What is tidally locked: Definition and 13 Discussions

Tidal locking between a pair of co-orbiting astronomical bodies occurs when one of the objects reaches a state where there is no longer any net change in its rotation rate over the course of a complete orbit. In the case where a tidally locked body possesses synchronous rotation, the object takes just as long to rotate around its own axis as it does to revolve around its partner. For example, the same side of the Moon always faces the Earth, although there is some variability because the Moon's orbit is not perfectly circular. Usually, only the satellite is tidally locked to the larger body. However, if both the difference in mass between the two bodies and the distance between them are relatively small, each may be tidally locked to the other; this is the case for Pluto and Charon, as well as for Eris and Dysnomia. Alternative names for the tidal locking process are gravitational locking, captured rotation, and spin–orbit locking.
The effect arises between two bodies when their gravitational interaction slows a body's rotation until it becomes tidally locked. Over many millions of years, the interaction forces changes to their orbits and rotation rates as a result of energy exchange and heat dissipation. When one of the bodies reaches a state where there is no longer any net change in its rotation rate over the course of a complete orbit, it is said to be tidally locked. The object tends to stay in this state because leaving it would require adding energy back into the system. The object's orbit may migrate over time so as to undo the tidal lock, for example, if a giant planet perturbs the object.
Not every case of tidal locking involves synchronous rotation. With Mercury, for example, this tidally locked planet completes three rotations for every two revolutions around the Sun, a 3:2 spin–orbit resonance. In the special case where an orbit is nearly circular and the body's rotation axis is not significantly tilted, such as the Moon, tidal locking results in the same hemisphere of the revolving object constantly facing its partner.
However, in this case the exact same portion of the body does not always face the partner on all orbits. There can be some shifting due to variations in the locked body's orbital velocity and the inclination of its rotation axis.

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  1. Strato Incendus

    Tidally-locked planets flipping around their axes

    Recently, I’ve heard from Anton Petrov on YouTube that some tidally-locked planets around red-dwarf stars (such as TRAPPIST-1) have been suspected to flip around their own axis every once in a while — so that the former day side becomes the night side, and vice versa. This is presumed to happen...
  2. A

    B Does being tidally locked affect a planet's magnetic field?

    Quick question from an amateur: Assume you have a tidally locked rocky planet around a red dwarf star. I've heard that a magnetic field is or can be dependent upon the rotation of a planet (I've read that the lack of rotation of Venus was a possible reason for the weak magnetic field among...
  3. A

    I Tides on a nearly tidally locked planet

    Non-Scientist here. Assume that you had a two body system (planet-moon, double-planet) where one of the bodies was very close to tidally locked. Say they are closely co-orbiting at 6 to ten diameters in a relatively quick 20 to 50 hour orbit. One of the planets is close to being tidally...
  4. A

    I Question about a magnetic dynamo on tidally locked moons

    All, Non-scientist amateur looking for an answer. I understand that planetary rotation aid is vital to maintain a magnetic dynamo. Venus' slow reverse rotation is often used as an explanation for its odd low magnetic field. Assume there is a moon or a double planet that is tidally locked...
  5. AstroCody

    B Tidally Locked Planets: How Close to the Star?

    How close must a planet be to its parent star in order for it to be tidally locked?
  6. H

    At what distance would binary planets not be tidally locked?

    Working on a story idea that involves binary worlds. To keep the math simple I'm assuming the planets to be Earth sized, and mass, orbiting a star identical to the sun. I managed to find someone to help with the first equations and discovered that based on the time it takes them to orbit each...
  7. Algr

    Tidally locked planet's sun still moves?

    Hi PF! So I was reading about the possibility of life on planets orbiting red dwarf stars, and it was said that the major problem was that the planet would become tidally locked. With half the planet in eternal darkness, the whole atmosphere would freeze on the dark side, leaving the planet...
  8. S

    Habitability of tidally locked exoplanets

    There's been a recent upsurge of interest in evidence for rocky planets orbiting red dwarf stars. Red dwarfs are very common in the the Milky Way and can live much longer than larger stars such as our Sun. The possible existence of several rocky planets in the habitable zone of Gliese 581 has...
  9. M

    How long will it take for the Earth to become tidally locked to the Moon?

    According to an article I read, http://www.badastronomy.com/bad/misc/tides.html the Earth will one day become tidally locked to the moon the way the moon is locked to the Earth today, so that the same face of the Earth always faces the moon. If this is true, how long will that process take?
  10. H

    Tidally locked planet - shouldn't the Goldilocks zone be wider?

    Okay, so planets in the Goldilocks zone are far enough from the star that it's less likely they are tidally locked. But assuming such a thing exists - shouldn't the habitable zone be wider if the planet is tidally locked to its star? At one extreme, the planet would have an Antarctica-sized...
  11. A

    Will the earth and sun ever be tidally locked?

    suppose there are two bodies,one revolving in an orbit around the other[like the Earth moon system]Differences in orbital and axial rotation of a small body results in a torque applied on it by the larger body. This results in the smaller body being tidally locked. THEN WHY ISN'T THE EARTH...
  12. L

    How can you tell that a planet in another star system is tidally locked?

    My question was piqued by this article: http://news.discovery.com/space/earth-like-planet-life.html The article claims that the planet discovered is tidally locked, but does not explain how this was determined. Presumably the planet is much too far away to be resolved by any telescope, and...
  13. R

    How Far Must a Planet Be to Become Tidally Locked with Different Star Types?

    Is there an equation to predict the distance of a hypothetical planet from it's host star to be locked? (For G, K, M stars)