What would microbes on another planet look like from Earth?

In one of the finest understatements of this very young century, some researchers have written that “The great distances that separate us from even the most nearby stars dictate that all measurements of the exoplanet must be made through remote sensing techniques for the foreseeable future.” Considering we struggle to put the funding together to go anywhere else in this Solar System, that foreseeable future seems to be stretched out for a long time.

But, if we’re limited to remote sensing, then there’s no excuse for not taking the time to think about what we should be looking for. When looking for life on Earth, we tend to look for green, since that’s the color of chlorophyl, the molecule that provides most of the energy for life here. As these researchers point out, green plants are a relatively recent arrival on Earth, only showing up about 450 million years ago. For 3 billion years prior to that, life was microbial.

And, while some microbial organisms get their energy through photosynthesis, a lot of others harvest light using different pigments or simply produce colored chemicals as an incidental byproduct of their metabolic activities. Microbes can range from a rich red to the dark purple of some salt-loving bacteria. So, if we’re looking to directly image signs of life on other planets, then we should think more carefully about what it might look like.