Looking for signs of life in exoplanet atmospheres is fraught with uncertainty. But now that we can detect polarised light reflected directly off other worlds, we could spot unmistakable evidence of photosynthesis
PALM trees with crimson fronds sway in the breeze as waves lap at a shore warmed by an alien sun. Rock pools are lined with something kelp-like, blue lichens carpet every boulder and strange flowers spring from the dunes beyond.
Astronomers are in little doubt that a plant-filled planet exists beyond our solar system, even if they aren’t entirely sure what the flora would look like. The universe isn’t short of worlds that could host life. Extrapolating from the 4000 or so exoplanets we have identified so far, NASA researchers recently estimated that there could be around 5 billion habitable planets in our galaxy alone. The challenge is to show that one of them is indeed inhabited.
A small army of astronomers are devoted to the task, scouring light that passes through alien atmospheres for hints of bacteria or plants. It is a vibrant enterprise, but at best it provides circumstantial evidence. Astronomers have long known there is a better way – that searching for light reflected off the surface of an exoplanet offers greater chances of success. “It gives you the chance to look directly for living material itself,” says William Sparks at the SETI Institute in Mountain View, California.
The problem was that it was impossible. But recent breakthroughs suggest we can finally tease out the portion of reflected light that would betray unambiguous signs of photosynthesis from other worlds. The telescopes we need are already under construction. In the meantime, the race is on to figure out just what to look for. Astronomers are now working to identify the reflected signatures of verdant …