Tidal locking

Tidal locking is a special way that two space objects, like a planet and its moon, move around each other. When two objects are tidally locked, one of them stays in the same position as it goes around the other. This means we always see the same side of the Moon from Earth.

This happens because of the strong pull of gravity between the two objects. Over a very long time, millions of years, their gravity changes how they spin until they match their orbits. Once this happens, they stay that way because it would take a lot of energy to change it back.

Sometimes, both objects can be locked to each other. This is true for Pluto and its moon Charon. They both spin in a way that they always show each other the same face. Other objects, like Mercury, have a different kind of lock where it spins three times for every two times it goes around the Sun.

Mechanism

Further information: Centers of gravity in non-uniform fields

Imagine two objects, like a planet and its moon, moving around each other. Over time, one object can end up always showing the same face to the other. This happens because gravity pulls on parts of the object, creating small bumps called tidal bulges. These bulges move as the object spins, but they don't line up perfectly with the direction of the other object.

Because the bulges are slightly off-center, gravity from the other object pulls on them, slowing down or speeding up the spin of the first object. This pull acts like a brake, gradually making the spin match the orbit. Eventually, the object spins at just the right speed to keep the same face turned toward its partner.

Orbital changes

When one object slows down its spin, the space between them actually gets a little bigger. This happens because the total spin and orbit of the two objects must stay balanced. If one speeds up, the other slows down to keep everything moving smoothly.

Locking of the larger body

See also: Synchronous orbit

The bigger object can also be affected, but more slowly, because the smaller one pulls less on it. For example, the Moon is very slowly changing how fast Earth spins. Over millions of years, this has made our days a little longer. In the far future, both Earth and the Moon might end up always showing the same face to each other — though this will take longer than the Sun will exist.

For bodies of similar size the effect may be of comparable size for both, and both may become tidally locked to each other on a much shorter timescale. An example is the dwarf planet Pluto and its satellite Charon. They have already reached a state where Charon is visible from only one hemisphere of Pluto and vice versa.

Eccentric orbits

For orbits that are not perfect circles, the object may not always show the exact same face. Instead, it might spin a little faster or slower depending on where it is in its orbit. Some objects, like the planet Mercury, spin in a pattern called a resonance — for Mercury, it spins three times for every two times it goes around the Sun.

Many exoplanets might also have similar patterns, spinning in steps rather than perfectly matching their orbit.

Occurrence

Moons

All nineteen known moons in the Solar System that are big enough to be round stay in the same position relative to the planet they orbit. This happens because they are very close to their planet, and the pull of gravity gets stronger the closer they are. However, most of the smaller, distant moons of big planets are not locked this way.

Pluto and Charon are a special case where both are locked to each other. Charon is quite large compared to Pluto and orbits very close, so both worlds always show the same face to each other. Pluto’s other moons move in uneven ways because of Charon’s influence. Similarly, Eris and Dysnomia are also locked together, as are Salacia and Actaea. It’s possible that Orcus and Vanth are locked together, but we don’t have enough information to be sure.

Earth's Moon

The Moon’s rotation and orbit are locked together. This means that, no matter when you look at the Moon from Earth, you always see the same side. Most of the far side of the Moon wasn’t seen until 1959, when a Soviet spacecraft called Luna 3 sent back pictures.

When you look at Earth from the Moon, Earth seems to stay in almost the same spot in the sky. It shows almost its whole surface as it spins.

Even though the Moon’s rotation and orbit are locked, we can see about 59 percent of the Moon’s surface from Earth. This is because of movements called libration and parallax. Librations happen because the Moon’s path around Earth isn’t a perfect circle, which lets us see a little more of its edges. Parallax is a viewing effect that lets us see small changes in the Moon’s surface depending on where on Earth we are watching from.

Planets

For a while, people thought Mercury always showed the same face to the Sun. But in 1965, radar showed that Mercury actually spins three times for every two times it goes around the Sun. This is called a 3:2 spin–orbit resonance.

The time between times when Venus gets closest to Earth is almost exactly five Venus days. This makes it seem like Venus always shows us the same face, but we don’t know if this is because of tidal locking or just chance.

The planet Proxima Centauri b, found in 2016, is almost certainly locked to its star, either showing the same face or in a 3:2 resonance like Mercury.

Stars

Close pairs of stars are expected to be locked to each other. Planets that orbit very close to their stars are also thought to be locked. One possible example is the star Tau Boötis, which may be locked to its planet Tau Boötis b. If so, the locking would likely work both ways.

Timescale

To figure out how long it takes for a space object to become tidally locked, scientists use a special math formula. This formula helps them estimate the time it might take, but it’s not always exact because some details are hard to measure.

The formula depends on several things, like how fast the object spins, how far it is from what it’s orbiting, and what it’s made of. Even with good information, the time can vary a lot. For example, bigger moons tend to become tidally locked faster than smaller ones if they’re at the same distance. But other factors, like nearby objects, can also affect this.

List of known tidally locked bodies

Solar System

All the objects listed below are tidally locked, meaning they always show the same face to the object they orbit. Except for Mercury, all these objects also take the same amount of time to spin once on their own as they do to go around their partner.

Extra-solar

Many ways to find planets far from us tend to find those very close to their stars. Because of this, most found planets are in areas where they would be tidally locked to their stars. One example is Tau Boötis, which is locked to the large planet Tau Boötis b.

Bodies likely to be locked

Solar System

Some moons are believed to be tidally locked to their planets based on how long it would take them to lock and how long they have been orbiting. However, we do not know enough about their rotations to be certain. These include:

Probably locked to Saturn

• Daphnis
• Aegaeon
• Methone
• Anthe
• Pallene
• Helene
• Polydeuces

Probably locked to Uranus

• Cordelia
• Ophelia
• Bianca
• Cressida
• Desdemona
• Juliet
• Portia
• Rosalind
• Cupid
• Belinda
• Perdita
• Puck
• Mab

Probably locked to Neptune

• Naiad
• Thalassa
• Despina
• Galatea
• Larissa

Probably mutually tidally locked

• Orcus and Vanth

Extrasolar

• Gliese 581c, Gliese 581b, and Gliese 581e may be tidally locked to their parent star Gliese 581.
• All planets in the TRAPPIST-1 system are likely to be tidally locked.