Think outside the box to find extraterrestrial life

Our Milky Way galaxy, and the billions of others beyond, is chock-a-block with extra-solar planets, scientists have learned in recent decades. But whether any of them can support life is a far more complex and contentious issue.

Many researchers hold that potentially "habitable" planets have to be rocky and within a limited zone in relation to their central sun-conditions that allow for the continuing presence of liquid water on their surfaces.

But in a provocative review article published this week in the journal Science, theoretical physicist Sara Seager of the Massachusetts Institute of Technology lays out a case for habitability being potentially more common than generally predicted.

As a pioneer in the study of exoplanet atmospheres, she paints a different picture of what kinds of planets might support life.

"Our basic premise is that to be habitable, a planet has to have liquid water," Seager said in an interview. "In addition, planets with thin atmospheres are mostly heated by their stars."

"But what primarily controls surface temperature is the greenhouse effect, what types of gases are in the atmosphere, and how massive a planet's atmosphere is. That's what we really have to understand."

Not your usual planet

With that in mind, Seager describes how large planets ten times farther from their stars than Earth is from the sun could also have liquid water and potentially life if, for instance, they had enough hydrogen gas in their atmospheres.

Hydrogen gas, she said, has a much more powerful greenhouse effect than what's in our atmosphere. So it could potentially keep a planet's surface warm with far less radiation from its star.

Relatively dry planets that are close to their suns could also be habitable, Seager said. They need to have less water to create suitable temperatures at their surface because atmospheric moisture is the most effective greenhouse gas of all, Kazinform quotes National Geographic.

She described Venus as an example of the dynamic run amok: The close-in planet once had a lot of water, but the presence of so much moisture set off a runaway greenhouse effect that ultimately made the planet unlivable. A drier early Venus, she said, might have evolved into a quite habitable planet.

Planets that aren't even orbiting stars-the many free-floating planets now known to exist-could support life, she said. They would need heat generated by radioactive or other processes at their cores, as well as the necessary atmospheric gases to keep the warmth in the atmosphere

"If there is one important lesson from exoplanets," Seager writes in her Sciencereview, "it is that anything is possible within the laws of physics and chemistry."

Astronomers have been extraordinarily successful in finding exoplanets in recent years, and have been finding many within what are considered to be classic habitable zones.

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