Geography
The Habitability of Tidally Locked Exoplanets in the Habitable Zone
Quick fact
Many of the most promising exoplanets for life, like TRAPPIST-1e, are tidally locked, meaning they always show the same face to their star. This creates a permanent dayside and nightside, with a narrow 'terminator' ring of twilight between them.
Why this is interesting
Imagine a planet where one side always faces its sun, forever baking, while the other side is trapped in eternal night. Could life survive on such a world?
Read the full explanation
Understanding The Habitability of Tidally Locked Exoplanets in the Habitable Zone
Tidal locking happens when a planet's rotation period matches its orbital period, so one hemisphere always faces the star. This occurs for planets close to their star, where the star's gravity stretches the planet and gradually slows its spin. For such a planet, the dayside receives all the starlight and can become scorching hot, while the nightside sees no sunlight at all and can be freezing. The habitable zone is the region where a planet could have liquid water on its surface, but for a tidally locked planet, the temperature difference between day and night is enormous. The question is whether the planet's atmosphere can redistribute heat to make the planet habitable despite the extreme contrast.
A deeper explanation
The key to habitability on a tidally locked planet is atmospheric circulation. If the planet has a thick atmosphere, winds can transport heat from the dayside to the nightside, moderating temperatures. For example, a dense atmosphere with a high heat capacity can create a 'heat runaway' loop that spreads warmth. The substellar point, where the star is directly overhead, receives the most energy and can become very hot, but if the atmosphere is efficient at moving heat, the nightside can remain above freezing. Computer models show that planets with at least 0.1 bar of CO2 can avoid a 'cold trap' on the nightside, allowing liquid water on the dayside and possibly the terminator. However, if the atmosphere is too thin, the nightside will freeze out all moisture, making the planet inhospitable. Thus, the habitability of tidally locked planets is a delicate balance of atmospheric pressure, composition, and circulation, and many factors like clouds and greenhouse gases can further tune the climate.