General Comments About Substellar Objects

One of the best-studied extrasolar planets is that about the star HD 209458. HD 209458b, as the planet is designated, was discovered through radial velocity measurements of its parent, HD 209458a, but light curves were obtained and the eclipses were observed (Charbonneau et al. 2000) permitting the radius, the inclination, and therefore, the mass and absolute dimensions of the planet and orbit to be determined. Table 16.2 in Milone & Wilson (2008) provides a list of known extrasolar planets.

Some fraction of the objects designated planets may be brown dwarfs, which are typically larger, more massive, and have larger internally generated radiative flux than planets. This may be expected of substellar objects detected through the radial velocity variation of their more massive parent stars, because the single-lined curve yields only projected masses. Nevertheless, not all of them can be brown dwarfs, because the distribution of projected masses with semi-major axis shows few of the yet higher mass objects that would be expected if most of those detected were brown dwarfs (Mayor et al. 1998).

Brown dwarfs have been shown to be absent in binary combinations with normal stars, the so-called brown dwarf desert . However, near-infrared surveys such as 2MASS, DENIS, and Sloan show them to be present both individually and in binaries with other low-mass, low-luminosity objects. If the "desert" is not due to a selection effect, the survey results imply that the dearth in binary systems is, at least, as Basri (2000) called it, a brown-dwarf "desert island."As the number of survey results continue to accumulate, however, the dearth looks more and more like a bonafide desert.

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