Figure 1: Artist’s impression of a giant planet.
Stars are objects with sufficient mass to sustain hydrogen fusion in their cores, while sub-stellar objects (i.e. brown dwarfs and planets) have masses below what is required to sustain hydrogen fusion. The mass boundary between stars and sub-stellar objects lie at about 80 times Jupiter’s mass and seems to be quite clear. However, the mass boundary between what is considered a planet and what is considered a brown dwarf is unclear. One definition is that if a planet is defined as a sub-stellar object that has not undergone deuterium burning at any point in its life; then the mass boundary between planets and brown dwarfs is about 13 times Jupiter’s mass. Unfortunately, this distinction is weak because deuterium burning only occurs for a brief period and has a negligible impact on the future evolution of a brown dwarf.
Hatzes & Rauer (2015) propose a new definition for planets, brown dwarfs and stars by presenting the mass-density relationship for objects ranging from planets with ~0.01 times Jupiter’s mass through stars with more than 80 times Jupiter’s mass. The mass-density diagram shows 3 distinct regions. Objects below 0.3 times Jupiter’s mass show considerable scatter, objects between 0.3 to 60 times Jupiter’s mass have a positive slope and objects over 60 times Jupiter’s mass follow a negative slope. Objects below 0.3 times Jupiter’s mass are simply referred to as low-mass planets. For objects between 0.3 to 60 times Jupiter’s mass, there appears to be no distinguishing characteristic to separate the lower mass members which are clearly giant planets from the higher mass members which are generally considered to be brown dwarfs. As a result, all objects between 0.3 to 60 times Jupiter’s mass are simply defined by Hatzes & Rauer (2015) as giant planets.
The slope becomes negative for objects over 60 times Jupiter’s mass. However, at 60 times Jupiter’s mass, an object is still not massive enough to sustain hydrogen fusion in its core. As a result, Hatzes & Rauer (2015) suggest that objects with 60 to 80 times Jupiter’s mass are considered bona fide brown dwarfs and objects over 80 times Jupiter’s mass are stars. The change in slope at 60 times Jupiter’s mass is a good mass boundary between what constitutes a planet and what constitutes a brown dwarf. This is because the change in slope indicates a change in the interior structure between planets and brown dwarfs. To summarise, this new definition goes like this - objects less than 0.3 times Jupiter’s mass are low-mass planets, objects between 0.3 to 60 times Jupiter’s mass are giant planets and objects between 60 to 80 times Jupiter’s mass are brown dwarfs.
Figure 2: The densities and masses of stars (red squares), giant planets and brown dwarfs, and low mass planets. Triangles represent Kepler discoveries and dots are CoRoT exoplanets. Ground-based discoveries for high mass giant planets are shown by pentagons. Hatzes & Rauer (2015).
Figure 3: The points from Figure 2 shown in the mass-radius plane. Hatzes & Rauer (2015).
Reference:
Hatzes & Rauer (2015), “A Definition for Giant Planets Based on the Mass-Density Relationship”, arXiv:1506.05097 [astro-ph.EP]