Sub-brown dwarf in the context of "Brown dwarf"

A satellite system is a set of gravitationally bound objects in orbit around a planetary mass object (incl. sub-brown dwarfs and rogue planets) or minor planet, or its barycenter. Generally speaking, it is a set of natural satellites (moons), although such systems may also consist of bodies such as circumplanetary disks, ring systems, moonlets, minor-planet moons and artificial satellites any of which may themselves have satellite systems of their own (see Subsatellites). Some bodies also possess quasi-satellites that have orbits gravitationally influenced by their primary, but are generally not considered to be part of a satellite system. Satellite systems can have complex interactions including magnetic, tidal, atmospheric and orbital interactions such as orbital resonances and libration. Individually major satellite objects are designated in Roman numerals. Satellite systems are referred to either by the possessive adjectives of their primary (e.g. "Jovian system"), or less commonly by the name of their primary (e.g. "Jupiter system"). Where only one satellite is known, or it is a binary with a common centre of gravity, it may be referred to using the hyphenated names of the primary and major satellite (e.g. the "Earth-Moon system").

Many Solar System objects are known to possess satellite systems, though their origin is still unclear. Notable examples include the Jovian system, with 95 known moons (including the large Galilean moons) and the largest overall, the Saturnian System, with 274 known moons (including Titan and the most visible rings in the Solar System alongside). Both satellite systems are large and diverse, in fact, all of the giant planets of the Solar System possess large satellite systems as well as planetary rings, and it is inferred that this is a general pattern. Several objects farther from the Sun also have satellite systems consisting of multiple moons, including the complex Plutonian system where multiple objects orbit a common center of mass, as well as many asteroids and plutinos. Apart from the Earth-Moon system and Mars' system of two tiny natural satellites, the other terrestrial planets are generally not considered satellite systems, although some have been orbited by artificial satellites originating from Earth.

This list covers all known stars, white dwarfs, brown dwarfs, and sub-brown dwarfs/rogue planets within 20 light-years (6.13 parsecs) of the Sun. So far, 131 such objects have been found. Only 22 are bright enough to be visible without a telescope, for which the star's visible light needs to reach or exceed the dimmest brightness visible to the naked eye from Earth, which is typically around 6.5 apparent magnitude.

The known 131 objects are bound in 94 stellar systems. Of those, 103 are main sequence stars: 80 red dwarfs and 23 "typical" stars having greater mass. Additionally, astronomers have found 6 white dwarfs (stars that have exhausted all fusible hydrogen), 21 brown dwarfs, as well as 1 sub-brown dwarf, WISE 0855−0714 (possibly a rogue planet). The closest system is Alpha Centauri, with Proxima Centauri as the closest star in that system, at 4.2465 light-years from Earth. The brightest, most massive and most luminous object among those 131 is Sirius A, which is also the brightest star in Earth's night sky; its white dwarf companion Sirius B is the hottest object among them. The largest object within the 20 light-years is Procyon.

A planetary-mass object (PMO), planemo, or planetary body (sometimes referred to as a world) is, by geophysical definition of celestial objects, any celestial object massive enough to achieve hydrostatic equilibrium and assume an ellipsoid shape, but not enough to sustain core fusion like a star.

The purpose of this term is to classify together a broader range of celestial objects than just "planet", since many objects similar in geophysical terms do not conform to conventional astrodynamic expectations for a planet. Planetary-mass objects can be quite diverse in origin and location, and include planets, dwarf planets, planetary-mass moons and free-floating planets, which may have been ejected from a system (rogue planets) or formed through cloud-collapse rather than accretion (sub-brown dwarfs).

WISE 0855−0714 (full designation WISE J085510.83−071442.5, or W0855 for short) is a sub-brown dwarf of spectral class Y4, located 7.4 light-years (2.3 parsecs) from the Sun in the constellation Hydra. It is the fourth-closest star or (sub-) brown dwarf system to the Sun and was discovered by Kevin Luhman in 2013 using data from the Wide-field Infrared Survey Explorer (WISE). It is the coldest brown dwarf found yet, having a temperature of about 285 K (12 °C; 53 °F). It has an estimated mass between 3 and 10 Jupiter masses, which makes it a planetary-mass object below the 13 Jupiter mass rough limit for deuterium fusion.

Kevin Luhman is a professor of astronomy and astrophysics from Pennsylvania State University who discovered both the third-closest stellar system, Luhman 16, and the fourth-closest stellar system, WISE 0855−0714, to the Sun. Both systems are composed of substellar objects (objects less massive than stars), falling into the category of brown dwarfs (Luhman 16) or even less massive objects (WISE 0855−0714) which are categorized as sub-brown dwarfs but also referred to as "free floating planets" or "planetary mass objects". WISE 0855−0714 (discovery published 2014) is the coldest massive object outside the Solar System that has been directly imaged.

Luhman 16 was named for its discoverer, following common practice for very nearby stars discovered in modern times.