an increasing radius as a function of decreasing wavelength corresponding to a change of Δ (Rp/R*) = 0.0071 from 0.33 to 4.5 μm. The spectrum suggests an effective extinction cross-section with a power law of index consistent with Rayleigh scattering, with temperatures of 973 ± 144K at the planetary terminator. We compare the transmission spectrum with hot-Jupiter atmospheric models including condensate-free and aerosol-dominated models incorporating Mie theory. While none of the clear-atmosphere models is found to be in good agreement with the data, we find that the complete spectrum can be described by models that include significant opacity from aerosols including Fe-poor Mg2 SiO4, MgSiO3, KCl and Na2S dust condensates.WASP- 6b is the second planet after HD 189733b which has equilibrium temperatures near ~1200K and shows prominent atmospheric scattering in the optical.

ATMOSPHERIC CHARACTERIZATION OF THE HOT JUPITER KEPLER-13Ab

Kepler-13Ab (= KOI-13.01) is a unique transiting hot Jupiter. It is one of very few known short-period planets orbiting a hot A-type star, making it one of the hottest planets currently known. The availability of Kepler data allows us to measure the planets occultation (secondary eclipse) and phase curve in the optical, which we combine with occultations observed by warm Spitzer at 4.5 μm and 3.6 μm and a ground-based occultation observation in the K_s band (2.1 μm). We derive a day-side hemisphere temperature of 2750 ± 160 K as the effective temperature of a black body showing the same occultation depths. Comparing the occultation depths with one-dimensional planetary atmosphere models suggests the presence of an atmospheric temperature inversion. Our analysis shows evidence for a relatively high geometric albedo, A_g = 0.33^(+0.04)_(-0.06). While measured with a simplistic method, a high A_g is supported also by the fact that the one-dimensional atmosphere models underestimate the occultation depth in the optical. We use stellar spectra to determine the dilution, in the four wide bands where occultation was measured, due to the visual stellar binary companion 1.15 ± 0.05 away. The revised stellar parameters measured using these spectra are combined with other measurements, leading to revised planetary mass and radius estimates of M_p = 4.94-8.09 M_J and R_p = 1.406 ± 0.038 R_J. Finally, we measure a Kepler midoccultation time that is 34.0 ± 6.9 s earlier than expected based on the midtransit time and the delay due to light-travel time and discuss possible scenarios.

THE ATMOSPHERIC CIRCULATION OF THE HOT JUPITER WASP-43b: COMPARING THREE-DIMENSIONAL MODELS TO SPECTROPHOTOMETRIC DATA

© 2015. The American Astronomical Society. All rights reserved. The hot Jupiter WASP-43b (2 MJ , 1 RJ , T orb = 19.5 hr) has now joined the ranks of transiting hot Jupiters HD 189733b and HD 209458b as an exoplanet with a large array of observational constraints. Because WASP-43b receives a similar stellar flux as HD 209458b but has a rotation rate four times faster and a higher gravity, studying WASP-43b probes the effect of rotation rate and gravity on the circulation when stellar irradiation is held approximately constant. Here we present three-dimensional (3D) atmospheric circulation models of WASP-43b, exploring the effects of composition, metallicity, and frictional drag. We find that the circulation regime of WASP-43b is not unlike other hot Jupiters, with equatorial superrotation that yields an eastward-shifted hotspot and large day-night temperature variations (600 K at photospheric pressures). We then compare our model results to Hubble Space Telescope (HST)/WFC3 spectrophotometric phase curve measurements of WASP-43b from 1.12 to 1.65 μm. Our results show the 5× solar model light curve provides a good match to the data, with a peak flux phase offset and planet/star flux ratio that is similar to observations; however, the model nightside appears to be brighter. Nevertheless, our 5× solar model provides an excellent match to the WFC3 dayside emission spectrum. This is a major success, as the result is a natural outcome of the 3D dynamics with no model tuning. These results demonstrate that 3D circulation models can help interpret exoplanet atmospheric observations, even at high resolution, and highlight the potential for future observations with HST, James Webb Space Telescope, and other next-generation telescopes.