Home→ Exoplanets FAQ → What are the properties and compositions of exoplanet atmospheres?
The observational study of exoplanet atmospheres is still a nascent and rapidly developing field, and at the moment is restricted mainly to hot Jupiters. Since 2002, more than two dozen gas-giant exoplanet atmospheres have been studied. The observational challengs are formidable, and theoretical modeling is difficult. Most of the exoplanet atmospheric studies so far are on giant exoplanets because their size makes them more sensitive to measurements. Data taken in the infrared band have been particularly amenable to this (because of the higher planet-to-star contrast compared to visible light). The close proximity of hot Jupiters to their host star also means that atmospheres will be subjected to intense heating and raised very high temperatures, and this complicates the physics that must be modeled even further.
In some cases specific molecules have been detected, the more robust results among them being for sodium, water vapor, methane, and carbon dioxide in gas-giant exoplanet atmospheres. More tentative claims exist in the literature. Direct imaging of exoplanets is highly desirable because it would yield more information with less ambiguity, but it is currently at the frontier of instrumental capability. Only a handful of examples exist at the moment, and those are exoplanets that are amongst the largest and brightest, and that are located at large distances from their host star. The latter property improves the planet-to-star brightness contrast.
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Two of the “poster-child” exoplanets which have been well-studied for many different reasons, including their atmospheres are the hot Jupiters HD 209458 b and HD 189733 b. In the former planer, there is a comet-like structure that extends to a distance of several times the planet's size and much has been written about what appears to be escape of the atmosphere. However, no satisfactory explanation that has a sound theoretical basis and that fits the data has yet been advanced.
The study of the atmospheres associated with smaller and less massive planets is riddled with the problem of degenerate theoretical scenarios (model solutions) being able to fit a given data set. The models must reflect the physics and composition of the planetary interior, not just the atmosphere. To illustrate the sort of language used in the scientific literature at the moment, a paper on the atmosphere of the by now quite well-known hot Neptune GJ 436b by Beaulieu et al. (2011) states that, “We show that the emission photometric data are not incompatible with the presence of abundant methane, but further spectroscopic data are desirable to confirm this scenario.” Don't worry about the details: the point is the use of the phrase “not incompatible with.”
For even smaller and less massive planets, the Super-Earths, the are even more prone to ambiguity. Foe example, a paper on the well-studied Super-Earth GJ 1214b by Bean et al. (2011) concludes that the planets atmosphere could consist of a very thick haze or cloud layer if it is made mostly of Hydrogen, or it could dominated by dense water vaopr. You may not be surprised to know that it is not uncommon to see results and conclusions in the literature that are different for the same exoplanet studied by different research groups and/or methods.
One of the ultimate goals is of course to study the atmospheres of rocky, terrestirial exoplanets, especially an Earth analog (no Earth analogs have yet been found). Studying the atmospheres of terrestrial exoplanets is much more challenging than it is for the gas giants, not only because the former are smaller, but because the atmospheric composition is expected to be much more diverse. Whereas the giant planets have retained their original atmospheres (which would have a composition similar to the host star), geological and biological activity can significantly change the atmospheric composition of terrestrial, rocky planets. Much of the original atmosphere could have been lost. The possibilities for the composition are so numerous that it is difficult to theorize what to expect. In other words, the possibilities are so wide open that it is hard to even know what to look for.
Despite the wide range of possibilities for the atmospheres of potential Earth analogs, it is clearly compelling to look for the sort of biosignatures that our own Earth would present if it were observed as an exoplanet. One of the strongest biosignatures of life on Earth is of course that due to oxygen. Even though oxygen is highly reactive, it makes up 21% of the Earth's atmosphere by volume. Without life, which is a continuous source of oxygen production on Earth, oxygen would be depleted. On the other hand, the converse is not necessarily true (the presence of Oxygen would not robustly imply that life had been found). Another strong indicator of life is the so-called “vegetation red edge.” This is a prominent feature at the red end of the reflection signal (spectrum) from Earth's atmosphere that is unique to plant life.
(Note for a more advanced readers: a state-of-the-art review of the exoplanet atmospheres and the various issues involved can be found in Seager and Deming (2010), ARA&A, 48, 631.)
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File under: Exoplanet Atmospheres; Molecules in exoplanet atmospheres; Escape of exoplanet atmospheres; Biosignature of exoplanets.