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A minor-planet moon is an astronomical object that orbits a minor planet as its natural satellite. It is thought that many asteroids and Kuiper belt objects may possess moons, in some cases quite substantial in size. Discoveries of minor-planet moons (and binary objects, in general) are important because the determination of their orbits provides estimates on the mass and density of the primary, allowing insights of their physical properties that is generally not otherwise possible.
Contents
- Terminology
- Discovery milestones
- Triple systems
- Commonality
- Origin
- Populations
- Dwarf planets
- Near Earth objects
- Mars crossers
- Asteroid belt
- Jupiter trojans
- Centaurs
- Trans Neptunian objects
- References
The first modern era mention of the possibility of an asteroid satellite was in connection with an occultation of the bright star Gamma Ceti by the minor planet (6) Hebe in 1977. The observer, amateur astronomer Paul D. Maley, detected an unmistakable 0.5 second disappearance of this naked eye star from a site near Victoria, Texas. Many hours later, several observations were reported in Mexico attributed to the occultation by (6) Hebe itself. Although not confirmed this documents the first formally documented case of a suspected companion of an asteroid. As of October 2016, there are over 300 minor planets known to have moons.
Terminology
In addition to the terms satellite and moon, the term "binary" (binary minor planet) is sometimes used for minor planets with moons, and "triple" for minor planets with two moons. If one object is much bigger it can be referred to as the primary and its companion as secondary. The term double asteroid is sometimes used for systems in which the asteroid and its moon are roughly the same size, while binary tends to be used independently from the relative sizes of the components. When binary minor planets are similar in size, the Minor Planet Center (MPC) refers to them as "binary companions" instead of referring to the smaller body as a satellite. A good example of a true binary is the 90 Antiope system, identified in August 2000. Small satellites are often referred to as moonlets.
Discovery milestones
As of February 2017, over 330 moons of minor planets have been discovered. These consist of:
Prior to the era of the Hubble Space Telescope and space probes reaching the outer Solar System, attempts to detect satellites around asteroids were limited to optical observations from Earth. For example, in 1978, stellar occultation observations were claimed as evidence of a satellite for the asteroid 532 Herculina. However, later more-detailed imaging by the Hubble Telescope did not reveal a satellite, and the current consensus is that Herculina does not have a significant satellite. There were other similar reports of asteroids having companions (usually referred to as satellites) in the following years. Also, a letter in Sky & Telescope magazine at this time pointed to apparently simultaneous impact craters on Earth (for example, the Clearwater Lakes in Quebec), suggesting that these craters were caused by pairs of gravitationally-bound objects.
In 1993, the first asteroid moon was confirmed when the Galileo probe discovered the small Dactyl orbiting 243 Ida in the asteroid belt. The second was discovered around 45 Eugenia in 1998. In 2001, 617 Patroclus and its same-sized companion Menoetius became the first known binary asteroids in the Jupiter trojans. The first trans-Neptunian binary after Pluto–Charon, 1998 WW31, was optically resolved in 2002.
Triple systems
Triple asteroids, or trinary asteroids, are known since 2005, when the asteroid 87 Sylvia was discovered to have two satellites, making it the first known triple system. This was followed by the discovery of a second moon orbiting 45 Eugenia. Also in 2005, the Kuiper belt object (KBO) Haumea was discovered to have two moons, making it the second KBO after Pluto known to have more than one moon.
Additionally, 216 Kleopatra and 93 Minerva were discovered to be trinary asteroids in 2008 and 2009 respectively. Since the first few trinary asteroids were discovered, more continue to be discovered at a rate of about one a year. Most recently discovered was a third moon orbiting the belt asteroid 130 Elektra, bringing the number of known trinary asteroids in the asteroid belt up to 6.
List of multiple minor planets:
Commonality
The data about the populations of binary objects are still patchy. In addition to the inevitable observational bias (dependence on the distance from Earth, size, albedo and separation of the components) the frequency appears to be different among different categories of objects. Among asteroids, an estimated 2% would have satellites. Among trans-Neptunian objects (TNO), an estimated 11% are thought to be binary or multiple objects, and the majority of the large TNOs have at least one satellite, including all four IAU-listed dwarf planets.
More than 50 binaries are known in each of the main groupings: near-Earth asteroids, belt asteroids, and trans-Neptunian objects, not including numerous claims based solely on light-curve variation.
Two binaries have been found so far among centaurs with semi-major axes smaller than Neptune. Both are double ring systems around 2060 Chiron and 10199 Chariklo, discovered in 1994–2011 and 2013 respectively.
Origin
The origin of minor-planet moons is not currently known with certainty, and a variety of theories exist. A widely accepted theory is that minor-planet moons are formed from debris knocked off of the primary by an impact. Other pairings may be formed when a small object is captured by the gravity of a larger one.
Formation by collision is constrained by the angular momentum of the components, i.e. by the masses and their separation. Close binaries fit this model (e.g. Pluto–Charon). Distant binaries however, with components of comparable size, are unlikely to have followed this scenario, unless considerable mass has been lost in the event.
The distances of the components for the known binaries vary from a few hundreds of kilometres (243 Ida, 3749 Balam) to more than 3000 km (379 Huenna) for the asteroids. Among TNOs, the known separations vary from 3,000 to 50,000 km.
Populations
What is "typical" for a binary system tends to depend on its location in the Solar System (presumably because of different modes of origin and lifetimes of such systems in different populations of minor planets).
Dwarf planets
Among the dwarf planets, it is 90 percent certain that Ceres has no moons larger than 1 km in size, assuming that they would have the same albedo as Ceres itself.
Pluto has five known moons. Its largest moon Charon is more than half the size of Pluto itself, and large enough to orbit a point outside Pluto's surface. In fact, each orbits the common barycenter between them, with Pluto's orbit entirely enclosed by Charon's; thus they form a binary system informally referred to as a double dwarf planet. Pluto's four other moons, Nix, Hydra, Kerberos, and Styx, are far smaller and orbit the Pluto–Charon system.
Haumea has two moons with radii estimated around 155 km (Hiʻiaka) and 85 km (Namaka).
Makemake has one known moon, S/2015 (136472) 1, estimated to be some 160 kilometers (100 mi) in diameter.
Eris has one known moon, Dysnomia. Its radius, based on its brightness, is estimated to be roughly between 150 and 350 km.
Near-Earth objects
There are 61 known near-Earth asteroids with moons (16 Amor, 37 Apollo, 7 Aten, and 1 Atira) with a total of 63 moons.
Mars crossers
There are 22 Mars-crossing asteroids with moons with a total of 23 moons.
Asteroid belt
There are 133 asteroids with 140 moons in the asteroid belt. Asteroids with light blue backgrounds are double asteroids, with similarly-sized components, and a Barycenter outside of the larger object. very light blue asteroids might be double asteroids, but due to errors in their size and orbit, it is uncertain.
Jupiter trojans
There are 18 Jupiter trojans with known moons.
Centaurs
There are only 2 centaurs with satellites, both of which are ring systems instead of moons.
Trans-Neptunian objects
There are 81 Trans-Neptunian objects with moons, with a total of 87 moons.