In the hope that this topic is of some interest, I have prepared the following chronological list of significant papers on exoplanetary clouds and atmospheres for your perusal, archived at ARXIV. In this post I provide links to several important papers submitted between 1997 and 2004.

1997

The first significant theoretical look at exoplanetary atmospheres on ARXIV that I am aware of:

Atmospheres of Giant Planets from Neptune to Gliese 229B

Mark S. Marley

Stratospheric heating, condensation, convective transport of non-equilibrium species, and deep radiative energy transport are important processes in the atmospheres of the solar jovian planets. They likely affect the atmospheres of extrasolar giant planets and brown dwarfs as well. Stratospheric temperatures control thermal fluxes in strong molecular bands, and may dramatically affect observed spectra. Condensation processes affect the appearance of an object, alter the abundances of atmospheric species, and influence the fluxes of both reflected and emitted radiation. Convection can dredge up non-thermochemical equilibrium species from the deep atmosphere to the observable atmosphere. Finally deep radiative zones can limit the effectiveness of deep convection and alter the boundary condition at which the planet radiates energy to space. Here I discuss the role these processes may play in the atmospheres of brown dwarfs and extrasolar giant planets.

1998

A first look by two of today's experts at exoplanetary atmospheres under the effects of intense irradiation:

Extrasolar Giant Planets under Strong Stellar Irradiation

S. Seager, D. D. Sasselov

We investigate the effects on extrasolar giant planets [EGPs] of intense irradiation by their parent stars, describing the issues involved in treating the model atmosphere problem correctly. We treat the radiative transfer in detail, allowing the flux from the parent star to interact with all relevant depths of the planetary atmosphere, with no need for a pre-assumed albedo. We present a low-resolution optical and near-IR spectrum of a close-in EGP, focusing on the differences from an isolated planet.

In our dust-free planetary atmospheres we find that Rayleigh scattering increases the EGP's flux by orders of magnitude shortward of the CaII H&K doublet (393 nm), and the spectral features of the parent star are exactly reflected. In the optical and near-IR the thermal absorption of the planet takes over, but the absorption features are changed by the irradiation. The inclusion of dust increases the reflected flux in the blue; the stellar spectral lines can be seen blueward of H-beta (486 nm).

An early look at chemical abundances in the atmospheres of giant exoplanets:

Chemical Equilibrium Abundances in Brown Dwarf and Extrasolar Giant Planet Atmospheres

A. Burrows, C. Sharp

We calculate detailed chemical abundance profiles for a variety of brown dwarf and extrasolar giant planet atmosphere models, focusing in particular on Gliese 229B, and derive the systematics of the changes in the dominant reservoirs of the major elements with altitude and temperature. We assume an Anders and Grevesse (1989) solar composition of 27 chemical elements and track 330 gas--phase species, including the monatomic forms of the elements, as well as about 120 condensates. We address the issue of the formation and composition of clouds in the cool atmospheres of substellar objects and explore the rain out and depletion of refractories. We conclude that the opacity of clouds of low--temperature (900 K), small--radius condensibles (specific chlorides and sulfides), may be responsible for the steep spectrum of Gliese 229B observed in the near infrared below 1 \mic. Furthermore, we assemble a temperature sequence of chemical transitions in substellar atmospheres that may be used to anchor and define a sequence of spectral types for substellar objects with Teff from 2200 K to 100 K.

A first attempt at modeling exoplanetary clouds under various irradiances:

Reflected Spectra and Albedos of Extrasolar Giant Planets I: Clear and Cloudy Atmospheres

M. S. Marley, C. Gelino, D. Stephens, J. Lunine, R. Freedman

The reflected spectra of extrasolar giant planets are primarily influenced by Rayleigh scattering, molecular absorption, and atmospheric condensates. We present model geometric albedo and phase integral spectra and Bond albedos for planets and brown dwarfs with masses between 0.8 and 70 Jupiter masses. Rayleigh scattering predominates in the blue while molecular absorption removes most red and infrared photons. Thus cloud-free atmospheres, found on giant planets with effective temperatures exceeding about 400 K, are quite dark in reflected light beyond 0.6 microns. In cooler atmospheres first water clouds and then other condensates provide a bright reflecting layer. Only planets with cloudy atmospheres will be detectable in reflected light beyond 1 micron. Thermal emission dominates the near-infrared for warm objects with clear atmospheres. However the presence of other condensates, not considered here, may brighten some planets in reflected near-infrared light and darken them in the blue and UV. Bond albedos, the ratio of the total reflected to incident power, are sensitive to the spectral type of the primary. Most incident photons from early type stars will be Rayleigh scattered, while most incident photons from late type stars will be absorbed. The Bond albedo of a given planet thus may range from 0.4 to 0.05, depending on the primary type. Condensation of a water cloud increases the Bond albedo of a given planet by up to a factor of two. The spectra of cloudy planets are strongly influenced by poorly constrained cloud microphysical properties, particularly particle size and supersaturation. Both Bond and geometric albedos are comparatively less sensitive to variations in planet mass and effective temperature.

1999

A must-read paper that established the foundations of the current nomenclature for exoplanetary atmospheres:

Albedo and Reflection Spectra of Extrasolar Giant Planets

David Sudarsky, Adam Burrows, Philip Pinto

We generate theoretical albedo and reflection spectra for a full range of extrasolar giant planet (EGP) models, from Jovian to 51-Pegasi class objects. Our albedo modeling utilizes the latest atomic and molecular cross sections, a Mie theory treatment of extinction by condensates, a variety of particle size distributions, and an extension of the Feautrier radiative transfer method which allows for a general treatment of the scattering phase function. We find that due to qualitative similarities in the compositions and spectra of objects within each of five broad effective temperature ranges, it is natural to establish five representative EGP albedo classes: a ``Jovian'' class (Teff < 150K; Class I) with tropospheric ammonia clouds, a ``water cloud'' class (Teff ~ 250K; Class II) primarily affected by condensed H2O, a ``clear'' class (Teff > 350K; Class III) which lacks clouds, and two high-temperature classes: Class IV (900K < Teff < 1500 K) for which alkali metal absorption predominates, and Class V (Teff > 1500K and/or low surface gravity for which a high silicate layer shields a significant fraction of the incident radiation from alkali metal and molecular absorption. The resonance lines of sodium and potassium are expected to be salient features in the reflection spectra of Class III, IV, and V objects. We derive Bond albedos and effective temperatures for the full set of known EGPs and explore the possible effects of non-equilibrium condensed products of photolysis above or within principal cloud decks. As in Jupiter, such species can lower the UV/blue albedo substantially, even if present in relatively small mixing ratios.

The following paper looks at the theoretical transmission spectra of HD 209458b:

Theoretical Transmission Spectra During Extrasolar Giant Planet Transits

S. Seager, D. D. Sasselov

The recent transit observation of HD 209458 b - an extrasolar planet orbiting a sun-like star - confirmed that it is a gas giant and determined that its orbital inclination is 85 degrees. This inclination makes possible investigations of the planet atmosphere. In this paper we discuss the planet transmission spectra during a transit. The basic tenet of the method is that the planet atmosphere absorption features will be superimposed on the stellar flux as the stellar flux passes through the planet atmosphere above the limb. The ratio of the planet's transparent atmosphere area to the star area is small, approximately 10^-3 to 10^-4; for this method to work very strong planet spectral features are necessary. We use our models of close-in extrasolar giant planets to estimate promising absorption signatures: the alkali metal lines, in particular the Na I and K I resonance doublets, and the He I triplet line at 1083.0 nm. If successful, observations will constrain the line-of-sight temperature, pressure, and density. The most important point is that observations will constrain the cloud depth, which in turn will distinguish between different atmosphere models. We also discuss the potential of this method for EGPs at different orbital distances and orbiting non-solar-type stars.

2001

A more detailed look at theoretical exoplanetary transmission spectra:

Transmission Spectra as Diagnostics of Extrasolar Giant Planet Atmospheres

Timothy M. Brown

Atmospheres of transiting extrasolar giant planets (EGPs) such as HD 209458 b must impose features on the spectra of their parent stars during transits; these features contain information about the physical conditions and chemical composition of the atmospheres. The most convenient observational index showing these features is the ``spectrum ratio'', defined as the wavelength-dependent ratio of spectra taken in and out of transit. I describe a model that estimates this ratio and its dependence upon parameters of the planetary atmosphere, including its cloud structure, temperature, chemical composition, and wind fields. For giant planets in close orbits, the depths of atomic and molecular features in the spectrum ratio may be as large as 0.001. Observations in visible and near-IR wavelengths using existing and planned spectrographs should be adequate to detect these features, and to provide some diagnostics of the conditions within the planetary atmosphere. I give numerous examples of such diagnostics, and I discuss their practicality.

A first look at the prospect of detecting atmospheres of Earthlike exoplanets:

Could We Detect Molecular Oxygen in the Atmosphere of a Transiting Extra-Solar Earth-Like Planet?

John K. Webb, Imma Wormleaton

Although the extra-solar planets discovered so far are of the giant, gaseous, type, the increased sensitivity of future surveys will result in the discovery of lower mass planets. The detection of O2 in the atmosphere of a rocky extra-solar planet would be a potential indicator of a life. In this paper we address the specific issue of whether we would be able to detect the O2 A-band absorption feature in the atmosphere of a planet similar to the Earth, if it were in orbit around a nearby star. Our method is empirical, in that we use observations of the Earth's O2 A-band, with a simple geometric modification for a transiting extra-solar planet, allowing for limb-darkening of the host star. We simulate the spectrum of the host star with the superposed O2 A-band absorption of the transiting planet, assuming a spectral resolution of 7 km/s (typical of current echelle spectrographs), for a range of spectral signal-to-noise ratios. The main result is that we could reliably detect the O2 A-band of the transiting planet for host stars with radii 0.3 solar or less. However, using existing instrumentation and 8m telescopes, this requires target M stars with m(V) of approximately 10 or brighter for integration times of about 10 hours or less. The number of such stars over the sky is small. Larger aperture telescopes and/or improved instrumentation efficiency would enable surveys of M stars down to m(V) = 13 and greatly improve the chances of discovering life elsewhere.

2002

An early look at the deep atmospheric dynamics of hot Jupiters:

Atmospheric Circulation and Tides of "51Peg b-like" Planets

Adam P. Showman, Tristan Guillot

We examine the properties of the atmospheres of extrasolar giant planets at orbital distances smaller than 0.1 AU from their stars. We show that these ``51Peg b-like'' planets are rapidly synchronized by tidal interactions, but that small departures from synchronous rotation can occur because of fluid-dynamical torques within these planets. Previous radiative-transfer and evolution models of such planets assume a homogeneous atmosphere. Nevertheless, we show using simple arguments that, at the photosphere, the day-night temperature difference and characteristic wind speeds may reach ~500 K and ~2 km/s, respectively. Substantial departures from chemical equilibrium are expected. The cloud coverage depends sensitively on the dynamics; clouds could exist predominantly either on the dayside or nightside, depending on the circulation regime. Radiative-transfer models that assume homogeneous conditions are therefore inadequate in describing the atmospheric properties of 51Peg b-like planets. We present preliminary three-dimensional, nonlinear simulations of the atmospheric circulation of HD209458b that indicate plausible patterns for the circulation and generally agree with our simpler estimates. Furthermore, we show that kinetic energy production in the atmosphere can lead to the deposition of substantial energy in the interior, with crucial consequences for the evolution of these planets. Future measurements of reflected and thermally-emitted radiation from these planets will help test our ideas.

A look at the expected spectra of Earthlike exoplanets at various points in their evolutionary history:

A Possible Aeronomy of Extrasolar Terrestrial Planets

W. A. Traub, K. W. Jucks

Terrestrial planetary systems may exist around nearby stars as the Earth-sized counterparts to the many giant planets already discovered within the solar neighborhood. In this chapter we first discuss the numerous techniques which have been suggested to search for extrasolar terrestrial planets. We then focus on the expected results from that technique in which an orbiting telescope or interferometer is used to obtain a visible or infrared spectrum of a planet, without contamination from the parent star. We show examples of such spectra for selected cases: the present Earth, the Neoproterozoic (snowball) Earth, a methane-rich Earth, and the present Mars and Venus. We conclude by discussing the implications of such spectra for the detection of life on an extrasolar terrestrial planet.

A first test of the detectability of Earth-like vegetation on exoplanets:

A test for the search for life on extrasolar planets: Looking for the terrestrial vegetation signature in the Earthshine spectrum

L. Arnold, S. Gillet, O. Lardiere, P. Riaud, J. Schneider

We report spectroscopic observations (400 to 800nm, R = approx 100) of Earthshine in June, July and October 2001 from which normalised Earth albedo spectra have been derived. The resulting spectra clearly show the blue colour of the Earth due to Rayleigh diffusion in its atmosphere. They also show the signatures of oxygen, ozone and water vapour. We tried to extract from these spectra the signature of Earth vegetation. A variable signal (4 to 10 +/-3%) around 700nm has been measured in the Earth albedo. It is interpreted as being due to the vegetation red edge, expected to be between 2 to 10% of the Earth albedo at 700nm, depending on models. We discuss the primary goal of the present observations: their application to the detection of vegetation-like biosignatures on extrasolar planets.

The first solid exo-atmospheric detection:

On the Indirect Detection of Sodium in the Atmosphere of the Planetary Companion to HD 209458

J.J. Fortney, D. Sudarsky, I. Hubeny, C.S. Cooper, W.B. Hubbard, A. Burrows, J.I. Lunine

Using a self-consistent atmosphere code, we construct a new model of the atmosphere of the transiting extrasolar giant planet HD 209458b to investigate the disparity between the observed strength of the sodium absorption feature at 589 nm and the predictions of previous models. For the atmospheric temperature-pressure profile we derive, silicate and iron clouds reside at a pressure of several mbar in the planet's atmosphere. These clouds have significant vertical extent and optical depth due to our slant viewing geometry and lead to increased absorption in bands directly adjacent to the sodium line core. Using a non-LTE sodium ionization model that includes photoionization by stellar UV flux, collisional processes with H2, and radiative recombination, we show that the ionization depth in the planet's atmosphere reaches ~1/2 mbar at the day/night terminator. Ionization leads to a slight weakening of the sodium feature. We present our baseline model, including ionization and clouds, which falls near the observational error bars. The sensitivity of our conclusions to the derived atmospheric temperature-pressure profile is discussed.

A more detailed atmospheric flow simulation of HD 209458b:

Changing Face of the Extrasolar Giant Planet, HD 209458b

James Y-K. Cho, Kristen Menou, Brad Hansen, Sara Seager

High-resolution atmospheric flow simulations of the tidally-locked extrasolar giant planet, HD 209458b, show large-scale spatio-temporal variability. This is in contrast to the simple, permanent day/night (i.e., hot/cold) picture. The planet's global circulation is characterized by a polar vortex in motion around each pole and a banded structure corresponding to ~3 broad zonal (east-west) jets. For very strong jets, the circulation-induced temperature difference between moving hot and cold regions can reach up to ~1000 K, suggesting that atmospheric variability could be observed in the planet's spectral and photometric signatures.

Modeling the temporal changes in the spectra of Earthlike exoplanets:

A Model of the Temporal Variability of Optical Light from Extrasolar Terrestrial Planets

Eric B. Ford, Sara Seager, Edwin L. Turner

The light scattered by an extrasolar Earth-like planet's surface and atmosphere will vary in intensity and color as the planet rotates; the resulting light curve will contain information about the planet's properties. Since most of the light comes from a small fraction of the planet's surface, the temporal flux variability can be quite significant, 10-100%. In addition, for cloudless Earth-like extrasolar planet models, qualitative changes to the surface (such as ocean fraction, ice cover) significantly affect the light curve. Clouds dominate the temporal variability of the Earth but can be coherent over several days. In contrast to Earth's temporal variability, a uniformly, heavily clouded planet (e.g. Venus), would show almost no flux variability. We present light curves for an unresolved Earth and for Earth-like model planets calculated by changing the surface features. This work suggests that meteorological variability and the rotation period of an Earth-like planet could be derived from photometric observations. The inverse problem of deriving surface properties from a given light curve is complex and will require much more investigation.

And the temporal changes on hot Jupiters:

"Weather" Variability Of Close-in Extrasolar Giant Planets

Kristen Menou, James Y-K. Cho, Sara Seager, Brad Hansen

Shallow-water numerical simulations show that the atmospheric circulation of the close-in extrasolar giant planet (EGP) HD 209458b is characterized by moving circumpolar vortices and few bands/jets (in contrast with ~10 bands/jets and absence of polar vortices on cloud-top Jupiter and Saturn). The large spatial scales of moving circulation structures on HD 209458b may generate detectable variability of the planet's atmospheric signatures. In this Letter, we generalize these results to other close-in EGPs, by noting that shallow-water dynamics is essentially specified by the values of the Rossby (Ro) and Burger (Bu) dimensionless numbers. The range of likely values of Ro (~ 0.01 - 10) and Bu (~ 1 - 200) for the atmospheric flow of known close-in EGPs indicates that their circulation should be qualitatively similar to that of HD 209458b. This results mostly from the slow rotation of these tidally-synchronized planets.

2003

More about the sodium detection on HD 209458b:

The Significance of the Sodium Detection in the Extrasolar Planet HD209458b Atmosphere

S. Seager

The Hubble Space Telescope (HST) detection of an extrasolar planet atmosphere in 2001 was a landmark step forward for the characterization of extrasolar planets. HST detected the trace element sodium, via the neutral atomic resonance doublet at 593 nm, in the transiting extrasolar giant planet HD209458b. In this paper I discuss the significance of this first ever extrasolar planet atmosphere detection. I explain how the sodium measurement can be used as a constraint on HD209458b atmosphere models and review recent interpretations of the lower-than-expected sodium line strength.

The first paper on an important effect on highly-irradiated atmospheres:

A Possible Bifurcation in Atmospheres of Strongly Irradiated Stars and Planets

I. Hubeny, A. Burrows, D. Sudarsky

We show that under certain circumstances the differences between the absorption mean and Planck mean opacities can lead to multiple solutions for an LTE atmospheric structure. Since the absorption and Planck mean opacities are not expected to differ significantly in the usual case of radiative equilibrium, non-irradiated atmospheres, the most interesting situations where the effect may play a role are strongly irradiated stars and planets, and also possibly structures where there is a significant deposition of mechanical energy, such as stellar chromospheres and accretion disks. We have presented an illustrative example of a strongly irradiated giant planet where the bifurcation effect is predicted to occur for a certain range of distances from the star.

An important paper on the use of linear polarization as a probe of exoplanetary atmospheres:

Uses of Linear Polarization as a Probe of Extrasolar Planet Atmospheres

S. Saar, S. Seager

We point out some advantages of making observations of extrasolar planets in linearly polarized (LP) light. Older cool stars have quite low levels (~ 10^-4 to 10^-5) of fractional LP, while extrasolar planets can have relatively high fractional LP (~0.1). Observations in LP light can therefore significantly enhance contrast between the planet and its parent star. Data on LP as a function of planetary orbital phase can be used to diagnose the properties (e.g., composition, size, and shape) of the scatterers in the planetary atmosphere. We discuss the feasibility of LP observations of extrasolar planets.

Detection of hydrogen atoms escaping from HD 209458b:

"Osiris" (HD209458b), an evaporating planet

Alfred Vidal-Madjar, Alain Lecavelier des Etangs

Three transits of the planet orbiting the solar type star HD209458 were observed in the far UV at the wavelength of the HI Ly-alpha line. The planet size at this wavelength is equal to 4.3 R_Jup, i.e. larger than the planet Roche radius (3.6 R_Jup). Absorbing hydrogen atoms were found to be blueshifted by up to -130 km/s, exceeding the planet escape velocity. This implies that hydrogen atoms are escaping this ``hot Jupiter'' planet. An escape flux of >~ 10^10g/s is needed to explain the observations. Taking into account the tidal forces and the temperature rise expected in the upper atmosphere, theoretical evaluations are in good agreement with the observed rate. Lifetime of planets closer to their star could be shorter than stellar lifetimes suggesting that this evaporating phenomenon may explain the lack of planets with very short orbital distance.

This evaporating planet could be represented by the Egyptian God ``Osiris'' cut into pieces and having lost one of them. This would give us a much easier way to name that planet and replace the unpleasant ``HD209458b'' name used so far.

Implications of this hydrogen blow-off and a new class of exoplanets:

Evaporation rate of hot Jupiters and formation of Chthonian planets

G. Hébrard, A. Lecavelier des Étangs, A. Vidal-Madjar, J.-M. Désert, R. Ferlet

Among the hundred of known extrasolar planets, about 15% are closer than 0.1 AU from their parent stars. But there are extremely few detections of planets orbiting in less than 3 days. At this limit the planet HD209458b has been found to have an extended upper atmosphere of escaping hydrogen. This suggests that the so-called hot Jupiters which are close to their parent stars could evaporate.

Here we estimate the evaporation rate of hydrogen from extrasolar planets in the star vicinity. With high exospheric temperatures, and owing to the tidal forces, planets evaporate through a geometrical blow-off. This may explain the absence of Jupiter mass planets below a critical distance from the stars. Below this critical distance, we infer the existence of a new class of planets made of the residual central core of former hot Jupiters, which we propose to call the ``Chthonian'' planets.

2004

Detection of oxygen and carbon in the HD 209458b blow-off:

Detection of oxygen and carbon in the hydrodynamically escaping atmosphere of the extrasolar planet HD209458b

A. Vidal-Madjar, J.-M. Désert, A. Lecavelier des Etangs, G. Hébrard, G.E. Ballester, D. Ehrenreich, R. Ferlet, J.C. McConnell, M. Mayor, C.D. Parkinson

Four transits of the planet orbiting the star HD209458 were observed with the STIS spectrograph on board HST. The wavelength domain (1180-1710A) includes HI as well as CI, CII, CIV, NV, OI, SI, SiII, SiIII and SiIV lines. During the transits, absorptions are detected in HI, OI and CII (5+/-2%, 13+/-4.5% and 7.5+/-3.5%, respectively). No absorptions are detected for other lines. The 5% mean absorption over the whole HI Lyman alpha line is consistent with the previous detection at higher resolution (Vidal-Madjar et al. 2003). The absorption depths in OI and CII show that oxygen and carbon are present in the extended upper atmosphere of HD209458b. These species must be carried out up to the Roche lobe and beyond, most likely in a state of hydrodynamic escape.

More theoretical results on hot Jupiter atmospheric composition:

On the Insignificance of Photochemical Hydrocarbon Aerosols in the Atmospheres of Close-in Extrasolar Giant Planets

Mao-Chang Liang, Sara Seager, Christopher D. Parkinson, Anthony Y.T. Lee, Yuk L. Yung

The close-in extrasolar giant planets (CEGPs) reside in irradiated environments much more intense than that of the giant planets in our solar system. The high UV irradiance strongly influences their photochemistry and the general current view believed that this high UV flux will greatly enhance photochemical production of hydrocarbon aerosols. In this letter, we investigate hydrocarbon aerosol formation in the atmospheres of CEGPs. We find that the abundances of hydrocarbons in the atmospheres of CEGPs are significantly less than that of Jupiter except for models in which the CH4 abundance is unreasonably high (as high as CO) for the hot (effective temperatures > 1000K) atmospheres. Moreover, the hydrocarbons will be condensed out to form aerosols only when the temperature-pressure profiles of the species intersect with the saturation profiles--a case almost certainly not realized in the hot CEGPs atmospheres. Hence our models show that photochemical hydrocarbon aerosols are insignificant in the atmospheres of CEGPs. In contrast, Jupiter and Saturn have a much higher abundance of hydrocarbon aerosols in their atmospheres which are responsible for strong absorption shortward of 600 nm. Thus the insignificance of photochemical hydrocarbon aerosols in the atmospheres of CEGPs rules out one class of models with low albedos and featureless spectra shortward of 600 nm.

A more detailed radiative model of HD 209458b:

A Time-Dependent Radiative Model of HD209458b

N. Iro, B. Bezard, T. Guillot

We present a time-dependent radiative model of the atmosphere of HD209458b and investigate its thermal structure and chemical composition. In a first step, the stellar heating profile and radiative timescales were calculated under planet-averaged insolation conditions. We find that 99.99% of the incoming stellar flux has been absorbed before reaching the 7 bar level. Stellar photons cannot therefore penetrate deeply enough to explain the large radius of the planet. We derive a radiative time constant which increases with depth and reaches about 8 hr at 0.1 bar and 2.3 days at 1 bar. Time-dependent temperature profiles were also calculated, in the limit of a zonal wind that is independent on height (i.e. solid-body rotation) and constant absorption coefficients. We predict day-night variations of the effective temperature of ~600 K, for an equatorial rotation rate of 1 km/s, in good agreement with the predictions by Showman &Guillot (2002). This rotation rate yields day-to-night temperature variations in excess of 600 K above the 0.1-bar level. These variations rapidly decrease with depth below the 1-bar level and become negligible below the ~5--bar level for rotation rates of at least 0.5 km/s. At high altitudes (mbar pressures or less), the night temperatures are low enough to allow sodium to condense into Na2S. Synthetic transit spectra of the visible Na doublet show a much weaker sodium absorption on the morning limb than on the evening limb. The calculated dimming of the sodium feature during planetary transites agrees with the value reported by Charbonneau et al. (2002).

First detection of high clouds in HD 209458b:

A new Search for Carbon Monoxide Absorption in the Transmission Spectrum of the Extrasolar Planet HD 209458b

Drake Deming, Timothy M. Brown, David Charbonneau, Joseph Harrington, L. Jeremy Richardson

We have revisited the search for carbon monoxide absorption features in transmission during the transit of the extrasolar planet HD 209458b. We acquired 1077 high resolution spectra at 2 microns using NIRSPEC on Keck II during three transits. Our sensitivity is sufficient to test the degree of CO absorption in the first overtone bands during transit, based on plausible models of the planetary atmosphere. We compare to theoretical tangent geometry absorption spectra, computed by adding height-invariant ad hoc temperature pertubations to the model atmosphere of Sudarsky et al., and by treating cloud height as an adjustable parameter. We do not detect CO absorption. Our analysis indicates a weakening similar to the case of sodium, suggesting that a general masking mechanism is at work in the planetary atmosphere. If this masking is provided by high clouds, our analysis defines the maximum cloud top pressure (i.e., minimum height) as a function of the model atmospheric temperature. For the relatively hot model used by Charbonneau et al. to interpret their sodium detection, our CO limit requires cloud tops at or above 3.3 mbar, and these clouds must be opaque at a wavelength of 2 microns. High clouds comprised of submicron-sized particles are already present in some models, but may not provide sufficient opacity to account for our CO result. Cooler model atmospheres, having smaller atmospheric scale heights and lower CO mixing ratios, may alleviate this problem to some extent. However, even models 500K cooler than the Sudarsky et al. model require clouds above the 100 mbar level to be consistent with our observations. Our null result therefore requires that clouds exist at an observable level in the atmosphere of HD 209458b, unless this planet is dramatically colder than current belief.

This looks like a good place to break off the listing. It's just at the start of the explosion of results and theory that continues to this day. Part 2 will be up when I have sufficient free time to compile it.

Bill