A new paper describing how an early Earth was likely dominated by purple bacteria may redirect scientists’ attention when looking for an inhabited exo-planet. The research was done by astrophysicists from Spain and was published earlier this month.
In the search for life elsewhere in the Universe, scientists have taken the “streetlamp” approach. As in, if a drunk loses his keys outside of the bar one night, where does he look? Under the streetlamp. That may not be where they most likely were dropped, but it is the place he can see. Along those same lines, scientists are looking for life that looks a lot like life on Earth, because that’s what we know how to detect.
Of course, just because we know what life looks like now does not mean life looks the same as it did throughout our nearly 4 billion year evolutionary history. Recent research has suggested at as many as 1 in 5 sun-like stars have rocky planets around the size of Earth in its habitable zone. While finding a planet about our size with the potential for life might not be too hard, the odds of finding one with life that is at the same evolutionary juncture as us might be too difficult right now.
In the paper, the researchers describe the Archaean eon: the period of time where life first emerged on Earth, spanning from 2.5-4 Ga (billions of years ago). At this time, photosynthetic microorganisms dominated the land and water alike. With such distinct organisms covering the planet, it is likely that they would have been detectable, even from great distances.
To test that hypothesis, the researchers modeled exoplanets with the same photosynthetic bacteria that once covered Earth. The bacteria were arranged in many different configurations; both in and out of the water, in various quantities and locations, and with different atmospheric obstacles. The team found that using visible and near-infrared radiation, the purple bacteria were incredibly reflective. They also discovered that cloud cover drastically reduced the reflection from bacteria in water. If we find an Earth-like planet covered in this type of bacteria, the very clouds that help make the planet hospitable for life may prevent scientists from finding it. Not only that, but the team might be able to differentiate between the purple bacteria and more complex lifeforms by shifting the spectrum on the instruments. This could essentially focus the proverbial streetlamp’s light and make a more targeted effort in detecting alien life.