When we think of alien life, images of little green men, Martians, or perhaps even spaceships carrying them to Earth are the first things that come to mind. Yet when you think about life on Earth, how it arose, and how it came to be the way it is today, you've got to recognize that the evidence of an inhabited world was present for billions of years before humans ever arose. The first signs of alien life that we're going to discover are unlikely to be sentient beings arriving at Earth, but rather the subtle signs we'd see if someone had been searching for Earth as it was hundreds of millions (or more) years ago.
While it's going to require some huge advances in technology before we can directly image the surface of an alien world for signs that it's inhabited, there's a surefire signature that organic processes have been happening on a planet's surface or in its oceans for a long period of time: the existence of certain chemicals in its atmospheric composition. And thanks to the unique properties of molecules, combined with the astronomical techniques we've pioneered over the last two decades, we have every reason to believe the first signs of life beyond our Solar System will show themselves in the coming two or three decades, tops.
Image credit: NASA / JPL-Caltech, via... [+]
When you have life on a planet, it's inevitably going to start by taking a source of energy -- perhaps sunlight, perhaps a hydro/geothermal vent, perhaps radioactively decaying elements -- and converting it into usable energy on a molecular scale. Based on the way we know chemistry works, that necessitates either the conversion of one molecule type into another or the production of a new molecule entirely. On our world, this is the mechanism by which huge amounts of oxygen were produced: something that would never have risen to become 21% of the atmosphere on Earth had we not been inhabited for billions of years.
So we can look for telltale signatures of these molecules that only come along if life is involved -- oxygen in the presence of non-reactive nitrogen, carbon dioxide, water vapor, methane, etc. -- on rocky worlds at the right distance from their stars to have liquid water. We even have a technique for how to measure these molecules under special circumstances: transit spectroscopy.
When a planet passes in front of its parent star relative to our line-of-sight, the light emitted from that star appears to dim ever so slightly, reaching a minimum brightness, and then brighten again as the planet completes its journey across the disk. This phenomenon is known as a planetary transit.
However, if that planet has an atmosphere, some of that starlight will be filtered through the planet's atmosphere, allowing us to detect not only what molecules and atoms are present, but what their relative abundances are. Since molecules and atoms absorb light of characteristic frequencies, it's only a question of technological implementation before we can measure Earth-like atmospheres around Sun-like stars and smaller. As it stands, we've already detected atmospheres (and water vapor) on Neptune-sized worlds around stars like our own.
Image credit: H. Rauer et al.: Potential Biosignatures in super-Earth Atmospheres. Astronomy &... [+]
As NASA's Chief Scientist, Ellen Stofan, said this past Tuesday, "We know where to look. We know how to look. In most cases, we have the technology, and we're on a path to implementing it." If the next generation of ground-and-space-based telescopes reaches the sensitivity they're designed to reach, and the existence of life is a relatively commonplace phenomenon in the Universe, one of the enduring philosophical questions for all of human history will finally have a definitive, affirmative answer: we are not alone.
