Developing a Color Code for Habitable Exoplanets

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There’s a growing number of exoplanets being found inside the habitable zones around their stars — the “sweet spot” where temperatures would allow for liquid water oceans on Earth-sized worlds.

These planets have largely been identified by detecting their ghostly silhouette caused by passing in front of their stars, or their invisible gravitational pull on their parent star. But determining if these worlds are inhabited (or, indeed, truly habitable) will require teasing out and dissecting the anemic amount of starlight filtered through a planet’s atmosphere, or reflected off of its surface. That’s a tall order.

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Siddharth Hegde of The Max Plank Institute for Astronomy, and colleagues, propose a “quick and dirty” way to sort out possibly inhabited worlds. His approach is to look at a planet’s reflected light through different colored filters. This is simpler than the more arduous task of spreading out a planet’s light into a spectrum. Such detailed spectroscopy of Earth-sized planets will have to wait for futuristic huge space telescopes.

The catch is that the target planets have to be largely cloudless and have rocky surfaces — not be smothered in thick atmospheres like Venus.

For starters, Hegde says that we should even consider environmental conditions where only extremophiles — microbes that can deal with extreme temperature, radiation, high salinity and acidity — can survive. “Extremophiles provide us with the minimum known envelope of environmental limits for life on our planet,” he writes.

Extremophiles on Earth live in environmental niches as long as there is liquid water, an energy source for metabolism, and a source of nutrients that helps in building and maintaining cellular structures. They dwell in Earth’s sand deserts, ice deserts and salt flats like the nearly bone dry Atacama Desert in Chile. They live inside rocks like sandstone that protect the organisms by filtering out destructive ultraviolet radiation.

But a challenge is that Earth’s extremophiles often live under the surface, so any telltale evidence of alien extremophiles’ presence could be blocked out.

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Looking at reflected colors collected from Earth-orbiting environmental reconnaissance satellites, Hegde studied three specific types of extremeophiles that would the color light to be reddish: lichens, bacterial mats and red algae in acid mine drainage. From that he built a model of Earth’s color signature as it would look to aliens using a similar scientific observations as us. By contrast, barren water, snow, sand and salt flats have little or no color for all practical purposes.

What’s more, previous interplanetary spacecraft observations of Earth have revealed a unique reddish tinge where plants using chlorophyll reflect a lot of near-infrared light back into space. This signature, called the “red edge” only became apparent on Earth 500 million years ago with the onset of multi-celled land organisms.

Any number of alien astronomers might use their own Kepler-like planet-hunting space telescopes to catalog Earth’s size, orbit, and density, among that of countless other terrestrial planets. If aliens applied a similar color study to Earth they might conclude that our planet has surface microorganisms. But that would only be convincing if the extraterrestrials also had photosynthetic carbon-based organisms.

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Hedge’s goal is to build a color diagram where various types of rock exoplanets can be binned (perhaps analogous to the color-magnitude diagram for classifying stars). Those that have colors similar to Earth’s would be prioritized targets for doing follow-up spectroscopic searches for atmospheric biosignatures of oxygen, methane, and carbon dioxide, among others.

Artwork credit/copyright: Lynette Cook

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