slave wrote:
Would it be accurate to think of gas(Hydrogen) planets on a continuum WITH stars??
Gas planets being collections of gas that never reached the mass required for fusion to occur.
Stars being gas giant planets that became massive enough for fusion to occur.
It is correct, surely?
http://exoplanets.org/methodology.htmlQuote:
The definition of "planet" is somewhat contentious. The IAU has defined the term "planet" for objects orbiting the Sun in terms of gravitational properties: massive enough for gravity to force it into a spheroidal shape, and to be the dominant gravitational object in its orbit. Asteroids share similar orbits, and so are not planets; Pluto shares part of its orbit with much larger Neptune. Not all astronomers agree with this definition, but it is "official".
Outside the Solar System, there is no formal definition.
Many astronomers feel that an object is a planet if it formed like the planets in our Solar System (i.e., from a bottom-up process in a disk of material around the young Sun). There may be other ways to form planets, though (from gravitational instabilities of said disks, or, "top down"), and we can rarely (if ever) tell how planets formed, so this definition is not that useful, in practice.
Ignoring formation mechanisms (and composition), one can also set up a mass spectrum from low to high: asteroid, dwarf planet, planet, brown dwarf, star. We cannot yet detect things much smaller than planets, so the relevant boundary to define is between planets and brown dwarfs. Brown dwarfs are basically low-mass stars, and so may have fundamentally different formation mechanisms than planets, but probably have signifacnt mass overlap with them (that is, the smallest brown dwarfs are less massive than the largest planets).
The IAU recommended a boundary at 13 Jupiter masses, that being roughly the mass at which a solar composition brown dwarf can just, briefly, fuse deuterium in its core. This recommendation also stated that "free-floating" exoplanets be called "sub-brown dwarfs".
So it's complicated. Basically, an exoplanet is a planet that orbits another star. We use a cutoff of 24 Jupiter masses to be on the safe (inclusive) side.
You can quibble over the exact thresholds, but somewhere around 13-15 Jupiter masses gets you a brown dwarf. Somewhere around 87 jupiter masses gets you a star.