Oxygen and methane should destroy each other when they are in the same atmosphere, breaking down into carbon monoxide and water. On Earth, however, these elements co-exist. That's because they're continually being replenished at a faster rate than they are destroyed. Oxygen is coming mainly from plants, while methane can be emitted by animals and volcanoes.

Seeing this co-existence from afar could be a "bio signature," an indication that our planet has life on it.

Finding such bio signatures in distant worlds is a challenge, however, because of the difficulty in getting spectral information from exoplanets which are faint and many light years away. Even if this obstacle is overcome, the discovery of methane and oxygen on a distant exoplanet could be a false positive, explains a new paper. It's possible that the planet could have oxygen on it, while an "exomoon" circling the planet has methane.

It would be all but impossible to distinguish between the two bodies because the moon is smaller, said lead author Hanno Rein, an assistant professor for the University of Toronto's department of physical and environmental sciences."People have not thought about this before, and the main reason ... is we don't know how likely such a scenario is in the first place," said Rein.

Rein's paper, published in the Proceedings of the National Academy of Sciences, is called "Some inconvenient truths about bio signatures involving two chemical species on Earth-like exoplanets."

It is also available on the pre-publishing site arXiv.

Spectral information
Earth's moon doesn't have an atmosphere, so from afar the methane and oxygen mix on our planet correctly shows there is life on it. Other moons, however, do have atmospheres. A notable example in our own solar system is Titan, a moon of Saturn with a thick hydrocarbon atmosphere that also has methane and ethane lakes on its surface.

To satisfy doubting minds that a planet indeed has these two elements in its atmosphere, two things must be known. There must be enough "information" coming from the planet to give it sufficient resolution. Information comes in the form of photons, or light particles, that reflect off the planet and are collected in telescopes.

But because exoplanets are so faint, very little light is coming off of these worlds in the first place. NASA's Kepler Space Telescope only detects them as minute points of light.

Gaining spectral information requires more photons, which in turn would require bigger telescopes. The size of the telescope required, Rein said, would be impractical, likely requiring a mirror that is kilometers or miles across."One can get fine spatial resolution, but one can't have large spectral resolution," he said.

Usually when astronomers try to make a telescope bigger at a smaller cost, they link the telescopes together using a technique called interferometery.

This is common in both optical astronomy and radio astronomy, with a recent notable example being the Atacama Large Millimeter/sub-millimeter Array (ALMA) in Chile. The observatory has 66 separate antennas that can combine forces to examine young solar systems in high resolution, peering through dust to see stars and planetary discs coming together.

This technique does not work well with spectroscopy, however. While linked telescopes can spatially resolve what an object looks like, it's more difficult to characterize what elements are on the surface because there are still not enough photons to make that analysis, Rein said.

What astronomers are looking for is not only the presence of oxygen or methane in the atmosphere, he added, but a deeper understanding of the rates at which these elements are produced and destroyed. To really interpret the ratio of these elements with respect to life, one also needs to know what non-biological sources could exist.

Finding a solution
The solution to this problem, Rein said, is not necessarily trying to build another telescope. The limitations he and his co-authors cited assume that the search for life zeros in on Earth-sized planets orbiting distant stars like our own sun. This is the stated goal of the Kepler mission and so far, that search has produced one Earth-sized candidate in the habitable zone of its sun-like star: Kepler-186f.

So there are two ways to solve the resolution problem. One is to find planets that are closer to Earth. The typical Kepler planet is hundreds of light years away, which makes it difficult to see anything at all."We can't take any spectrum of them or do good follow up observations, so finding planets close by is key," Rein said.

He advocated that future searches focus on stars that are much closer to us, perhaps a few dozen light-years away, to gain more information. The other way is to change the nature of the search itself by searching for stars that are dimmer than our own sun. These red dwarf stars would not give as much light as a sun-like star, and any habitable planets would have to be closer in. But because the dwarfs are not quite as bright, it would be easier to distinguish the planet's light from the star's light.

This is especially important given that planetary atmosphere composition is usually obtained from watching the planet orbit pass across the face of its star. If the spectral composition of the star changes when the planet passes across it, astronomers can assume that the planet has certain elements that the star does not. For this reason, Rein said it's important to see his study not so much as presenting problems, but more offering solutions to get around them. These techniques would make it easier to understand what a planet's atmosphere is really made of, he said.

Besides which, he's including the possibility of life in our own solar system. Saturn's Enceladus, for example, has a liquid ocean and tidal heat generated from its orbit around the giant planet, making it the right temperature to perhaps host microbes.

"So we don't really need to go so far [to look for life]. It's just really hard if we want to find an Earth-like planet around a sun-size star," Rein said. He added that our assumptions of life itself are also based on a single point of data - Earth - and it is difficult to know what the "canonical example" of a life-friendly planet would be.

One notable addition to the search for exoplanets will be NASA's James Webb Space Telescope, which launches in 2018. The observatory will be located in a stable spot (called L2) on the opposite side of the Earth to the side facing the sun. Among its goals will be to look at planetary systems in infrared light, learning more about how old and massive the systems are by examining their spectra.