One of the “holy grails” of exoplanet research is to be able to probe the chemical composition of a planet’s atmosphere if there is one. The motivation is of course, at least in part, to begin to assess the possibilities of life on a given exoplanet. The task is extremely challenging however, because an average density measurement is not enough, as there are too many ambiguities. The temperature is also not robustly known, and even when appropriate data of sufficient quality are available, there are still numerous ambiguities. In such situtaions a full-blown model of the planet and it’s atmosphere is needed, incorporating as much physics as possible, albeit with necessary approximations to make the problem tractable. You can then tweak the inputs to the model, including the chemical constituents, to see if you can match the data and the circumstances under which a match is obtained. If you are fortunate you may be able rule out particular scenarios, and if you are even more fortunate you may be able to narrow the possibilities down to only a few scenarios. The ultimate goal of a single, unique description of the atomosphere is currently elusive.
A preprint of a paper that appeared in March, not yet formally published, is one of those papers that you will never read about in the news because it is the kind of work that is deemed not exciting enough for popular consumption, yet it actually represents an extremely important development that will one day lead to highly sought-after results. The paper, by A. Howe and A. S. Burrows (Theoretical Transit Spectra for GJ 1214B and Other Super-Earths), describes the construction of a library of atmosphere models that can be compared with data in order to constrain possible scenarios for the structure and composition of an exoplanet atmosphere. What the title means is that during an eclipse of the host star (a transit), the light from the star broken down into different wavelengths (i.e., the spectrum) changes compared to the pre-eclipse data because chemical species in the atmosphere leave specific imprints on the light signatures. One of the complications in studying exoplanet atmosphere is the possible presence of a cloud cover (or “haze”) because it introduces ambiguity in the interpretation of the data. The advance is really in the comprehensiveness of the coverage of the possible parameter ranges, or as the authors put it, they have “developed a new capability to compute transit spectra of super-Earths that allows atmosphees with arbitrary proportions of common molecular species, along with hazes.”
The authors apply the new tool to what is by now quite a well-known super-Earth, GJ 1214b, which is estimated to have a mass of about 6.5 times the mass of the Earth. They first describe how conflicting results from previous studies have been the norm, particularly in deciding whether the atmosphere is hydrogen-rich or hydrogen-poor. Why is this important to know? The more hydorgen-rich the atmosphere is, the closer it is to the composition of a star or a gas giant, so there be some clues as to the planet’s history. It is also part of the journey towards eventually being able to develop the capability of unambiguously quantifying the presence of other key elements and molecular species. As the authors state in the paper, “No other spectral observations of super-Earths are available at this time of writing,” so this stuff really is at the frontier of knowledge.
So what is the result for GJ 1214b? Unfortunately some of the data are still conflicting even before interpretation (i.e., some of the data from different instruments at the same wavelength are inconsistent so there may be adverse effects making some of the data invalid but it is not known which data are invalid). Despite that, the authors are able to rule out a cloudless atmosphere (whatever the composition of the underlying atmosphere). What is interesting is that none of the models satisfactorily fit the entire set of the nonconflicting data. To some this may seem depressing but another way of looking at it is that it is actually quite exciting because of the prospect of learning something new and perhaps unexpected (by forcing the development of even more diverse model, widening the search). In the meantime, the authors say that, depsite the shortcomings, a hydrogen-rich atmosphere is preferred but a hydrogen-poor one cannot be ruled out. Finally, the authors come up with the important prediction that the hydrogen-rich models predict specific water signatures and suggest that observations be made at the predicted wavelengths to resolve the ambiguity between hydrogen-rich and hydrogen-poor atmospheres.
All in all, this is a very interesting development in probing exoplanet atmospheres, and it is a good illustration of how much effort must be expended in “grunge work” before the breakthrough discoveries can be made. However, it also illustrates that the “grunge work” itself can actually be very interesting in terms of figuring out how to solve the problems and face the challenges. It’s a bit like going into a forest that has not been traveled through by a human – the first people to get there have to hack away at the obstacles to clear the path.
