The diversity of alien colors: How to recognize life on distant planets

Some of the microorganisms hail from the most extreme environments on Earth; taken together, the samples should allow for a (cautious) estimate of the diversity of biological colors on planets other than Earth. The results are available in an online catalogue and have also been published in the Proceedings of the National Academy of Sciences of the United States of America (PNAS).

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 byproducts 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.

We observe planets by studying starlight reflected off their atmospheres or surfaces. When Jupiter or Venus shines brightly in the night sky, the light you see is sunlight reflected by those planets. 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 surface could be measured directly through the imprint left by the organism's pigmentation, the chemical makeup that determines its color. This imprint is the reflected light's spectrum: the light split up, rainbow-light, into component colors. It is the chemical analogue of a fingerprint, allowing for the identification of different types of microorganisms.

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 microorganisms: 36 from existing culture collections, 100 assembled by Paulino-Lima, and one isolated by Rocco Mancinelli of the BAER Institute at Ames. A primary concern in selecting species was diversity of pigmentation: The 137 life forms span a variety of colors 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 catalog. This biosignature catalog (which consists of reflectance spectra in the optical and near-infrared wavelength regions of the electromagnetic spectrum, 0.35-2.5 micrometers) is the most complete and diverse to date, and the first dedicated to surface biosignatures for exoplanets.

The team has plans to collect more samples and to add more fingerprints to the catalog, 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 highly technically 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 neighboring 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.”

Contact information

Siddharth Hegde (lead author)
Max Planck Institute for Astronomy
Phone: (+49|0) 6221 528-432
Email: hegde@mpia.de

Lisa Kaltenegger (co-author)
Institute for Pale Blue Dots, Cornell University
Phone: (+001) 607 255 35 07
Email: lkaltenegger@astro.cornell.edu

Lynn Rothschild (co-author)
NASA Ames Research Center
Phone: (+1) 650 604 65 25
Email: lynn.j.rothschild@nasa.gov

Anna Ho (public information officer)
Max Planck Institute for Astronomy
Phone: (+49|0) 6221528-237
Email: annaho@mpia.de

Markus Pössel (public information officer)
Max Planck Institute for Astronomy
Phone: (+49|0) 6221 528-261
Email: pr@mpia.de

Background information

The results described here have been published in the Proceedings of the National Academy of Sciences of the United States of America (PNAS) as Hegde et al.: “Surface biosignatures of exo-Earths: Remote detection of extraterrestrial life.”

The authors are Siddharth Hegde (MPIA), Ivan G. Paulino-Lima (NASA Postdoctoral Program Fellow, NASA Ames Research Center), Ryan Kent (UCSC UARC at NASA Ames), Lisa Kaltenegger (MPIA and Institute for Pale Blue Dots, Cornell University), and Lynn Rothschild (NASA Ames).

This work was carried out as part of a NASA Planetary Biology Internship Award (PBI) that Hegde received in 2013. Starting in May 2015, Hegde will be a post-doctoral Research Associate at the Institute for Pale Blue Dots at Cornell University, where the biosignature database is hosted:

The biosignature catalog is online at http://biosignatures.astro.cornell.edu

http://www.mpia.de/news/science/2015-03-biosignatures – Online version of the press release; includes additional image material and text