Just as hopes for detecting life outside the solar system have risen, a problem has appeared that may leave a modicum of doubt about even the most optimistic observations.
Construction is about to start on the European Extremely Large Telescope (E-ELT) whose giant mirror will allow it to study planets orbiting other stars (exoplanets) and hopefully pick up signs of life in their atmospheres.
However, in Proceedings of the National Academy of Sciences three scientists have pointed to a problem: If the planet being studied has a moon which also has an atmosphere we might get false positives, results that suggest the presence of life when it is not really there.
The basis of E-ELT's quest is the idea that some gasses do not last long in planetary atmospheres. They tend to react with other gasses or surface minerals. Consequently, if they are detected it is probably a sign of continuous replenishment, called chemical disequilibrium. Any aliens able to get a chemical signal on our own atmosphere would notice the presence of oxygen and methane in particular, and suspect the Earth of hosting life.
Finding one such gas would be encouraging, but finding two would be better still. Oxygen and methane interact with each other even faster than with surface rocks. If they're both present over any period of time the source must be continuous, not an occasional outbreak such as a volcano.
However, Assistant Professor Hanno Rein of the University of Toronto points out that there could be a confounding factor. The favored method of detection will occur when light from the parent star shines through the planet's atmosphere, in the process picking up the spectral signal of any reasonably common gasses.
From our perspective, an exoplanet and its moon are so close together that we would struggle to distinguish light that has passed through the atmosphere of the planet from that of the moon. So if there was oxygen in the atmosphere of the planet, while the moon had methane, it would look to us as if one of them had both. In the authors' words, “Our results show that the integrated spectrum of the planet and the moon closely resembles that of a single object in strong chemical disequilibrium.”
Rein and his colleagues suggest that spectral detection will never be enough on its own to make us confident a planet has life. Instead they think we will have to wait for signs such as radio waves that would signal the presence of not just life, but an advanced civilization.
On the other hand, with only one moon in our solar system having an appreciable atmosphere, and that around a planet too large to host life, systems of Earth-like planets with atmosphere-bearing moons may be very uncommon. Consequently, while the detection of two incompatible gasses might not confirm life on an exoplanet with certainty, it would remain a very powerful indication.