Planetary-mass objects

A planetary mass object, PMO, or planemo is a celestial object with a mass that falls within the range of the definition of a planet – i.e. a mass greater than that of a minor object, yet smaller than that of a nuclear reactive brown dwarf or star. By definition all planets are planetary mass objects but the purpose of the term is to describe objects which do not conform to typical expectations for a planet. Free floating planets not orbiting a star may be rogue planets ejected from their system, or objects that have formed through cloud-collapse rather than accretion (sometimes called sub-brown dwarfs).

Rogue planets

Several computer simulations of stellar and planetary system formation have suggested that some objects of planetary mass would be ejected into interstellar space. Some scientists have argued that such objects found roaming in deep space should be classed as "planets", although others have suggested that they could be low-mass stars.

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, or orbiting a larger object such as 2MASS J04414489+2301513.
For a brief time in 2006, astronomers believed they had found a binary system of such objects, Oph 162225-240515, which the discoverers described as "planemos", or "planetary mass objects". However, recent analysis of the objects has determined that their masses are probably each greater than 13 Jupiter-masses, making the pair brown dwarfs.

Satellite planets and belt planets

Some large satellites are of similar size or larger than the planet Mercury, e.g. Jupiter's Galilean moons and Titan. Alan Stern has argued that location should not matter and only geophysical attributes should be taken into account in the definition of a planet, and proposes the term satellite planet for a planet-sized object orbiting another planet. Likewise planet-sized objects in the asteroid belt or Kuiper belt should also be planets according to Stern.