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.

/pdf/a-continuum-from-clear-to-cloudy-hot-jupiter-exoplanets-2w8jo5oknx.pdf

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

First Measurements of Methyl Radical (CH 3 ) in Jupiter's atmosphere using TEXES/IRTF and EXES/SOFIA

Evolution of Stratospheric Chemistry in the Saturn Storm Beacon

Photochemistry of a Volcanically Driven Atmosphere on Io

Jovian Auroral Chemistry

Sub-Neptunes (Rp~1.25-4 REarth) remain the most commonly detected exoplanets to date. However, it remains difficult for observations to tell whether these intermediate-sized exoplanets have surfaces and where their surfaces are located. Here we propose that the abundances of trace species in the visible atmospheres of these sub-Neptunes can be used as proxies for determining the existence of surfaces and approximate surface conditions. As an example, we used a state-of-the-art photochemical model to simulate the atmospheric evolution of K2-18b and investigate its final steady-state composition with surfaces located at different pressures levels (Psurf). We find the surface location has a significant impact on the atmospheric abundances of trace species, making them deviate significantly from their thermochemical equilibrium and "no-surface" conditions. This result arises primarily because the pressure-temperature conditions at the surface determine whether photochemically-produced species can be recycled back to their favored thermochemical-equilibrium forms and transported back to the upper atmosphere. For an assumed H2-rich atmosphere for K2-18b, we identify seven chemical species that are most sensitive to the existence of surfaces: ammonia (NH3), methane (CH4), hydrogen cyanide (HCN), acetylene (C2H2), ethane (C2H6), carbon monoxide (CO), and carbon dioxide (CO2). The ratio between the observed and the no-surface abundances of these species, can help distinguish the existence of a shallow surface (Psurf 100 bar). This framework can be applied together with future observations to other sub-Neptunes of interest.

Julianne I. Moses

Papers

First Measurements of Methyl Radical (CH 3 ) in Jupiter's atmosphere using TEXES/IRTF and EXES/SOFIA

Evolution of Stratospheric Chemistry in the Saturn Storm Beacon

Photochemistry of a Volcanically Driven Atmosphere on Io

Jovian Auroral Chemistry

How to identify exoplanet surfaces using atmospheric trace species in hydrogen-dominated atmospheres