Research trio suggests exomoon atmospheres could cause false-positive signs of life on exoplanets

April 29, 2014 by Bob Yirka report
This artist's conception illustrates Kepler-22b, a planet known to comfortably circle in the habitable zone of a sun-like star. Credit: NASA/Ames/JPL-Caltech

( —A trio of space scientists has published a paper in Proceedings of the National Academy of Sciences in which they suggest that current assumptions regarding using spectral signatures as a means to identify exoplanets that may harbor life, has a major flaw—a false positive could occur if the planet has a moon with an atmosphere that contaminates the spectrum. In their paper, Hanno Rein, Yuka Fujii and David Spiegel of the University of Toronto, the Tokyo Institute of Technology and MIT respectively, point out a major problem with using spectral signatures as a means for finding out if life exists on other planets—moons which can cause the false impression of chemical disequilibrium.

Spectral signatures are found by comparing the light that passes through the atmosphere of an exoplanet with its host star. Doing so allows scientists to discern which gases are in the planet's atmosphere. If there are two gases that exist, that likely wouldn't in the absence of life, than logic suggests there might be life there. An example would be an atmosphere that holds both oxygen and methane. Because they react with one another (causing one to dissipate), the only way an atmosphere could hold both is if the supply of one of them is being continuously replenished—most likely (because of the types of gases involved) by a living creature on the surface below. The logic up to that point, is fine, the researchers suggest, except for that it doesn't take into account what happens if for example, the planet's atmosphere has oxygen and its moon has an that holds methane. They're too far apart to react with one another but so close together that to us, they'd appear as one spectral signature, representing a false positive.

The team points out that current telescope technology is not accurate enough to allow for separating out moons with atmospheres from their host planets, and won't be in the near future, thus using as a means for identifying possible life-supporting candidate exoplanets (such as the recently discovered Kepler-186f) cannot be viewed as a viable option. They conclude that because of such a flaw, it might be beyond our current abilities to identify the existence of on other , unless it can be identified in some other way, such as the detection of "intelligent" radio signals.

Explore further: Quest for extraterrestrial life not over, experts say

More information: Some inconvenient truths about biosignatures involving two chemical species on Earth-like exoplanets, Hanno Rein, PNAS, 2014. DOI: 10.1073/pnas.1401816111

The detection of strong thermochemical disequilibrium in the atmosphere of an extrasolar planet is thought to be a potential biosignature. In this article we present a previously unidentified kind of false positive that can mimic a disequilibrium or any other biosignature that involves two chemical species. We consider a scenario where the exoplanet hosts a moon that has its own atmosphere and neither of the atmospheres is in chemical disequilibrium. Our results show that the integrated spectrum of the planet and the moon closely resembles that of a single object in strong chemical disequilibrium. We derive a firm limit on the maximum spectral resolution that can be obtained for both directly imaged and transiting planets. The spectral resolution of even idealized space-based spectrographs that might be achievable in the next several decades is in general insufficient to break the degeneracy. Both chemical species can only be definitively confirmed in the same object if absorption features of both chemicals can be unambiguously identified and their combined depth exceeds 100%.

Press release

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not rated yet Apr 29, 2014
I see some problems with this.
1. The moons spectral signature would be a fraction of the planet's, because the planet would be blocking most of the light going through the atmosphere of the moon.

2. What are the numbers for a moon forming in the habitable zone with an appreciable atmosphere?

3. The presence of free oxygen itself is already a bioindicator. Not a slam dunk, not as conclusive as Oxygen + Methane, but a pointer.
not rated yet Apr 29, 2014
If a spectrum is "contaminated" by an exomoon, the spectrum from the next pass in front of the star could be different. Having several spectra that all agree would only occur if the exomoon was in the same relative position each time the exoplanet passed in front of the star. Possible, yes, but not likely.
1 / 5 (2) Apr 29, 2014
This strengthens the case for interstellar exploration. Certainly the view will be different from the Oort cloud, let alone a nearby star. There will also be the opportunity to use our own sun a gravitational lense, the ability to do triangulation and ultimately the chance to enter a different interstellar cloud. Remember that Columbus found the Americas only by travelling into the unknown. We must do the same or accept backwardness.
4 / 5 (2) Apr 29, 2014
If humans, as a species, are really dedicated to finding exoplanets with potential habitability, we need to consider sending out a fast moving probe to get far enough outside our solar system to do tests to see if the Earth itself qualifies as a habitable planet using the same tools as are intended to be used to detect similar exoplanets. Such a probe would have to be equipped with an advanced telescope, spectrometer, etc. and would be expensive, but it would verify what the spectral signature of a known habitable and life-hosting planet should be like. That data could become our baseline for discovery of habitable exoplanets. Without such a reliable baseline, the search is going to be fraught with possibility for error, IMHO.

A bonus would be that this same probe could be used to search for exoplanets from a post just outside our solar system instead of from the Earth's surface.
1 / 5 (1) Apr 29, 2014
We can do this with 1950s/60s tech...we just can't get past the politics...

3 / 5 (2) Apr 29, 2014
Okay, a large planet and a large moon, both with volatile but different atmospheres.

Yes, that will be a very recurrent problem ... especially compared to say, large volcanic episodes establishing disequilibrium on tectonic active planets ... not. =D

And as SG notes, oxygen on a habitable (low temperature) is pretty much good enough.

These types of results are useful to specify uncertainties, but will be a small factor.

@LariAnn: That is work already under progress. Astronomers have detected life on Earth from reflected Earthlight on the Moon with the techniques proposed to be used elsewhere.

@MM: "do this".

Do what? Astrobiologists are discussing remote observations, you are discussing travel options. I don't think probes can compete economically or in science turnaround times.
not rated yet Apr 30, 2014
I agree with LariAnn, we need to establish a baseline derived from our own planet. Reflected Earth light off the moon is a start but its not a pure, standard measurement of Earths atmosphere as it passes in front of our star. Even directly reflected light from earth seen at a distance will give us clues as to better our understanding of what to expect.
not rated yet May 01, 2014
Do what? Astrobiologists are discussing remote observations, you are discussing travel options. I don't think probes can compete economically or in science turnaround times.

Did you bother to read the article? The economics are perfectly feasible. It's policy that's the problem. That was my whole POINT.

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