Primary (astronomy)
Encyclopedia
A primary is the main physical body
of a gravitationally-bound, multi-object system. This body contributes most of the mass
of that system and will generally be located near its center of mass
.
In the solar system
, the Sun
is the primary for all objects that orbit around it. In the same way, the primary of all satellites (be they natural satellite
s (moons) or artificial satellites
) is the planet
they orbit. The word primary is often used to avoid specifying whether the object near the center of mass is a planet, a star or any other astronomical object
. In this sense, primary is always used as a noun
.
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 center of mass is very close to the center of the Sun. However, the gas giants are far enough from the sun that the center of mass of the solar system can be outside the Sun, despite the Sun having most of the mass of the solar system.
An interesting example of what can be called a primary is Pluto
and its moon, Charon
. The center of mass (or barycenter) of those two bodies is always outside Pluto's surface. This has led some astronomers to call the Pluto-Charon system a binary dwarf planet or a double planet
rather than simply a dwarf planet (the primary) and its moon. In 2006, the International Astronomical Union
briefly considered a formal definition of the term double planet that could have formally included Pluto and Charon but this definition was not ratified.
Beyond the solar system, the use of the noun primary, in reference to an exoplanet, is dubious. Astronomers have not yet detected any bodies that orbit an exoplanet. The use of primary to refer to the supermassive black hole
s in the center of most galaxies has not occurred in scientific journal
s.
Physical body
In physics, a physical body or physical object is a collection of masses, taken to be one...
of a gravitationally-bound, multi-object system. This body contributes most of the mass
Mass
Mass can be defined as a quantitive measure of the resistance an object has to change in its velocity.In physics, mass commonly refers to any of the following three properties of matter, which have been shown experimentally to be equivalent:...
of that system and will generally be located near its center of mass
Center of mass
In physics, the center of mass or barycenter of a system is the average location of all of its mass. In the case of a rigid body, the position of the center of mass is fixed in relation to the body...
.
In the solar system
Solar System
The Solar System consists of the Sun and the astronomical objects gravitationally bound in orbit around it, all of which formed from the collapse of a giant molecular cloud approximately 4.6 billion years ago. The vast majority of the system's mass is in the Sun...
, the Sun
Sun
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...
is the primary for all objects that orbit around it. In the same way, the primary of all satellites (be they natural satellite
Natural satellite
A natural satellite or moon is a celestial body that orbits a planet or smaller body, which is called its primary. The two terms are used synonymously for non-artificial satellites of planets, of dwarf planets, and of minor planets....
s (moons) or artificial satellites
Satellite
In the context of spaceflight, a satellite is an object which has been placed into orbit by human endeavour. Such objects are sometimes called artificial satellites to distinguish them from natural satellites such as the Moon....
) is the planet
Planet
A planet is a celestial body orbiting a star or stellar remnant that is massive enough to be rounded by its own gravity, is not massive enough to cause thermonuclear fusion, and has cleared its neighbouring region of planetesimals.The term planet is ancient, with ties to history, science,...
they orbit. The word primary is often used to avoid specifying whether the object near the center of mass is a planet, a star or any other astronomical object
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...
. In this sense, primary is always used as a noun
Noun
In linguistics, a noun is a member of a large, open lexical category whose members can occur as the main word in the subject of a clause, the object of a verb, or the object of a preposition .Lexical categories are defined in terms of how their members combine with other kinds of...
.
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 center of mass is very close to the center of the Sun. However, the gas giants are far enough from the sun that the center of mass of the solar system can be outside the Sun, despite the Sun having most of the mass of the solar system.
An interesting example of what can be called a primary is Pluto
Pluto
Pluto, formal designation 134340 Pluto, is the second-most-massive known dwarf planet in the Solar System and the tenth-most-massive body observed directly orbiting the Sun...
and its moon, Charon
Charon (moon)
Charon is the largest satellite of the dwarf planet Pluto. It was discovered in 1978 at the United States Naval Observatory Flagstaff Station. Following the 2005 discovery of two other natural satellites of Pluto , Charon may also be referred to as Pluto I...
. The center of mass (or barycenter) of those two bodies is always outside Pluto's surface. This has led some astronomers to call the Pluto-Charon system a binary dwarf planet or a double planet
Double planet
In astronomy, double planet and binary planet are informal terms used to describe a binary system of two astronomical objects that each satisfy the definition of planet and that are near enough to each other to have a significant gravitational effect on each other compared with the effect of the...
rather than simply a dwarf planet (the primary) and its moon. In 2006, the International Astronomical Union
International Astronomical Union
The International Astronomical Union IAU is a collection of professional astronomers, at the Ph.D. level and beyond, active in professional research and education in astronomy...
briefly considered a formal definition of the term double planet that could have formally included Pluto and Charon but this definition was not ratified.
Beyond the solar system, the use of the noun primary, in reference to an exoplanet, is dubious. Astronomers have not yet detected any bodies that orbit an exoplanet. The use of primary to refer to the supermassive black hole
Supermassive black hole
A supermassive black hole is the largest type of black hole in a galaxy, in the order of hundreds of thousands to billions of solar masses. Most, and possibly all galaxies, including the Milky Way, are believed to contain supermassive black holes at their centers.Supermassive black holes have...
s in the center of most galaxies has not occurred in scientific journal
Scientific journal
In academic publishing, a scientific journal is a periodical publication intended to further the progress of science, usually by reporting new research. There are thousands of scientific journals in publication, and many more have been published at various points in the past...
s.