
A planetary-mass object (PMO), planemo,
or planetary body is by
geophysical definition of celestial objects any celestial object massive enough to achieve
hydrostatic equilibrium
In fluid mechanics, hydrostatic equilibrium (hydrostatic balance, hydrostasy) is the condition of a fluid or plastic solid at rest, which occurs when external forces, such as gravity, are balanced by a pressure-gradient force. In the planetar ...
(to be rounded under its own gravity), but not enough to sustain core fusion like a
star
A star is an astronomical object comprising a luminous spheroid of plasma held together by its gravity. The nearest star to Earth is the Sun. Many other stars are visible to the naked eye at night, but their immense distances from Earth make ...
.
The purpose of this term is to refer to a broader range of celestial objects than the concept of
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 ...
, since many objects similar in geophysical terms do not conform to typical expectations for a planet. Planetary-mass objects can be quite distinguished in origin and location. Planetary-mass objects include
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 ...
s,
planetary-mass satellite or free-floating planemos, which may have been ejected from a system (
rogue planet
A rogue planet (also termed a free-floating planet (FFP), interstellar, nomad, orphan, starless, unbound or wandering planet) is an interstellar object of planetary-mass, therefore smaller than fusors (stars and brown dwarfs) and without a ...
s) or formed through cloud-collapse rather than accretion (sometimes called
sub-brown dwarf
A sub-brown dwarf or planetary-mass brown dwarf is an astronomical object that formed in the same manner as stars and brown dwarfs (i.e. through the collapse of a gas cloud) but that has a planetary mass, therefore by definition below the li ...
s).
Types
Planetary-mass satellite

The three largest satellites
Ganymede,
Callisto Callisto most commonly refers to:
* Callisto (mythology), a nymph
*Callisto (moon), a moon of Jupiter
Callisto may also refer to:
Art and entertainment
*'' Callisto series'', a sequence of novels by Lin Carter
*''Callisto'', a novel by Torsten ...
, and
Titan
Titan most often refers to:
* Titan (moon), the largest moon of Saturn
* Titans, a race of deities in Greek mythology
Titan or Titans may also refer to:
Arts and entertainment
Fictional entities
Fictional locations
* Titan in fiction, fictiona ...
are of similar size or larger than the planet
Mercury; these and four more –
Io,
Moon
The Moon is Earth's only natural satellite. It is the fifth largest satellite in the Solar System and the largest and most massive relative to its parent planet, with a diameter about one-quarter that of Earth (comparable to the width ...
,
Europa, and
Triton – are larger than the largest dwarf planet,
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 ...
. Another dozen smaller satellites are large enough to be round through their own gravity, tidal forces from their parent planets, or both. In particular, Titan has a thick atmosphere and stable bodies of liquid on its surface, like Earth (though for Titan the liquid is
methane
Methane ( , ) is a chemical compound with the chemical formula (one carbon atom bonded to four hydrogen atoms). It is a group-14 hydride, the simplest alkane, and the main constituent of natural gas. The relative abundance of methane on Ear ...
rather than water). Proponents of the geophysical definition of planets argue that location should not matter and that only geophysical attributes should be taken into account in the definition of a planet. The term ''satellite planet'' is sometimes used for planet-sized satellites.
Dwarf planets

A dwarf planet is a planetary-mass object that is neither a true planet nor a natural satellite; it is in direct orbit of a star, and is massive enough for its gravity to compress it into a hydrostatically equilibrious shape (usually a spheroid), but has not cleared the neighborhood of other material around its orbit. Planetary scientist and New Horizons principal investigator
Alan Stern, who proposed the term 'dwarf planet', has argued that location should not matter and that only geophysical attributes should be taken into account, and that dwarf planets are thus a subtype of planet. The IAU accepted the term (rather than the more neutral 'planetoid') but decided to classify dwarf planets as a separate category of object.
Planets and exoplanets
Former stars
In close
binary star
A binary star is a system of two stars that are gravitationally bound to and in orbit around each other. Binary stars in the night sky that are seen as a single object to the naked eye are often resolved using a telescope as separate stars, in ...
systems one of the stars can lose mass to a heavier companion.
Accretion-powered pulsars may drive mass loss. The shrinking star can then become a planetary-mass object. An example is a Jupiter-mass object orbiting the
pulsar
A pulsar (from ''pulsating radio source'') is a highly magnetized rotating neutron star that emits beams of electromagnetic radiation out of its magnetic poles. This radiation can be observed only when a beam of emission is pointing toward E ...
PSR J1719-1438. These shrunken white dwarfs may become a
helium planet or
carbon planet
A carbon planet is a theoretical type of planet that contains more carbon than oxygen. Carbon is the fourth most abundant element in the universe by mass after hydrogen, helium, and oxygen.
Marc Kuchner and Sara Seager coined the term "carbon ...
.
Sub-brown dwarfs
Stars form via the gravitational collapse of gas clouds, but smaller objects can also form via cloud-collapse. Planetary-mass objects formed this way are sometimes called sub-brown dwarfs. Sub-brown dwarfs may be free-floating such as
Cha 110913-773444[
*] and
OTS 44,
or orbiting a larger object such as
2MASS J04414489+2301513.
Binary systems of sub-brown dwarfs are theoretically possible;
Oph 162225-240515
Oph 162225-240515, often abbreviated Oph 1622, and also known as Oph 11, is a pair of brown dwarfs that have been reported as orbiting each other. The bodies are located in the constellation Scorpius and are about 400 light years away. Mas ...
was initially thought to be a binary system of a
brown dwarf
Brown dwarfs (also called failed stars) are substellar objects that are not massive enough to sustain nuclear fusion of ordinary hydrogen (hydrogen-1, 1H) into helium in their cores, unlike a main sequence, main-sequence star. Instead, they have ...
of 14 Jupiter masses and a sub-brown dwarf of 7 Jupiter masses, but further observations revised the estimated masses upwards to greater than 13 Jupiter masses, making them brown dwarfs according to the IAU working definitions.
Captured planets
Rogue planet
A rogue planet (also termed a free-floating planet (FFP), interstellar, nomad, orphan, starless, unbound or wandering planet) is an interstellar object of planetary-mass, therefore smaller than fusors (stars and brown dwarfs) and without a ...
s in
stellar clusters have similar velocities to the stars and so can be recaptured. They are typically captured into wide orbits between 100 and 10
5 AU. The capture efficiency decreases with increasing cluster volume, and for a given cluster size it increases with the host/primary mass. It is almost independent of the planetary mass. Single and multiple planets could be captured into arbitrary unaligned orbits, non-coplanar with each other or with the stellar host spin, or pre-existing planetary system.
Rogue planets
Several
computer simulation
Computer simulation is the process of mathematical modelling, performed on a computer, which is designed to predict the behaviour of, or the outcome of, a real-world or physical system. The reliability of some mathematical models can be dete ...
s of stellar and planetary system formation have suggested that some objects of planetary mass would be ejected into
interstellar space
Outer space, commonly shortened to space, is the expanse that exists beyond Earth and its atmosphere and between celestial bodies. Outer space is not completely empty—it is a near-perfect vacuum containing a low density of particles, predo ...
.
Such objects are typically called ''rogue planets''.
See also
*
Planetary mass
*
List of gravitationally rounded objects of the Solar System
This is a list of most likely gravitationally rounded objects of the Solar System, which are objects that have a rounded, ellipsoidal shape due to their own gravity (but are not necessarily in hydrostatic equilibrium). Apart from the Sun itself, ...
*
List of Solar System objects by size
This article includes a list of the most massive known objects of the Solar System and partial lists of smaller objects by observed mean radius. These lists can be sorted according to an object's radius and mass and, for the most massive objects, ...
References
{{Reflist
Planets