Studying hazy, distant exoplanets for signs of life: This is what happened on Archean Earth

New Delhi: To help better understand which hazy, distant planets have habitable conditions suitable for life, a team of researchers has been studying the Earth - specifically Earth during the Archean era, an epic 1-1/2-billion-year period early in our planet’s history.

For years, astronomers have been trying to find out which distant planets might have habitable conditions, the role of atmospheric haze has been hazy.

 

As per a new finding from NASA, Earth’s atmosphere seems to have been quite different then, probably with little available oxygen but high levels of methane, ammonia and other organic chemicals.

Geological evidence suggests that haze might have come and gone sporadically from the Archean atmosphere - and researchers aren’t quite sure why.

To learn better about hazy earthlike exoplanets, the team looked to haze formation during the Archean era.

“We like to say that Archean Earth is the most alien planet we have geochemical data for,” said Giada Arney of NASA’s Goddard Spaceflight Center in Greenbelt, Maryland, and a member of the NASA Astrobiology Institute’s Virtual Planetary Laboratory based at the University of Washington, Seattle.

Researchers said in the best case, haze in a planet’s atmosphere could serve up a smorgasbord of carbon-rich, or organic, molecules that could be transformed by chemical reactions into precursor molecules for life, and might also screen out much of the harmful UV radiation that can break down DNA.

However, in the worst case, haze could become so thick that very little light gets through. This could turn the surface to become so cold and freezed completely.

The researchers put together sophisticated computer modeling to look at how haze affected the surface temperature of Archean Earth and, in turn, how the temperature influenced the chemistry in the atmosphere.

“The new modeling indicates that as the haze got thicker, less sunlight would have gotten through, inhibiting the types of sunlight-driven chemical reactions needed to form more haze. This would lead to the shutdown of haze-formation chemistry, preventing the planet from undergoing runaway glaciation due to a very thick haze,” said the report.

The team calls this phenomenon 'self-limiting haze', and their work is the first to make the case that this is what occurred on Archean Earth.

The researchers concluded that self-limiting haze could have cooled Archean Earth by about 36 degrees Fahrenheit (20 Kelvins) - enough to make a difference but not to freeze the surface completely.

“Our modeling suggests that a planet like hazy Archean Earth orbiting a star like the young sun would be cold,” said Shawn Domagal-Goldman, a Goddard scientist and a member of the Virtual Planetary Laboratory. “But we’re saying it would be cold like the Yukon in winter, not cold like modern-day Mars.”

Such a planet might be considered habitable, even if the mean global temperature is below freezing, as long as there is some liquid water on the surface, the report added.

“Haze may turn out to be very helpful as we try to narrow down which exoplanets are the most promising for habitability,” said Arney, who is also the lead author of two related papers published by the team.

The findings have been published in the November 2016 issue of the journal Astrobiology.