Can an exoplanet’s orange haze indicate life?

Astronomers often think of Earth-like exoplanets as "pale blue dots, but with this haze, Earth would have been a 'pale orange dot'," says Giada Arney. Above, an image of Saturn's haze-shrouded moon Titan taken by the Cassini spacecraft. (Credit: NASA)

An atmospheric haze around a distant planet—like the one which probably shrouded and cooled the young Earth—could show that the world is potentially habitable, or even be a sign of life itself.

Astronomers often use the Earth as a proxy for hypothetical exoplanets in computer modeling to simulate what such worlds might be like and under what circumstances they might be hospitable to life.

For a new study, researchers chose to study Earth in its Archean era, about 2½ billion years ago, because it is “the most alien planet we have geochemical data for,” says Giada Arney, a doctoral student at the University of Washington.

The new work builds on geological data from other researchers that suggest the early Earth was intermittently shrouded by an organic pale orange haze that came from light breaking down methane molecules in the atmosphere into more complex hydrocarbons, organic compounds of hydrogen and carbon.

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“Hazy worlds seem common both in our solar system and in the population of exoplanets we’ve characterized so far,” Arney says. “Thinking about Earth with a global haze allows us to put our home planet into the context of these other worlds, and in this case, the haze may even be a sign of life itself.”

Researchers used photochemical, climate, and radiation simulations to examine the early Earth shrouded by a “fractal” hydrocarbon haze, meaning that the imagined haze particles are not spherical, as used in many such simulations, but agglomerates of spherical particles, bunched together like grapes, but smaller than a raindrop. A fractal haze, they found, would have significantly lowered the planetary surface temperature.

However, they also found the cooling would be partly countered by concentrations of greenhouse gases that tend to warm a planet. This combination would result in a moderate, possibly habitable average global temperature.

Such a haze would also have absorbed ultraviolet light so well as to effectively shield the Archean Earth from deadly radiation before the rise of oxygen and the ozone layer, which now provides that protection. The haze was a benefit to just-evolving surface biospheres on Earth, as it could be to similar exoplanets.

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The researchers also found that, based on the early Earth data, it’s unlikely such a haze would be formed by abiotic, or nonliving means. So for exoplanets with Earth-like amounts of carbon dioxide in their atmospheres, “organic haze might be a novel type of biosignature, Arney says.

“However, we know these hazes can also form without life on worlds like Saturn’s moon Titan, so we are working to come up with more ways to distinguish biological hazes from abiotic ones.”

“Giada’s work shows that the haze could have intertwined with life in more ways than we previously suspected,” says coauthor Shawn Domagal-Goldman of the NASA Goddard Space Flight Center in Greenbelt, Maryland.

Astronomers often think of Earth-like exoplanets as “pale blue dots”—after a famous photo of Earth taken by the Voyager spacecraft—”but with this haze, Earth would have been a ‘pale orange dot,'” Arney says.

Researchers from the University of Colorado at Boulder and the University of St. Andrews are coauthors of the study that was presented at the American Astronomical Society’s Division of Planetary Sciences conference. The NASA Astrobiology Institute funded the work.

Source: University of Washington