Livable Exoplanets Could Be 'Very Different From Earth,' MIT Scientist Says

Despite the fact that Earth-like planets in habitable zones are being identified at a faster and faster rate, there's still no confirmation that life exists or could exist on any of them. One scientist is arguing that's because we need to think outside the box more.

Sara Seager, a professor of Planetary Science and Physics at MIT, has been a pioneer in the field of exoplanet atmospheres and urges scientists to look at more factors when determining habitability. She published her thoughts in the journal Science.

"Even with the requirement that a planet's surface temperature must be compatible with liquid water (because all life on Earth requires liquid water), a new emerging view is that planets very different from Earth may have the right conditions for life," writes Seager. "The broadened possibilities will increase the future chances of discovering an inhabited world."

The key to a planet's surface temperature, and therefore its ability to sustain liquid water, according to Seager, really lies in the atmosphere's greenhouse capabilities.

Planets with thin atmospheres, such as our very own, need their parents stars to provide the heat for them, however, a large planet lying far beyond the habitable zone could still keep warm enough for liquid water if it had copious amounts of hydrogen in its atmosphere. Hydrogen has an incredibly potent greenhouse effect and could, in theory, slowly bake a planet to the right temperature.

Atmospheric water is also important in creating the right mood for liquid water. The closer a planet is to its star, the drier it has to be in order to properly house liquid water, Seager says. She cites Venus, which might have turned out more livable if it had been drier in its initial stages. Instead, its high moisture levels led to an perpetuating greenhouse effect which has made the planet as unlivable and toxic as it is today.

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

"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."

You can learn more about exoplanets and the habitable zone through NASA's Kepler mission.