The state of exoclime studies


A review paper on the highlights in exoplanet atmosphere studies has just been published in Nature by Adam Burrows from Princeton (arguably the most experienced specialist in the field).

The five highlights selected are:

1. The detection of atoms and molecules (and hazes) in transmission spectra

Only three features are deemed to have been reliably detected in transmission spectra: the narrow lines of sodium and potassium, and water-vapour bands. The effect of clouds and hazes is inferred from the absence of features.

[Fig. 1 from the paper]

Transmission spectrum of HD 189733b, showing the core of the sodium and potassium lines, and the signature of aerosols. [Fig. 1 from the paper].

In addition to the detection of specific features, one highlight of transit spectroscopy is the featureless spectrum of GJ 1214b, indicating the presence of high clouds:

The transit spectrum of GJ 1241b. The flat spectrum require high clouds masking all molecular features. [Fig. 2 from the paper]

The transit spectrum of GJ 1241b. The flat spectrum require high clouds masking all molecular features. [Fig. 2 from the paper]

On the topic of transit spectra, Burrow’s review is as remarkable for what it leaves out as for what it includes, and is a tacit indication of the somewhat erratic early history of the field. In particular, numerous detections of molecular features claimed from noisy spectra have since been shown to be spurious (e.g. methane in HD 189733b, carbon dioxide in XO-1b, and many others). Of more consequence, the review also leaves out the very numerous secondary eclipse depth measurements with Spitzer, on which dozens of paper have been written, but which in the end have proven frustratingly unreliable. The relevant passage in the Nature review is worth quoting:

Of course, the mere detection of an exoplanet is a victory, and the efforts that have gone into winning these data should not be discounted. Nevertheless, with nearly fifty such campaigns and detections “in the can”, one has learned that it is only with next-generation spectra using improved (perhaps dedicated) spectroscopic capabilities that the desired thermal and compositional information will be forthcoming.

2. Exo-exospheres

Atmospheric escape was detected around some hot Jupiters, measured by the presence of strong atomic lines at UV wavelengths in the transit spectrum. HD 189733b for instance is opaque in the Lyman-alpha hydrogen line over a region many times larger than the planet itself (n.b. the exosphere is the extended layer above a planetary atmosphere where atoms and molecules can leak into space).

3. Phase light curves and planet maps

HD 18977b is, again, the main character in this story, with the spectacular mapping with Spitzer phase curves (see this post). These maps showed that the hottest point was shifted eastwards by winds, confirming one of the fundamental predictions of hot-Jupiter weather models.

4. High spectral resolution techniques

Narrow molecular lines detected in transit spectroscopy have been used to measure the speed of the planet, and even the speed of the winds on the planet. Narrow lines were also used to produce a rough map of the closest brown dwarf (see here).

5. High contrast imaging

The last highlight is the study of planets detected with direct imaging, in particular the four known planets around HR8799. Photometry and spectroscopy of these objects have revealed the presence of clouds (see here). This is the most recent of the five highlights, and is undergoing very rapid development .

Thus at the end of 2014, depending whether we notice the empty or full half of our glass, we might throw a puzzled look at the imposing scrap heap of discarded results, or bask in the glory of solidly established inferences on exoplanet atmosphere that we thought, not so long ago, to be far our of the reach of our telescopes.

Featured Image: temperature map of HD 189733b inferred from the 8-micron phase curve with Spitzer from the seminal study of Knutson et al. 2007.

Figure original references: Fig. 1 from Pont, Sing, Gibson et al. (2013), Fig. 2 from Kreidberg, Bean, Désert et al. (2014)


About Author

I am a professor of planetary science at the University of Exeter. My specialty is the study of exoplanets, in particular the observation and modelling of exoplanet atmospheres. I have done my PhD a the University of Geneva and worked in Chile, France and Switzerland.