A
primary body
?? also called a
central body
,
host body
,
gravitational primary
, or simply
primary
?? is the main
physical body
of a
gravitationally
bound, multi-object system. This object constitutes most of that system's
mass
and will generally be located near the system's
barycenter
.
Motion of the Solar System's
barycenter
relative to the
Sun
In the
Solar System
, 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
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
. In this sense, the word
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 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,
[1]
despite the Sun comprising most of the Solar System's mass.
A disputed example of a system that may lack a primary is
Pluto
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
or
binary
dwarf 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.
The use of the noun
primary
to refer to an
extrasolar planet
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
at the center of most galaxies has not occurred in
scientific journals
.