Tidal locking is a phenomenon where a planet of an astronomical object that always faces one side in its orbit. It is most common on moons and planets orbiting red dwarfs. This is known when a body in space orbits another body in a way that the body’s orbital and rotational periods are equal in length. The body spins around its own axis once for each time it orbits around another, specific body in space. By having equal years and days, this orbiting body shows the same side of its face to those looking at it from the other body, or the body that being rotated around. With some exceptions such as the Pluto-Charon system, only the satellite/moon is tidally locked to the larger body. Like Earth and the moon, the same side of the moon always faces Earth, however, the orbit is not perfectly circular, like the vast majority of orbits.
A demonstration of tidal locking - one side of the Moon always faces Earth.
Examples[]
Moons[]
Most moons are tidally locked with their greatest gravitational attractors, as they orbit very closely, and tidal force decreases with increasing distance. Pluto and Charon are a special example of a tidal lock, where Charon is a large moon compared to its orbital attractor, and has a relatively close orbit. Pluto's other moons are more chaotically tidally locked due to the effect of Charon. The rotational and orbital periods of Earth's Moon are nearly identical to each other, so no matter when the Moon is observed from Earth, the same hemisphere of the Moon is always seen. When the Earth is observed from the Moon, the Earth appears to remain in the same place, rotating on its own axis.
The tidal locking of asteroidal moons is currently largely unknown.
Planets[]
Mercury has a 3:2 spin–orbit , rotating three times for every two revolutions around the Sun, which results in the same positioning at those observation points. Mercury has been in the 3:2 rotation state early after its formation. Venus' rotational and orbital periods only differ by approximately 13 Earth days in length; it is unknown if this is a result of tidal locking.
On the topic of exoplanets, planets orbiting in the habitable zone of a red dwarf star, or perhaps late (relatively cool) orange dwarfs, are very likely tidally locked, which poses an issue for habitability. That being said, there is still good reason to be optimistic:
- 1997 studies by NASA's Ames Research Center have shown that a planet's atmosphere (assuming it included greenhouse gases CO2 and H2O) need only be 100 millibar, or 10% of Earth's atmosphere, for the star's heat to be effectively carried to the night side, a figure well within the bounds of photosynthesis.
- Seawater, too, could effectively circulate without freezing solid if the ocean basins were deep enough to allow free flow beneath the night side's ice cap.
- A 2010 study concluded that Earth-like water worlds tidally locked to their stars would still have temperatures above 240 K (−33°C) on the night side; theoretically, organisms could sustain themselves on supercooled water sourced from the ice, and an ocean which is saline enough might be just on the edge of freezing.
- Climate models constructed in 2013 indicate that cloud formation on tidally locked planets would minimize the temperature difference between the day and the night side, greatly improving habitability prospects for red dwarf planets. Further research, including a consideration of the amount of photosynthetically active radiation, has suggested that tidally locked planets in red dwarf systems might at least be habitable for higher plants.
For instance, the exoplanet Proxima Centauri b orbits its parent star close enough to be tidally locked. That being said though, its orbit is probably elliptical, which would create a 3:2 spin–orbit resemblance like Mercury, or a spin-orbit resonance as high as 1:2 like Dione–Enceladus.
Stars[]
Close binary stars are found to be tidally locked, before being formed into 1 star. Tau Boötis, an unusual example, is a star that might be tidally locked to its planet Tau Boötis b.
Tidally locked Solar System objects[]
| Parent body | Tidally locked satellites |
|---|---|
| Sun | Mercury (3:2 rotation) |
| Earth | Moon |
| Mars | Phobos, Deimos |
| Jupiter | Metis, Andrastea, Callisto, Ganymede, Europa, Io, Thebe, Amalthea |
| Saturn | Pan, Atlas, Prometheus, Pandora, Epimetheus, Janus, Mimas |
| Uranus | Miranda, Ariel, Umbriel, Titania, Oberon |
| Neptune | Proteus, Triton |