Astronomers are gearing up for a new phase of research on exoplanets (planets outside our solar system), teaming up with biologists to formulate search strategies for life on these distant planets. So far, these efforts have focused on what are known as indirect biosignatures, such as by-products of life that could be detectable in a host planet's atmosphere. But if the surface of an exoplanet were dominated by one particular life form, a more direct form of detection might be possible: a detection based on light reflected by that life form, taking on a characteristic tint in the process.
Eight of the 137 microorganism samples used to measure biosignatures for the catalogue. In each panel, the top is a regular photograph of the sample and the bottom is a micrograph, a 400x zoomed-in version of the top image. The scientists were aiming to achieve diversity in colour and pigmentation. (Image: Hegde et al./MPIA) (click on image to enlarge)
We observe planets by studying starlight reflected off their atmospheres or surfaces, because the composition of this reflected light provides us with information about what is to be found on the planet. Alien astronomers making detailed observations of Earth would notice a greenish tint as sunlight reflected by trees and other vegetation reaches their telescopes.
Similarly, the presence of an alien organism covering large swathes of an exoplanet’s surface could be measured directly through the imprint left by the organism's pigmentation, the chemical makeup that determines its colour. This imprint is the reflected light's spectrum: the light split up, rainbow-light, into component colours. It is the chemical analogue of a fingerprint, allowing for the identification of different types of microorganism.
Now, a group of astronomers and biologists led by Siddharth Hegde has teamed up to explore what these fingerprints might look like and how diverse they could be. Hegde, then a graduate student at the Max Planck Institute for Astronomy, and astronomer Lisa Kaltenegger (Director of the Institute for Pale Blue Dots at Cornell University) teamed up with biologist Lynn Rothschild, postdoctoral fellow Ivan Paulino-Lima and research associate Ryan Kent, all of the NASA Ames Research Center, to explore the full range of possibilities for what chemical fingerprints – and therefore exoplanet surface biosignatures – could look like.
To this end, the team assembled cultures of 137 different species of microorganism. A primary concern in selecting species was diversity of pigmentation: the 137 life forms span a variety of colours and are residents of a variety of environments, ranging from the Atacama Desert in Chile, to seawater in Hawaii, to some old woodwork at Salt Spring in Boone’s Lick State Park, Missouri.
The team reflected light off samples from each microorganism culture, measured their chemical fingerprints, and assembled their findings in an online catalogue. This biosignature catalogue (which consists of reflectance spectra in the optical and near-infrared wavelength regions, 0.35-2.5 micrometres) is the most complete and diverse to date, and the first dedicated to surface biosignatures for exoplanets.
For now, the catalogue primarily serves to illustrate the potential diversity of extrasolar life. It also illustrates the potential diversity of extrasolar planets, because particular pigments arise out of particular environmental conditions and can thus provide clues regarding the nature of the planet.
In this composite satellite image from NASA, you can see a greenish tint in the reflected sunlight, a direct signature of plant life present on Earth's surface. Similarly, if microbial life with a particular pigmentation covered larges swathes of an exoplanet's surface, its presence could in principle be measured directly through its tint in reflected starlight viewed through our telescopes. (Image: NASA Earth Observatory (click on image to enlarge)
In addition, because the surface of a planet affects its atmosphere, these biosignatures could be used as initial conditions for models of exoplanet atmospheres (called “atmospheric radiative transfer models”). More precisely, the surface composition determines how much radiation is reflected back from the surface and used in chemical processes in the atmosphere.
When the team assembled their catalogue, they observed that the fingerprint of a microorganism is primarily determined by its pigmentation makeup. This makeup is a result of secondary metabolic processes, which are unique to life forms and play important roles in photosynthesis, in screening harmful ultraviolet radiation, and in preventing oxidative damage. Thus, recognizing a particular pigmentation makeup through a biosignature corresponds to recognizing a type of living being.
The team has plans to collect more samples and to add more fingerprints to the catalogue, in order to further enhance the diversity of the microorganisms represented. They hope that it will be helpful not only to astrobiologists, but also to astronomers who are trying to make models of planetary atmospheres. However, even with the next generation of telescopes, detecting the fingerprints of organisms living on planetary surfaces will be technically highly challenging.
At the moment, it is not possible to directly measure light from an Earth-sized planet, because this light is drowned out by the much brighter neighbouring starlight. For now, Kaltenegger says, “this (database) gives us for the first time a glimpse into the detectable signatures of the fascinating diversity of worlds that could exist out there.”