Searching for alien life on exoplanets: Researchers study Archean Earth during 1-billion-year period

Researchers have been searching for life on planets other than Earth since long. A team of researchers studied the Earth, specifically the Archean Earth during 1-1/2-billion-year period early in the planet’s history in order to better understand which hazy and distant exoplanet could have habitable conditions to sustain life.

For years, astronomers have been trying to look out for alien life by studying distant planets for habitable conditions.

New NASA study suggests that the atmosphere of the Earth appears to have been different during the Archean era. The reason could be little availability of oxygen and high levels of methane, ammonia and other organic chemicals.

The haze might have come and disappeared sporadically from the Archean atmosphere, geological evidence suggests. But researchers are not quite sure why.

In order to better understand the hazy earth-like exoplanets, the researchers studied the 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.

In the best case, said the researchers, haze in the atmosphere of a planet could serve up a smorgasbord of carbon-rich, or organic, molecules that could be transformed by chemical reactions into precursor molecules for life. It might also push 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.

In order to see how haze affected the surface temperature of Archean Earth in turn and in turn how the temperature influenced the chemistry in the atmosphere, the researchers put together sophisticated computer modelling.  

“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.

This phenomenon is called 'self-limiting haze' as named by the team. Their work is to first make the case that this is what happened on Archean Earth.

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, the researchers concluded.

“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 could sustain life and can 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 study was published in the November 2016 issue of the journal Astrobiology.