Planetary-mass satellite The three largest satellites
Ganymede,
Titan, and
Callisto are of similar size or larger than the planet
Mercury; these and four more –
Io,
the Moon,
Europa, and
Triton – are larger and more massive than the largest and most massive dwarf planets,
Pluto and
Eris. Another dozen smaller satellites are large enough to have become round at some point in their history through their own gravity, tidal heating 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 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
International Astronomical Union (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 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
PSR J1719−1438. These shrunken white dwarfs may become a
helium planet or
carbon planet.
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 was initially thought to be a binary system of a
brown dwarf 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 planets in
stellar clusters have similar velocities to the stars and so can be recaptured. They are typically captured into wide orbits between 100 and 105 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 simulations of stellar and planetary system formation have suggested that some objects of planetary mass would be ejected into
interstellar space. Such objects are typically called
rogue planets. ==See also==