In one of the best understatements of this very young century, some researchers have written: “The great distances separating us from even the nearest stars dictate that all measurements of the exoplanet must be made using remote sensing techniques for the foreseeable future.” Given that we’re struggling to raise the funding to go anywhere else in this solar system, that near future seems a long way off.
But if we limit ourselves to remote sensing, then there’s no excuse for not taking the time to think about what we should be looking for. When we look for life on Earth, we tend to look for green because that’s the color of chlorophyll, the molecule that provides the most energy for life here. As these researchers point out, green plants are a relatively recent arrival to Earth, only about 450 million years ago. Three billion years before that, life was microbial.
And while some microbial organisms get their energy from photosynthesis, many others harvest light using various pigments or simply produce colored chemicals as a by-product of their metabolic activities. Microbes can range from a rich red to the dark purple of some salt-loving bacteria. So if we want to directly image signs of life on other planets, we need to think more carefully about what it might look like.
The authors of the new paper chose a sample of 137 different microorganisms and tested them in a spectrogram that measures the light they reflect at different wavelengths. This population included organisms known for their pigments, as well as a number of specimens that live in extreme environments. To make sure other researchers know what to think about, the authors have put all the resulting data online.
The individual results are not particularly interesting. In the shortwave 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’s true of any carbon-based life form. It is only when you enter the visible that typical characteristic features of pigments become apparent.
And that’s largely what you’d expect. If you were to harvest energy with a pigment, you would have to harvest it at the wavelengths that are not absorbed by the water vapor and carbon dioxide in the atmosphere. And we define what’s visible based on what we can see, which evolved in response to the information that’s also being sent 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 aren’t so many clouds that light never reaches the surface anyway.
Where there may be an exception is near a small, faint, red star, where the output is pointed strongly toward the infrared. There, much of the available energy could reach the surface as infrared, in which case the organisms would absorb it using molecules that are inconspicuous among the masses of other hydrocarbons within the organism itself.
We are still a long way from being able to image exoplanets suitable for life as we commonly recognize it. But that problem can be solved on the order of decades rather than centuries. And when we do, data like this, along with the general principles it reveals, could aid in the analysis of these planets.
PNAS2015. DOI: 10.1073/pnas.1421237112 (About DOIs).