Earth-Moon Blending

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An outline of the paper: 
Modeling the Infrared Spectrum of the Earth-Moon System: Implications for the Detection and Characterization of Earthlike Extrasolar Planets and Their Moonlike Companions
 by Tyler D. Robinson

Although still years off, the analysis of a terrestrial exoplanet’s atmosphere is a tantalising prospect. Going from our single datum of planet Earth, current thinking holds that terrestrial planets covered in bodies of liquid water are the most promising places to search for evidence of extraterrestrial life. One way of doing this will be to search for tell-tale signs of biological activity in the atmosphere’s chemical composition that would be difficult to explain with abiotic chemical processes. For instance, molecular oxygen only comprises 21% of our own atmosphere because it is produced by plants and photosynthetic bacteria. If it was not constantly replenished by these biological sources, any gaseous oxygen would quickly react with other atoms, forming new chemical species such as silicates and carbon dioxide. For this reason, if one day we detect significant amounts of oxygen in the atmosphere of another exoplanet, it would provide strong evidence that some kind of biological activity was underway.

Detecting the spectral signature of a molecule such as oxygen in an Earth-like planet’s atmosphere, however, is an extremely challenging measurement to make for many reasons, such as the relative faintness of the planet compared to the host star, instrumental systematic effects and chromatic contamination by star spots. If a satellite similar to our own Moon were present around the exoplanet, such a detection would be further complicated. Tyler Robinson (University of Washington) has recently investigated this situation.

The concern is that because such a satellite, or ‘exomoon’, would not be spatially resolvable, its light would be blended into the light we receive from the planet. As a result, the thermal emission from an exomoon could potentially wash out certain spectral absorption features that we are searching for in the planetary emission spectrum, such as those due to oxygen.

(Top) A true colour image of the Earth-Moon system taken by the Deep Impact satellite as part of NASA's Epoxi mission. (Bottom) A simulated image of the Earth-Moon system at 10 microns, illustrating how the substellar point on the Moon is brighter than any feature on the Earth. For an observation of an exoplanet-exomoon system, the two bodies would be unresolved, and the blended exomoon contribution would need to be accounted for in order to obtain an accurate spectrum for the planet. The colour bar indicates the equivalent blackbody temperature. Taken from Robinson (2011).

To investigate this possibility, Robinson calculated what an alien astronomer would measure for the combined infrared spectrum of the Earth-Moon system. Sure enough, he found that the blended Moon contribution would significantly affect the measurements, and would need to be removed in order to accurately determine the composition of the Earth’s atmosphere. For instance, the depth of a particular water absorption band at 6.3 microns would be reduced so much that if the alien astronomer didn’t account for the blended Moon light, then it might underestimate the Earth’s atmospheric water content by a whopping factor of 10!

Robinson suggests that a way around this problem might be to observe a given system at different times, so that we see different fractions of the illuminated exomoon surface. Then by studying how the integrated exoplanet-exomoon spectrum varies as the exomoon’s phase changes, it could be possible to disentangle the moonlight from the planetary signal. Hence, not only would this phase-differencing technique allow the true planetary emission spectrum to be recovered, but it may also be a viable means for discovering exomoons. In any case, the contamination of exoplanetary emission spectra by exomoons is one of the many obstacles that we must keep in mind as we continue to hone in on the prize of characterising the atmosphere of another planet like the Earth.

 

Feature Image:

Image courtesy of NASA, found here.

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About Author

I'm a PhD student at the University of Oxford. My work focuses on transiting exoplanets and, in particular, what we can learn about the atmospheres of these systems. A large part of this involves getting a better handle on the various instrumental systematics that contaminate the small signals we're trying to measure, and devising methods to remove them from the data. I'm also investigating ways of correcting for the effect of star spots on planetary transmission and emission spectroscopy measurements. My supervisor is Suzanne Aigrain.