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A primary (also called a gravitational primary, primary body, or central body) is the main 
The center of mass is the average position of all the objects weighed by mass. The Sun is so massive that the Solar System's barycenter frequently lies very near the Sun's center but owing to the mass and distance of the gas giant planets, the Solar System's barycenter occasionally lies outside the Sun as well, despite the Sun comprising most of the Solar System's mass.
A disputed example of a system that may lack a primary is
physical body
In common usage and classical mechanics, a physical object or physical body (or simply an object or body) is a collection of matter within a defined contiguous boundary in three-dimensional space. The boundary must be defined and identified by ...
of a gravitationally bound, multi-object system. This object constitutes most of that system's mass
Mass is an intrinsic property of a body. It was traditionally believed to be related to the quantity of matter in a physical body, until the discovery of the atom and particle physics. It was found that different atoms and different element ...
and will generally be located near the system's barycenter.
In the Solar System
The Solar System Capitalization of the name varies. The International Astronomical Union, the authoritative body regarding astronomical nomenclature, specifies capitalizing the names of all individual astronomical objects but uses mixed "Solar ...
, the Sun is the primary for all objects that orbit the star. In the same way, the primary of all satellites (be they natural satellites (moons) or artificial ones) is the planet
A planet is a large, rounded astronomical body that is neither a star nor its remnant. The best available theory of planet formation is the nebular hypothesis, which posits that an interstellar cloud collapses out of a nebula to create a ...
they orbit. The term ''primary'' is often used to avoid specifying whether the object near the barycenter is a planet, a star, or any other astronomical object
An astronomical object, celestial object, stellar object or heavenly body is a naturally occurring physical entity, association, or structure that exists in the observable universe. In astronomy, the terms ''object'' and ''body'' are often us ...
. In this sense, the word ''primary'' is always used as a noun
A noun () is a word that generally functions as the name of a specific object or set of objects, such as living creatures, places, actions, qualities, states of existence, or ideas.Example nouns for:
* Organism, Living creatures (including people ...
.
The center of mass is the average position of all the objects weighed by mass. The Sun is so massive that the Solar System's barycenter frequently lies very near the Sun's center but owing to the mass and distance of the gas giant planets, the Solar System's barycenter occasionally lies outside the Sun as well, despite the Sun comprising most of the Solar System's mass.
A disputed example of a system that may lack a primary is Pluto
Pluto (minor-planet designation: 134340 Pluto) is a dwarf planet in the Kuiper belt, a ring of bodies beyond the orbit of Neptune. It is the ninth-largest and tenth-most-massive known object to directly orbit the Sun. It is the largest k ...
and its moon Charon. The barycenter of those two bodies is always ''outside'' Pluto's surface. This has led some astronomers to call the Pluto–Charon system a ''double
A double is a look-alike or doppelgänger; one person or being that resembles another.
Double, The Double or Dubble may also refer to:
Film and television
* Double (filmmaking), someone who substitutes for the credited actor of a character
* Th ...
'' or ''binary'' dwarf planet
A dwarf planet is a small planetary-mass object that is in direct orbit of the Sun, smaller than any of the eight classical planets but still a world in its own right. The prototypical dwarf planet is Pluto. The interest of dwarf planets to ...
, rather than simply a dwarf planet (the primary) and its moon. In 2006, the International Astronomical Union
The International Astronomical Union (IAU; french: link=yes, Union astronomique internationale, UAI) is a nongovernmental organisation with the objective of advancing astronomy in all aspects, including promoting astronomical research, outreach ...
briefly considered a formal definition of the term ''double planet'' that could have formally included Pluto and Charon, but this definition was not ratified.
The use of the noun ''primary'' to refer to an extrasolar planet
An exoplanet or extrasolar planet is a planet outside the Solar System. The first possible evidence of an exoplanet was noted in 1917 but was not recognized as such. The first confirmation of detection occurred in 1992. A different planet, init ...
is dubious. Astronomers have not yet detected any bodies ( exomoons) that orbit an exoplanet. The use of ''primary'' to refer to the supermassive black hole
A supermassive black hole (SMBH or sometimes SBH) is the largest type of black hole, with its mass being on the order of hundreds of thousands, or millions to billions of times the mass of the Sun (). Black holes are a class of astronomical obj ...
at the center of most galaxies has not occurred in scientific journal
In academic publishing, a scientific journal is a periodical publication intended to further the progress of science, usually by reporting new research.
Content
Articles in scientific journals are mostly written by active scientists such ...
s.
See also
* Double planet * * Natural satellite * ''n''-body problem ** Two-body problem **Three-body problem
In physics and classical mechanics, the three-body problem is the problem of taking the initial positions and velocities (or momenta) of three point masses and solving for their subsequent motion according to Newton's laws of motion and Newton's ...
* Orbiting body
Notes
{{DEFAULTSORT:Primary (Astronomy) Orbits