Definition of Exoplanets and Brown Dwarfs
Thousands of substellar objects (that is, brown dwarfs and planets) have been detected since 1989 (see, for instance, the following web sites: exoplanet.eu and https://exoplanetarchive.ipac.caltech.edu). Their masses run from 0.02 times the mass of the Earth (PSR 1257 + 20 b) to about 63 Jupiter masses (CoRoT-15 b), about five orders of magnitude in mass. More than 2700 confirmed planets have been detected by the photometric transit technique (e.g., see the chapter by Dunham in this Handbook of Exoplanets), which led to an accurate determination of their size. Their radii run from 0.32 Earth radius (Kepler-37 b) to 2.1 Jupiter radius (HAT-P-67 b), about two orders of magnitude in size. The corresponding mass-radius diagram is represented in Fig. 1. One could be tempted to think that the more massive the object is, the larger it is in size and that there is some limit in mass and/or radius that distinguishes planets from everything else, even if this mass limit is below the well accepted substellar borderline at 72 Jupiter masses (minimum mass required for stable nuclear fusion of hydrogen in the interiors of solar metallicity stars). The objects between planets and very low-mass stars have been named brown dwarfs since the paper by Jill Tarter in 1973.
KeywordsBrown Dwarf Planet Definition
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