But you dont need a twilight zone to support life if you have the right atmosphere, says Abbot.

An atmosphere transports heat around the planet, making conditions that could support liquid water and perhaps life more widespread. It wouldn't just be at the terminator, Abbot says, it would be all over the place.

Theres a balancing act: The atmosphere needs to be dense enough to transport heat, but not so dense as to become smothering. Conversely, if the nightside becomes too cold, it could freeze the gases out, stealing an atmosphere entirely.

A 2016 simulation developed by Abbot and Koll indicates that the right balance is achievable: Some tidally locked exoplanets could host just right atmospheres that move heat around efficiently enough to keep even the night side warm.

It would be strange because it would be permanently night but you might still have conditions where life similar to ours could exist, says Koll. Consider the poles on Earth, where life persists, even though sunlight is scarce. It doesnt get too deadly cold, in great part because wind or motions of the ocean actually redistribute heat.

On Earth, oceans are key players in the global circulation of heat; water holds more heat energy than air and is far more efficient at moving that heat around. So, along with an atmosphere, oceans could play an important role in keeping the day and night sides of a tidally locked exoplanet temperate.

Oceans also lead to evaporation, spurring the formation of clouds that can also play an important role in regulating conditions on a planet, says Feng Ding, a researcher at Harvard studying exoplanet atmospheres. Clouds that form as water collects in the atmosphere can serve as a kind of reflective blanket, bouncing incoming stellar radiation back out and helping to cool a planet.

Some computer simulations suggest that clouds could keep temperatures low enough for oceans to form even on exoplanets that might otherwise be boiling hot. And those clouds would create rain. On the planets day side, strong updrafts created where sunlight is most intense would move warm, wet air upwards, creating torrential rainstorms, says Pierrehumbert.

Read more:

Beyond the twilight zone: Living on a two-faced world - Astronomy Magazine