TL;DR - Your “biosignature” might just be a photobomb.
In simulations of an Earth-sized planet orbiting a Sun-like star, the atmosphere of a 0.4 Earth-radius moon was shown to affect the system’s spectrum, in ways that could be observationally identical to signs of disequilibrium on a single body. In other words, your fancy methane-and-oxygen “biosignature” might just be a moon jumping into the frame, and it’s probably impossible to tell the difference. Resolution is the main limiting factor - not just present resolution, but projected best-case-scenario resolutions. Direct imaging allows us higher resolutions than infrared spectroscopy, but clouds and hazes make interpreting those images a complicated task.
The authors suggest two methods for unambiguously crossing off an exomoon from your laundry list of possible false-positives:
First, you could look for absorption features that together exceed 100% where they shouldn’t. This would suggest that at least some of one of the species in question is actually on the planet itself.
Alternately, you could discover a single-species silver-bullet biosignature that’s abundant enough to be observable and spectroscopically unique enough to be identifiable. #soundsfakebutok
PAPER: Rein, H., et al. 2014, “Some inconvenient truths about biosignatures involving two chemical species on Earth-like exoplanets.” Proceedings of the National Academy of Sciences, 111, 19 (link)
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BONUS COMMENTARY BELOW, for those who don’t mind reading more...
A few study highlights, limitations, and opportunities for more work:
The authors focused specifically on an Earth-like planet orbiting a Sun-like star. These are among the systems most likely to be habitable (regardless of detection). However, it seems the authors’ primary concern is the detectability and disentanglement of spectral signatures from multiple small bodies. That task is likely to be much easier with small stars, which are much more common and - so far - much more likely to host rocky planets. If we suspect that resolution and signal-to-noise ratios will be limiting factors, I’d be curious to see a comparable analysis around a fairly quiescent M-dwarf.
The authors gave the moon an oddly dense atmosphere, with a surface pressure of 1500 mbar (versus the Earth’s 1013 mbar). This fits nicely with their suggested method of detecting the moon’s interference - namely, looking for absorption features that together exceed 100% once you’ve disentangled the spectra. In a way, it also suggests that fainter signals might be harder to see for what they are (a lunar photobomb). But...
Just how big does a moon have to be to retain an atmosphere in the first place, assuming it formed alongside its planet (i.e. by impact, not capture)? Are habitable-zone Earth-like planets in stable orbits likely to have moons that size? In other words, is this particular moon - large, with a thick atmosphere - a strawman?
Open questions: If the moon didn’t form in situ, what are the odds of atmosphere-hosting moon capture by the largest possibly-habitable rocky planets? How long would those systems take to stabilize? Would the instability be detectable? And what would the stabilization process do to their atmospheres? Could instability or tidal heating, alongside heightened flux from inward migration, even create an atmosphere from previously locked-in material? What would be the loss rate of such an atmosphere on such a small body?
Most of all, I’d be curious to see mechanisms by which these degeneracies could be resolved - i.e. by obtaining the occultation and transit spectra of each body independently. I can imagine seeking and timing spectral variations of the planet-moon system as a whole and finding the end-members within, either as a way to resolve the satellite’s orbital period or as a follow-up to transit-based disentanglement like Teachey, et al. suggested this year. Unlike the more distant habitable planets orbiting sun-like stars discussed here, habitable-zone planets orbiting close to cool stars have short enough orbital periods to permit continual observation... with the reasonable expectation of observing a satellite occultation (if a large, atmosphere-retaining satellite is actually present).
In short, while I appreciate the authors’ focus on our balmiest, most Earth-like scenario, and while I understand that this is just proof-of-concept... if you’re going to claim a task is impossible because of observational limitations, you should probably try to simulate that task using configurations that are (a) physically likely and (b) more easy to observe than the ones chosen here.
2087
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