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.

The authors of the new paper picked a sampling of 137 different microorganisms and put them to the test in a spectrogram that measured the light they reflect at different wavelengths. This population included organisms known for their pigments, as well as a sampling of some that live in extreme environments. To make sure that other researchers know what they should be considering, the authors have placed all the resulting data online.

The individual results aren't especially interesting. In the short-wave infrared, the spectra are dominated by the water content of the living cells. As you move into the near-infrared, the internal contents—the lipids and proteins that structure the cell—begin to dominate, something that will be true for any carbon-based life form. It's only once you move into the visible that distinctive signatures of pigments typically become apparent.

And that's largely what you'd expect. If you were harvesting energy with a pigment, you'd need to harvest it at the wavelengths that aren't absorbed by the water vapor and carbon dioxide in the atmosphere. And we define what's visible based on what we can see, which has evolved in response to the information that also transmits through the atmosphere. So, what we consider visible is simply where the most energy is available. This will probably be true for just about any planet near a sun-like star, provided there's not so many clouds that light never reaches the surface anyway.

Where there may be an exception is near a small, dim, red star, where the output is heavily biased toward the infrared. There, a lot of the available energy could reach the surface as infrared, in which case the organisms would be absorbing it using molecules that don't stand out from the mass of other hydrocarbons in the organism itself.

We're still a long way from being able to image exoplanets that are suitable for life as we typically recognize it. But that problem may be solved on the order of decades, rather than centuries. And, when we do, data like this, along with the general principles it reveals, may help guide the analysis of these planets.

PNAS, 2015. DOI: 10.1073/pnas.1421237112 (About DOIs).