Co-orbital configuration

In astronomy, a co-orbital configuration is a collection of celestial objects (such as asteroids, moons, or planets) that orbit at the same (or very similar) distance from their parent object as each other. Co-orbital objects are in a 1:1 mean motion resonance.

A trojan object shares an orbit with a larger object, but does not collide with it because it orbits around one of the two Lagrangian points of stability (Trojan points), L₄ and L₅, which lie 60° ahead of and behind the larger body.

An exchange orbit is a where a pair of objects swap semimajor axis or eccentricity when they approach each other.

Trojan asteroids

There are several thousand known Trojan asteroids which orbit the Sun at the same distance as a planet.

Co-orbital moons

The only known co-orbital moons are those in the Saturn system, which has three sets of co-orbital moons.

Saturn's moon Tethys has two Trojan moons (Telesto and Calypso), and Dione also has two Trojan moons (Helene and Polydeuces).

The Saturnian moons Janus and Epimetheus swap orbits when they approach each other—for details see Orbital relationship between Epimetheus and Janus.

Co-orbital planets

The discovery of a pair of co-orbital exoplanets has been reported but later retracted.^[1]

In addition to swapping semi-major axis like Saturn's moons Epimetheus and Janus, another possibility is to share the same axis, but swap eccentricity instead.^[2]

One possibility for the habitable zone is a trojan planet of a gas giant close to its star.^[3]

Formation of Earth–Moon system

According to the giant impact hypothesis, Earth's Moon was formed after a collision between two co-orbiting objects – Theia, believed to be about as massive as Mars, and proto-Earth – whose orbits were perturbed by other planets, bringing Theia out of its trojan position and causing the collision.

See also

• Kordylewski cloud
• Quasi-satellite
• Double planet

References

1. ^ Two planets found sharing one orbit, New Scientist, 24 February 2011
2. ^ Exchange orbits: a possible application to extrasolar planetary systems?, B. Funk, R. Schwarz, R. Dvorak, M. Roth
3. ^ Extrasolar Trojan Planets close to Habitable Zones, R. Dvorak, E. Pilat-Lohinger, R. Schwarz, F. Freistetter

• Eric B. Ford and Matthew J. Holman (2007). "Using Transit Timing Observations to Search for Trojans of Transiting Extrasolar Planets". The Astrophysical Journal Letters 664 (1): L51–L54. Bibcode 2007ApJ...664L..51F. doi:10.1086/520579. http://www.iop.org/EJ/article/0004-637X/571/1/528/55113.html. 

External links

• A Search for Trojan Planets Web page of group of astronomers searching for extra-solar Trojan Planets at Appalachian State University