We present the results of a new series of nongray calculations of the atmospheres, spectra, colors, and evolution of extrasolar giant planets (EGPs) and brown dwarfs for effective temperatures below 1300 K This theory encompasses most of the mass/age parameter space occupied by substellar objects and is the first spectral study down to 100 K These calculations are in aid of the multitude of searches being conducted or planned around the world for giant planets and brown dwarfs and reveal the exotic nature of the class Generically, absorption by H2 at longer wavelengths and H2O opacity windows at shorter wavelengths conspire to redistribute flux blueward Below 1200 K, methane is the dominant carbon bearing molecule and is a universal diagnostic feature of EGP and brown dwarf spectra We find that the primary bands in which to search are Z (~105 ?m), J (~12 ?m), H (~16 ?m), K (~22 ?m), M (~5 ?m), and N (~10 ?m), that enhancements of the emergent flux over blackbody values, in particular in the near infrared, can be by many orders of magnitude, and that the infrared colors of EGPs and brown dwarfs are much bluer than previously believed In particular, relative to J and H, the K band flux is reduced by CH4 and H2 absorption Furthermore, we conclude that for Teff's below 1200 K most or all true metals may be sequestered below the photosphere, that an interior radiative zone is a generic feature of substellar objects, and that clouds of H2O and NH3 are formed for Teff's below ~400 and ~200 K, respectively This study is done for solar-metallicity objects in isolation and does not include the effects of stellar insulation Nevertheless, it is a comprehensive attempt to bridge the gap between the planetary and stellar realms and to develop a nongray theory of objects from 03MJ (Saturn) to 70MJ (~007 M?) We find that the detection ranges for brown dwarf/EGP discovery of both ground- and space-based telescopes are larger than previously estimated

https://iopscience.iop.org/article/10.1086/305002/pdf

A Nongray Theory of Extrasolar Giant Planets and Brown Dwarfs

Data Reduction And Error Analysis For The Physical Sciences

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.

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

Earth is over 4,500 million years old. Massive bombardment of the planet took place for the first 500–700 million years, and the largest impacts would have been capable of sterilizing the planet. Probably until 4,000 million years ago or later, occasional impacts might have heated the ocean over 100 °C. Life on Earth dates from before about 3,800 million years ago, and is likely to have gone through one or more hot-ocean 'bottlenecks'. Only hyperthermophiles (organisms optimally living in water at 80–110 °C) would have survived. It is possible that early life diversified near hydrothermal vents, but hypotheses that life first occupied other pre-bottleneck habitats are tenable (including transfer from Mars on ejecta from impacts there). Early hyperthermophile life, probably near hydrothermal systems, may have been non-photosynthetic, and many housekeeping proteins and biochemical processes may have an original hydrothermal heritage. The development of anoxygenic and then oxygenic photosynthesis would have allowed life to escape the hydrothermal setting. By about 3,500 million years ago, most of the principal biochemical pathways that sustain the modern biosphere had evolved, and were global in scope.

The habitat and nature of early life

The C/O ratio is predicted to regulate the atmospheric chemistry in hot Jupiters Recent observations suggest that some exoplanets, eg, Wasp 12-b, have atmospheric C/O ratios substantially different from the solar value of 054 In this Letter, 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 phases In particular, between the H2O and CO snowlines most oxygen is present in icy grains—the building blocks of planetary cores in the core accretion model—while 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 a C/O ~ 1, while atmospheres significantly contaminated by evaporating planetesimals will have a stellar or substellar 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

https://iopscience.iop.org/article/10.1088/2041-8205/743/1/L16/pdf

The Effects of Snowlines on C/O in Planetary Atmospheres

Photochemistry Saturn's Atmosphere

Photochemical Models of Saturn's Atmosphere: Comparisons with ISO Observations

Comparatively little is known about atmospheric chemistry on Uranus and Neptune, because remote spectral observations of these cold, distant ``Ice Giants'' are challenging, and each planet has only been visited by a single spacecraft during brief flybys in the 1980s Thermochemical equilibrium is expected to control the composition in the deeper, hotter regions of the atmosphere on both planets, but disequilibrium chemical processes such as transport-induced quenching and photochemistry alter the composition in the upper atmospheric regions that can be probed remotely Surprising disparities in the abundance of disequilibrium chemical products between the two planets point to significant differences in atmospheric transport The atmospheric composition of Uranus and Neptune can provide critical clues for unravelling details of planet formation and evolution, but only if it is fully understood how and why atmospheric constituents vary in a three-dimensional sense and how material coming in from outside the planet affects observed abundances Future mission planning should take into account the key outstanding questions that remain unanswered about atmospheric chemistry on Uranus and Neptune, particularly those questions that pertain to planet formation and evolution, and those that address the complex, coupled atmospheric processes that operate on Ice Giants within our solar system and beyond

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Atmospheric chemistry on Uranus and Neptune

Introduction: Infrared spectra from Jupiter exhibit numerous emission features due to higher-order hydrocarbons produced from methane photolysis in the upper stratosphere. Photochemical models to date [e.g., 1,2,3] have done a poor job of reproducing the observed hydrocarbon abundances, especially if one considers the new high-quality observational constraints provided by ISO, the Infrared Space Observatory [see 4,5,6,7]. Recent advances in our knowledge of the stratospheric temperature profile, species abundances, and chemical kinetics data have provided important new model constraints and input parameters, and updated photochemical models are needed in order to explain the observed composition. We use the observational constraints provided by ISO to construct new one-dimensional steady-state models of Jovian stratospheric photochemistry. The models include coupled hydrocarbon and oxygen photochemistry, vertical eddy and molecular diffusion, and radiative transport (including multiple Rayleigh scattering by H2, He, and CH4). We focus on determining the eddy diffusion coefficient profile in the stratosphere, as this information is currently poorly constrained or contradictory [e.g., 8,9, and Fig. 1]. We also discuss the implications with regard to hydrocarbon photochemistry on all the giant planets. Results: The upper-stratospheric eddy diffusion coefficient derived from observations of Jupiter’s 584 A brightness [e.g., 10] is inconsistent with that derived from the Voyager ultraviolet stellar occultation experiment [8] or infrared CH4 ν3-band florescence data [11]. Therefore, we have created two models (A & B) with very different high-altitude diffusion profiles (see Fig. 1). We also test the eddy diffusion profile used by [1] (Model C). The resulting mixing-ratio profiles for several observed constituents are shown in Fig. 2. Our main conclusions are as follows: (a) Models A and B both provide a good fit to the ISO C2H2, C2H6, CH3C2H, C4H2, and C6H6 data. The He profile from Model B is consistent the 584 A data, but the Model B CH4 profile is inconsistent with the methane observations of [8,11]. The Model A CH4 profile is consistent with [8,11], but the He profile is inconsistent with [10]. A reanalysis of all the above data sets [8,10,11], new UV occultation data (i.e., the Cassini flyby), and/or a better determination of the upper atmospheric temperature profile as a function of latitude are needed before this issue can be resolved. (b) Model C and the model of [1] are inconsistent with the C4H2 upper limit and the C2H6/C2H2 ratio inferred from ISO data [4], suggesting that the eddy diffusion coefficient adopted in these models is too large at pressures of 1-100 mbar and/or the hydrocarbon chemistry from [1] is incorrect or incomplete. (c) The large abundance of benzene on Jupiter and the other giant planets [e.g., 5] most probably results from the long dissociation lifetime of the C6H6 molecule. Upon absorption of a UV photon, electronically excited C6H6 can be collisionally quenched at lower stratospheric pressures before it dissociates.

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Photochemistry and Diffusion in Jupiter's Stratosphere

Meteoroid ablation in the Neptune atmosphere can influence the chemistry of the upper atmosphere through the supply of oxygen, in the case of water-ice meteoroid ablation, which will eventually be converted to CO in the upper atmosphere, and through the ablation and recondensation of relatively refractory meteoroid material, which leads to the production of dust particles that can act as sites for lower-atmosphere condensation of hydrocarbons. An analysis is presently made of ablation rate calculation uncertainties. The ablation equations presented are applicable to other planets.

Julianne I. Moses

Papers

Photochemistry Saturn's Atmosphere

Photochemical Models of Saturn's Atmosphere: Comparisons with ISO Observations

Atmospheric chemistry on Uranus and Neptune

Photochemistry and Diffusion in Jupiter's Stratosphere

Meteoroid ablation in Neptune's atmosphere