Why scientists need to search for alien life on purple planets

Billions of years ago, when microbial life first emerged on Earth, our planet would have appeared purple from space. Armed with this knowledge, scientists now say we should be on the lookout for exoplanets tinged in a similar purple hue — a possible sign of extraterrestrial life.

Back during the Archean era, some three billion years ago, one of the more widespread forms of life were purple bacteria — photosynthetic microorganisms that inhabited both aquatic and terrestrial environments. These conditions would have been similar to the one recently discovered by Australian scientists, an ecosystem dating back 3.5 billion years.

Related: Scientists find a pink planet.

A team of astrobiologists, curious to know if these signatures could be both visible and detectable from space, recently conducted an investigation to simulate the visible and near-infrared radiation reflected by Earth. Their radiative transfer model took several scenarios into account as they simulated an early version of Earth, including the possible distribution of purple bacteria as it would have appeared over continents and oceans, and in consideration of cloud cover.

Their results showed that purple bacteria would indeed have had a noticeable reflective spectrum and a strong reflectivity increase similar to the red edge of leafy plants, though shifted towards the color red. This would have produced a detectable signal in the visual spectra of our planet, though it would have depended on the amount of cloud cover and the concentration and distribution of the purple bacteria.

The next step, says team member Lisa Kaltenegger of the Max Planck Institute for Astronomy in Heidelberg, Germany, is to use multi-color photometric techniques to search for planets similar to an Archean Earth — a search that would look for large swaths of purple bacteria inhabiting vast extensions of an exoplanet. The same technique could be used to locate a planet with present-day Earth-like conditions, one that's covered by deserts, vegetation, and microbial mats. We could even start to look for life around non-traditional candidates, like white dwarfs. Realistically, this search could start in 2018 with the launch of the James Webb Space Telescope.

Read the entire study at the preprint archive arXiv: "Characterizing the purple Earth: Modelling the globally-integrated spectral variability of the Archean Earth."

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