The Dawes Review 3: The Atmospheres of Extrasolar Planets and Brown Dwarfs

Abstract

The last few years has seen a dramatic increase in the number of exoplanets known and in the range of methods for characterising their atmospheric properties. At the same time, new discoveries of increasingly cooler brown dwarfs have pushed down their temperature range which now extends down to Y-dwarfs of < 300 K. Modelling of these atmospheres has required the development of new techniques to deal with the molecular chemistry and clouds in these objects. The atmospheres of brown dwarfs are relatively well understood, but some problems remain, in particular the behavior of clouds at the L/T transition. Observational data for exoplanet atmosphere characterisation is largely limited to giant exoplanets that are hot because they are near to their star (hot Jupiters) or because they are young and still cooling. For these planets there is good evidence for the presence of CO and H2O absorptions in the IR. Sodium absorption is observed in a number of objects. Reflected light measurements show that some giant exoplanets are very dark, indicating a cloud free atmosphere. However, there is also good evidence for clouds and haze in some other planets. It is also well established that some highly irradiated planets have inflated radii, though the mechanism for this inflation is not yet clear. Some other issues in the composition and structure of giant exoplanet atmospheres such as the occurrence of inverted temperature structures, the presence or absence of CO2and CH4, and the occurrence of high C/O ratios are still the subject of investigation and debate.

Could photosynthesis function on Proxima Centauri b?

“…Since Proxima Centauri b is our closest extrasolar planet it is an obvious target for projects that aim at observing extrasolar planets by direct imaging seeking to detect biogenic atmospheric and surface signatures (Turnbull et al 2012; Seager 2014; Turbet et al 2016). Stellar transits of Proxima Centauri b would greatly help in characterizing the planet if we are fortunate enough that they do occur (Kaltenegger & Traub 2009; van Belle & von Braun 2009; Jones & Sleep 2010; Kaltenegger et al 2010; Turnbull et al 2012; Bailey 2014).…”

Retrieving Temperatures and Abundances of Exoplanet Atmospheres with High-resolution Cross-correlation Spectroscopy

“…The field of exoplanet characterization has matured to the point where we can begin to answer fundamental questions regarding planetary climate, composition, and formation, and provide context for understanding our own solar system planets Burrows 2014;Bailey 2014;Crossfield 2015). The community has leveraged the power of ground-and spacebased observatories to find and characterize a diverse range of planets, ranging from hot giants to terrestrialsized, potentially habitable worlds.…”

Atlas of Experimental and Theoretical High-temperature Methane Cross Sections from T = 295 to 1000 K in the Near-infrared

“…Considering the cosmic abundance of carbon and hydrogen, and the thermodynamic stability of methane at low temperatures, it is no surprise that methane is found in numerous astronomical sources. In substellar objects, methane appears when the temperature drops below about 1500 K (Bailey 2014). The spectrum of hot methane in the near-infrared is the defining feature of T-type brown dwarfs (Oppenheimer et al 1995;Nakajima et al 2001;Kirkpatrick 2005;Legget et al 2007).…”