Planetary-mass object

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.^[11]

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.^[12]

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.^[14] These shrunken white dwarfs may become helium planet or carbon planet.

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^[16] and OTS 44,^[17] or orbiting a larger object such as 2MASS J04414489+2301513.

準褐色矮星の連星系は理論的には可能である。Oph 162225-240515 は当初、木星質量の 14 倍の褐色矮星と木星質量の 7 倍の準褐色矮星の連星系と考えられていたが、その後の観測により推定質量が木星質量の 13 倍以上に修正され、IAU の作業定義によれば褐色矮星となった。^{[ 18 ] [ 19 ] [ 20 ]}

星団内の浮遊惑星は恒星と似た速度を持つため、再捕獲される可能性があります。通常、100 から10 ⁵ AUの間の広い軌道に捕獲されます。捕獲効率は星団の体積が増加するにつれて低下し、特定の星団サイズでは、主星の質量とともに増加します。惑星の質量にはほとんど依存しません。単独または複数の惑星が、互いにまたは主星の自転軸と共面ではない任意の非整列軌道、または既存の惑星系に捕獲される可能性があります。^{[ 21 ]}

恒星系や惑星系の形成に関するいくつかのコンピュータシミュレーションでは、惑星質量を持ついくつかの天体が星間空間に放出されることが示唆されている。^{[ 22 ]}このような天体は通常、放浪惑星 と呼ばれる。