Astronomy
The Tidal Locking of Exoplanets and Its Effect on Climate
Quick fact
Tidal locking does not automatically create a molten dayside and frozen nightside. Climate simulations of possible Proxima Centauri b atmospheres include temperate cases because winds, clouds and oceans can move heat away from the illuminated hemisphere.
Why this is interesting
A planet can orbit so that one hemisphere continually faces its star. Does that condemn one side to an inferno and the other to permanent ice, or can an atmosphere turn two extremes into a habitable world?
Read the full explanation
Understanding The Tidal Locking of Exoplanets and Its Effect on Climate
A synchronously rotating planet turns once during each orbit, so almost the same hemisphere faces the star. Gravitational tides can drive a close-in planet toward that state by exerting a torque on tidal bulges, although the final spin can also depend on orbital eccentricity, interior properties and atmospheric effects. The climate then depends on energy transport. With little or no atmosphere, the surface can develop a large day-night temperature contrast. With a substantial atmosphere, winds carry warm air toward the dark side and return cooler air toward the lit side. An ocean can move and store additional heat, while dayside clouds can reflect incoming starlight. These processes can keep the nightside from collapsing into extreme cold and can prevent the substellar region from becoming excessively hot. Proxima Centauri b is often modelled in synchronous or other spin-orbit states, but its actual surface conditions and atmosphere have not been measured. It is therefore incorrect to describe its dayside as known to melt metal.
A deeper explanation
Tidal forces arise because a star pulls slightly more strongly on the near side of a planet than on the far side. If the resulting tidal bulge is not aligned with the star, gravity applies a torque that exchanges rotational and orbital angular momentum and dissipates energy inside the planet. Over time this can favour a spin-orbit resonance. A circular close orbit often makes 1:1 synchronous rotation plausible, but it is not a universal outcome: eccentricity, the body's response to deformation, companion planets and atmospheric tides can alter the evolution. Climate must then be calculated rather than inferred from the word 'locked.' The temperature pattern reflects the balance between absorbed starlight, infrared cooling and transport by the atmosphere or ocean. Atmospheric mass, radiative gases, cloud formation, surface pressure and circulation speed all affect that balance. General-circulation models for Proxima Centauri b have produced scenarios with liquid-water-compatible temperatures under some assumed atmospheres and oceans, alongside colder or less habitable cases under other assumptions. A permanent terminator can be climatically interesting, but it is not necessarily the planet's only moderate region; efficient transport can broaden temperate conditions. Conversely, a thin atmosphere may move too little heat. Tidal locking therefore sets the geography of illumination, while atmospheric and surface physics determine the climate.