Q:Information: - In physics, mass is a property of a physical body. It is the measure of an object's resistance to acceleration (a change in its state of motion) when a net force is applied. It also determines the strength of its mutual gravitational attraction to other bodies. The basic SI unit of mass is the kilogram (kg). - A planet is an astronomical body orbiting a star or stellar remnant that The term "planet" is ancient, with ties to history, astrology, science, mythology, and religion. Several planets in the Solar System can be seen with the naked eye. These were regarded by many early cultures as divine, or as emissaries of deities. As scientific knowledge advanced, human perception of the planets changed, incorporating a number of disparate objects. In 2006, the International Astronomical Union (IAU) officially adopted a resolution defining planets within the Solar System. This definition is controversial because it excludes many objects of planetary mass based on where or what they orbit. Although eight of the planetary bodies discovered before 1950 remain "planets" under the modern definition, some celestial bodies, such as Ceres, Pallas, Juno and Vesta (each an object in the solar asteroid belt), and Pluto (the first trans-Neptunian object discovered), that were once considered planets by the scientific community, are no longer viewed as such. - The dwarf planet Pluto has five moons down to a detection limit of about 1 km in diameter . In order of distance from Pluto , they are Charon , Styx , Nix , Kerberos , and Hydra . Charon , the largest of the five moons , is mutually tidally locked with Pluto , and is massive enough that Pluto -- Charon is sometimes considered a double dwarf planet . - Pluto (minor-planet designation: 134340 Pluto) is a dwarf planet in the Kuiper belt, a ring of bodies beyond Neptune. It was the first Kuiper belt object to be discovered. - A shape is the form of an object or its external boundary, outline, or external surface, as opposed to other properties such as color, texture, or material composition. - A dwarf planet is a planetary-mass object that is neither a planet nor a natural satellite. That is, it is in direct orbit of the Sun, and is massive enough for its gravity to crush it into a hydrostatic equilibrium shape (usually a spheroid), but has not cleared the neighborhood of other material around its orbit. - A natural satellite or moon is, in the most common usage, an astronomical body that orbits a planet or minor planet (or sometimes another Small Solar System Body). - Neptune is the eighth and farthest known planet from the Sun in the Solar System. In the Solar System, it is the fourth-largest planet by diameter, the third-most-massive planet, and the densest giant planet. Neptune is 17 times the mass of Earth and is slightly more massive than its near-twin Uranus, which is 15 times the mass of Earth and slightly larger than Neptune. Neptune orbits the Sun once every 164.8years at an average distance of . It is named after the Roman god of the sea and has the astronomical symbol , a stylised version of the god Neptune's trident. - The Kuiper belt or , sometimes called the EdgeworthKuiper belt, is a circumstellar disc in the Solar System beyond the planets, extending from the orbit of Neptune (at 30AU) to approximately 50AU from the Sun. It is similar to the asteroid belt, but it is far larger20 times as wide and 20 to 200 times as massive. Like the asteroid belt, it consists mainly of small bodies, or remnants from the Solar System's formation. Although many asteroids are composed primarily of rock and metal, most Kuiper belt objects are composed largely of frozen volatiles (termed "ices"), such as methane, ammonia and water. The Kuiper belt is home to three officially recognized dwarf planets: Pluto, Haumea, and Makemake. Some of the Solar System's moons, such as Neptune's Triton and Saturn's Phoebe, are also thought to have originated in the region. - In fluid mechanics, a fluid is said to be in hydrostatic equilibrium or hydrostatic balance when it is at rest, or when the flow velocity at each point is constant over time. This occurs when external forces such as gravity are balanced by a pressure gradient force. For instance, the pressure-gradient force prevents gravity from collapsing Earth's atmosphere into a thin, dense shell, whereas gravity prevents the pressure gradient force from diffusing the atmosphere into space. - A spheroid, or ellipsoid of revolution, is a quadric surface obtained by rotating an ellipse about one of its principal axes; in other words, an ellipsoid with two equal semi-diameters. Given the information above, choose from the list below the object entity that exhibits the relation 'subclass of' with the subject 'moons of pluto'. Choices: - acceleration - asteroid - average - belt - change - community - definition - diameter - dwarf - dwarf planet - eye - fluid - formation - giant - god - home - human - mass - metal - natural satellite - number - object - orbit - perception - physics - planet - point - pressure - property - quadric - revolution - ring - satellite - sea - shape - spheroid - star - state - system - third - thought - time - trident - velocity - water
A:
natural satellite