Co-orbital configuration
Encyclopedia
In astronomy
, a co-orbital configuration refers to two or more celestial objects
(such as asteroids, moons, or planets) that orbit at the same, or very similar, distance from their parent object as each other, i.e. they are in a 1:1 mean motion resonance
.
There are several classes of co-orbital objects, depending on their point of libration
. The most common and best-known class is the trojan
, which librate around one of the two stable Lagrangian point
s (Trojan points), and , 60° ahead of and behind the larger body respectively. Another class is the horseshoe orbit
, in which objects librate around 180° from the larger body. Objects librating around 0° are called quasi-satellite
s.
An exchange orbit is a where a pair of objects swap semi-major axis
or eccentricity when they approach each other.
. Most of these orbit near Jupiter
's Lagrangian points, the traditional Jupiter Trojans. Neptune
has 8 known trojan objects
, Mars
4 known ones, and Earth
one
, .
and Dione
have two trojan moons. Telesto
and Calypso
in Tethys's and respectively, and Helene
and Polydeuces
in Dione's and respectively.
One possibility for the habitable zone
is a trojan planet of a gas giant
close to its star
.
, Earth's Moon
was formed after a collision between two co-orbiting objects – Theia, believed to have had about 10% of the mass of Earth (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.
ian moons Janus
and Epimetheus
swap orbits when they approach each other—for details see Orbital relationship between Epimetheus and Janus.
Astronomy
Astronomy is a natural science that deals with the study of celestial objects and phenomena that originate outside the atmosphere of Earth...
, a co-orbital configuration refers to two or more celestial objects
Astronomical object
Astronomical objects or celestial objects are naturally occurring physical entities, associations or structures that current science has demonstrated to exist in the observable universe. The term astronomical object is sometimes used interchangeably with astronomical body...
(such as asteroids, moons, or planets) that orbit at the same, or very similar, distance from their parent object as each other, i.e. they are in a 1:1 mean motion resonance
Orbital resonance
In celestial mechanics, an orbital resonance occurs when two orbiting bodies exert a regular, periodic gravitational influence on each other, usually due to their orbital periods being related by a ratio of two small integers. Orbital resonances greatly enhance the mutual gravitational influence of...
.
There are several classes of co-orbital objects, depending on their point of libration
Libration
In astronomy, libration is an oscillating motion of orbiting bodies relative to each other, notably including the motion of the Moon relative to Earth, or of Trojan asteroids relative to planets.-Lunar libration:...
. The most common and best-known class is the trojan
Trojan (astronomy)
In astronomy, a Trojan is a minor planet or natural satellite that shares an orbit with a larger planet or moon, but does not collide with it because it orbits around one of the two Lagrangian points of stability , and , which lie approximately 60° ahead of and behind the larger body,...
, which librate around one of the two stable Lagrangian point
Lagrangian point
The Lagrangian points are the five positions in an orbital configuration where a small object affected only by gravity can theoretically be stationary relative to two larger objects...
s (Trojan points), and , 60° ahead of and behind the larger body respectively. Another class is the horseshoe orbit
Horseshoe orbit
A horseshoe orbit is a type of co-orbital motion of a small orbiting body relative to a larger orbiting body . The orbital period of the smaller body is very nearly the same as for the larger body, and its path appears to have a horseshoe shape in a rotating reference frame as viewed from the...
, in which objects librate around 180° from the larger body. Objects librating around 0° are called quasi-satellite
Quasi-satellite
A quasi-satellite is an object in a 1:1 orbital resonance with its planet that stays close to the planet over many orbital periods.A quasi-satellite's orbit around the Sun takes exactly the same time as the planet's, but has a different eccentricity , as shown in the diagram on the right...
s.
An exchange orbit is a where a pair of objects swap semi-major axis
Semi-major axis
The major axis of an ellipse is its longest diameter, a line that runs through the centre and both foci, its ends being at the widest points of the shape...
or eccentricity when they approach each other.
Trojans
Trojan minor planets
There are several thousand known trojan minor planets orbiting the SunSun
The Sun is the star at the center of the Solar System. It is almost perfectly spherical and consists of hot plasma interwoven with magnetic fields...
. Most of these orbit near Jupiter
Jupiter
Jupiter is the fifth planet from the Sun and the largest planet within the Solar System. It is a gas giant with mass one-thousandth that of the Sun but is two and a half times the mass of all the other planets in our Solar System combined. Jupiter is classified as a gas giant along with Saturn,...
's Lagrangian points, the traditional Jupiter Trojans. Neptune
Neptune
Neptune is the eighth and farthest planet from the Sun in the Solar System. Named for the Roman god of the sea, it is the fourth-largest planet by diameter and the third largest by mass. Neptune is 17 times the mass of Earth and is slightly more massive than its near-twin Uranus, which is 15 times...
has 8 known trojan objects
Neptune Trojan
Neptune trojans are Kuiper belt object-like bodies in solar orbit that have the same orbital period as Neptune and follow roughly the same orbital path...
, Mars
Mars
Mars is the fourth planet from the Sun in the Solar System. The planet is named after the Roman god of war, Mars. It is often described as the "Red Planet", as the iron oxide prevalent on its surface gives it a reddish appearance...
4 known ones, and Earth
Earth
Earth is the third planet from the Sun, and the densest and fifth-largest of the eight planets in the Solar System. It is also the largest of the Solar System's four terrestrial planets...
one
Earth Trojan asteroid
Earth trojans are asteroids that orbit in the vicinity of the Earth-Sun Lagrangian points and . They are named after the Trojan asteroids that are associated with the analogous Lagrangian points of Jupiter....
, .
Trojan moons
The Saturnian system contains two sets of trojan moons. Both TethysTethys (moon)
Tethys or Saturn III is a mid-sized moon of Saturn about across. It was discovered by G. D. Cassini in 1684 and is named after titan Tethys of Greek mythology. Tethys is pronounced |Odysseus]] is about 400 km in diameter, while the largest graben—Ithaca Chasma is about 100 km wide and...
and Dione
Dione (moon)
Dione is a moon of Saturn discovered by Cassini in 1684. It is named after the titan Dione of Greek mythology. It is also designated Saturn IV.- Name :...
have two trojan moons. Telesto
Telesto (moon)
Telesto is a moon of Saturn. It was discovered by Smith, Reitsema, Larson and Fountain in 1980 from ground-based observations, and was provisionally designated '. In the following months, several other apparitions were observed: , , and ....
and Calypso
Calypso (moon)
Calypso is a moon of Saturn. It was discovered in 1980, from ground-based observations, by Dan Pascu, P. Kenneth Seidelmann, William A. Baum, and Douglas G. Currie, and was provisionally designated ' . Several other apparitions of it were recorded in the following months: , , , and...
in Tethys's and respectively, and Helene
Helene (moon)
Helene is a moon of Saturn. It was discovered by Pierre Laques and Jean Lecacheux in 1980 from ground-based observations at Pic du Midi Observatory, and was designated . In 1988 it was officially named after Helen of Troy, who was the granddaughter of Cronus in Greek mythology...
and Polydeuces
Polydeuces (moon)
Polydeuces is a very small natural satellite of Saturn that is co-orbital with Dione and librates around the trailing Lagrangian point . Its diameter is estimated to be about 3.5 km....
in Dione's and respectively.
Trojan planets
The discovery of a pair of co-orbital exoplanets has been reported but later retracted.One possibility for the habitable zone
Habitable zone
In astronomy and astrobiology, a habitable zone is an umbrella term for regions that are considered favourable to life. The concept is inferred from the empirical study of conditions favourable for Life on Earth...
is a trojan planet of a gas giant
Gas giant
A gas giant is a large planet that is not primarily composed of rock or other solid matter. There are four gas giants in the Solar System: Jupiter, Saturn, Uranus, and Neptune...
close to its star
Star
A star is a massive, luminous sphere of plasma held together by gravity. At the end of its lifetime, a star can also contain a proportion of degenerate matter. The nearest star to Earth is the Sun, which is the source of most of the energy on Earth...
.
Formation of the Earth–Moon system
According to the giant impact hypothesisGiant impact hypothesis
The giant impact hypothesis states that the Moon was created out of the debris left over from a collision between the young Earth and a Mars-sized body. The colliding body is sometimes called Theia for the mythical Greek Titan who was the mother of Selene, the goddess of the moon.The giant impact...
, Earth's Moon
Moon
The Moon is Earth's only known natural satellite,There are a number of near-Earth asteroids including 3753 Cruithne that are co-orbital with Earth: their orbits bring them close to Earth for periods of time but then alter in the long term . These are quasi-satellites and not true moons. For more...
was formed after a collision between two co-orbiting objects – Theia, believed to have had about 10% of the mass of Earth (about as massive as Mars
Mars
Mars is the fourth planet from the Sun in the Solar System. The planet is named after the Roman god of war, Mars. It is often described as the "Red Planet", as the iron oxide prevalent on its surface gives it a reddish appearance...
), and proto-Earth – whose orbits were perturbed by other planets, bringing Theia out of its trojan position and causing the collision.
Horseshoe orbits
Objects in a horseshoe orbit librate around 180° from the primary. Their orbits encompass both Lagrangian points.Co-orbital moons
The SaturnSaturn
Saturn is the sixth planet from the Sun and the second largest planet in the Solar System, after Jupiter. Saturn is named after the Roman god Saturn, equated to the Greek Cronus , the Babylonian Ninurta and the Hindu Shani. Saturn's astronomical symbol represents the Roman god's sickle.Saturn,...
ian moons Janus
Janus (moon)
Janus is an inner satellite of Saturn. It is also known as Saturn X . It is named after the mythological Janus.-Discovery and orbit:Janus occupies practically the same orbit as the moon Epimetheus...
and Epimetheus
Epimetheus (moon)
Epimetheus is an inner satellite of Saturn. It is also known as Saturn XI. It is named after the mythological Epimetheus, brother of Prometheus.-Discovery:Epimetheus occupies essentially the same orbit as the moon Janus...
swap orbits when they approach each other—for details see Orbital relationship between Epimetheus and Janus.
Quasi-satellite
Quasi-satellites are objects librating around 0° from the primary. Low-eccentricity quasi-satellite orbits are highly unstable, but for moderate to high eccentricities such orbits can be stable.Exchange orbits
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.External links
- Quicktime animation of co-orbital motion from Murray and Dermott
- Cassini Observes the Orbital Dance of Epimetheus and Janus The Planetary Society
- A Search for Trojan Planets Web page of group of astronomers searching for extrasolar trojan planets at Appalachian State University