Tidal locked planet

Tidal locked planets always show the same face to their star, creating a scorching dayside and a freezing nightside.

About this subject

Tidal locking, or synchronous rotation, is common among exoplanets orbiting close to their stars. The star's gravity stretches the planet, slowing its spin until the orbital and rotational periods match. The most famous example in our Solar System is the Moon, which always faces Earth. Among exoplanets, Proxima Centauri b, in the habitable zone of the nearest star, is likely tidally locked. This creates extreme contrasts: a permanent dayside with scorching temperatures that could boil oceans, and a permanent nightside with ice and possible nitrogen deposits. The terminator line between day and night may have temperate conditions, especially if an atmosphere redistributes heat. Planets around red dwarfs, like those in the TRAPPIST-1 system, are also subject to this effect. Direct observation is challenging, but climate models suggest strong winds can transport energy and moderate temperature differences. Interestingly, Venus almost became tidally locked to the Sun but its slow retrograde rotation prevents equilibrium.

Frequently Asked Questions

What is a tidally locked planet?

It is a planet whose rotation period equals its orbital period, so the same side always faces the star. One side is permanent day, the other permanent night.

How does tidal locking affect the possibility of life?

Theoretically, the terminator zone (day-night boundary) could have mild temperatures. If a thick atmosphere exists, heat can be redistributed, potentially allowing habitable conditions.

Are all close-in exoplanets tidally locked?

No, but most with very short orbits (less than 50 days) likely are. The time needed for locking depends on distance and the planet's internal rigidity.