Tidal Locking Explained: Worlds That Face One Side to Their Star

Tidal locking happens when the gravity of a nearby body slows a world’s rotation until it no longer changes over the course of one orbit. In the simplest case, called synchronous rotation, the rotation period equals the orbital period, so the same hemisphere always faces the partner. The Moon is the familiar example: from Earth we see the same face of it every night.

This article explains the mechanism, shows how it plays out in the Solar System, and describes why astronomers care about it when they study planets around other stars.

How tides slow a rotation

Gravity pulls harder on the near side of a body than on the far side. That difference stretches the body slightly into an egg shape, creating tidal bulges. If the body spins faster than it orbits, the bulges are dragged ahead of the line joining the two bodies, and the partner’s gravity pulls back on them. The result is a small torque that gradually slows the spin.

Friction inside the body turns the lost rotational energy into heat. Over a long enough time the rotation settles into a stable pattern where the bulge stays pointed at the partner and the torque vanishes. The process works in both directions: the larger body is also slowed by the smaller one, though far more slowly.

How long it takes depends very strongly on the separation between the two bodies. According to Wikipedia, estimates of locking time are rough and can be wrong by orders of magnitude, because they depend on poorly known properties such as how the interior of the planet deforms.

The Moon and other Solar System examples

• The Moon. It keeps one face toward Earth. Because its orbit is not a perfect circle and because of small wobbles called libration, observers over time can see about 59 percent of the lunar surface, according to Wikipedia, rather than exactly half.
• Pluto and Charon. The two are mutually locked. They are close in mass and orbit close together, so each keeps the same face toward the other.
• Many large moons of the giant planets. Several of the large moons of Jupiter and Saturn are also considered synchronously locked.

Locked is not the only outcome

Tidal locking is often described as “one face always toward the star”, but that holds only for 1:1 synchronous rotation. Some scientific sources use the term only for this case, while others include other stable resonances.

Mercury shows why. It is not in synchronous rotation. It completes three rotations for every two orbits around the Sun, a 3:2 spin-orbit resonance. The planet’s eccentric orbit is part of why this resonance is stable, and the pattern means one sunrise-to-sunrise day on Mercury spans two of its years.

Why tidal locking matters for exoplanets

Planets that orbit close to their stars feel strong tides, so many are likely to be locked. This is especially true around red dwarfs, which are cooler than the Sun and have habitable zones at small distances. Wikipedia describes Proxima Centauri b as “almost certainly tidally locked”, either synchronised or in a 3:2 resonance, and says the planets of the TRAPPIST-1 system are likely to be locked too.

Detection bias complicates the picture. The same source notes that about 85 percent of detected exoplanets lie inside the zone where tidal locking is expected, which probably reflects in part that close-in planets are easier to find. The true proportion of locked planets in the galaxy is unclear.

A permanent dayside and nightside

If a planet is locked and has a roughly circular orbit, the stellar heating is always on the same hemisphere. This produces some distinctive features:

• Dayside. The hemisphere facing the star is continually heated.
• Nightside. The other hemisphere never receives direct starlight.
• Terminator. The boundary between them is a ring of permanent twilight.
• Atmospheric circulation. If a planet has an atmosphere, winds can carry heat from the day side to the night side and affect how hot or cold each region becomes.

Astronomers have built idealised models of these climates. One hypothetical class often described in the literature is the “eyeball planet”, where a patch of ocean or ice faces the star and the rest of the planet is frozen or dry. These models remain theoretical.

For gas giants that orbit very close to their stars, observations can measure how brightness changes as the planet moves around its orbit, which gives some information on the difference between the two hemispheres. How astronomers read such light is described in the overview of reading exoplanet atmospheres.

Spin, orbit and the study of planets

Rotation is one of the properties that shape what we can learn about a world, along with mass, radius and atmosphere. A doctoral thesis overview on spinning worlds shows how planetary spin can be probed with spectroscopy, and the publications page lists research on exoplanet spectroscopy.

Frequently asked questions

Does a tidally locked planet have no day and night?

It has a permanent day side and a permanent night side if it is in 1:1 synchronous rotation. The star does not move across the sky for an observer on the surface. A world in a different resonance, such as Mercury, still has a day-night cycle.

Is the far side of the Moon always dark?

No. The Moon keeps one hemisphere toward Earth, but the far side receives sunlight as the Moon orbits. The names “near side” and “far side” describe the view from Earth, not the lighting.

Is Mercury tidally locked?

Not in the strict 1:1 sense. Mercury turns three times for every two orbits, a 3:2 spin-orbit resonance, so it has a day-night cycle even though tides have shaped its rotation.

Can tidally locked planets be habitable?

Scientists regard this as an open question. Models suggest that an atmosphere that moves heat around could help, but such results depend on assumptions that remain uncertain.

Key points

Tidal locking is a consequence of gravity acting on a deformable body over a long time. The Moon is locked, Mercury is in a different resonance, and many planets around small stars are expected to be locked.