Until we develop the warp drive, looking for life on planets spotted beyond our solar system is going to be done from here on Earth. It's just as well: The spaceship Earth is obviously the grooviest spaceship, and we can tell quite a bit about the growing number of exoplanets we've spotted with the Kepler telescope. Relying on principles like a planet's orbital distance and size are—admittedly imperfect—ways of ruling out some planets.
Another is looking at a planet's atmospheric composition. If you're looking for life, the thinking goes, look for what life needs and produces—oxygen in the atmosphere on Earth comes from plant life, so it's in the atmosphere elsewhere, maybe it's from a similar source. Other molecules, such as methane or industrial pollution, have also been proposed as possible indicators of life, coming from, for example, animal farts and our factories, respectively.
Astronomers are able to look at molecules in exoplanet atmospheres from Earth, just by looking at the light of the planet's star passing through its atmosphere as it transits between the star and Earth.
Problem is, finding one of these gases alone can be misleading, and can lead to a false positive. Sending a probe hundreds and hundreds of light years away, just to discover that an atmosphere is all ozone (O3) without O2, or is just methane from volcanoes, while the planet is still incapable of supporting life, would be beyond disappointing.
So researchers at NASA Astrobiology Institute's Virtual Planetary Laboratory have been running thousands of simulations, tweaking the atmospheres, trying to figure out what a habitable planet really looks like.
"We tried really, really hard to make false-positive signals for life, and we did find some, but only for oxygen, ozone, or methane by themselves," said Shawn Domagal-Goldman of NASA's Goddard Space Flight Center, one of the lead authors of the paper that was published in Astrophysical Journal.
"However, our research strengthens the argument that methane and oxygen together, or methane and ozone together, are still strong signatures of life," he said.
That's because oxygen and methane abhor each other. An atmosphere heavy in one of these gases has to have its supplies of the other continually replenished, and the most reliable way that happens on Earth is through the mechanisms of life. Ergo, if its got methane AND oxygen, the odds are much better that the planet supports something to respiring, or I guess, farting gases into the atmosphere.
"It's like college students and pizza," Domagal-Goldman said, going into the greatest metaphor ever. "If you see pizza in a room, and there are also college students in that room, chances are the pizza was freshly delivered, because the students will quickly eat the pizza. The same goes for methane and oxygen."
There are a lot of variables in this modeling to work with, not just on the planet, but also for the star that the planet orbits. Oxygen and ozone can be produced when ultraviolet light breaks apart carbon dioxide, so the types and proportions of light coming from the star is going to impact what's seen during a transit.
"If there is more ultraviolet light hitting the atmosphere, it will drive these photochemical reactions more efficiently," Domagal-Goldman said. "To confirm life is making oxygen or ozone, you need to expand your wavelength range to include methane absorption features. Ideally, you'd also measure other gases like carbon dioxide and carbon monoxide [a molecule with one carbon atom and one oxygen atom]. So we're thinking very carefully about the issues that could trip us up and give a false-positive signal, and the good news is by identifying them, we can create a good path to avoid the issues false positives could cause."
As for a good path, I'd recommend coming up with a catchy rhyme to help scientists remember something like:
If you spot some Methane, or O2, or Ozone
There may not be life, if it appears alone
But if two or three types are found there
Then life of some kind might just abound there.
Just spitballing, here.