process of gravitational collapse and fragmentation. A planet, however, is built up by accretion in a circumstellar disk of gas and dust.

Although it is generally assumed that the average planet will be of lower mass than the average brown dwarf, the upper mass limit for planet formation and the lower mass limit for brown dwarf formation are not yet known. Indeed, these mass limits may overlap, making it very difficult to know whether a particular object is a planet or a brown dwarf.

An alternative distinction is sometimes drawn between planets and brown dwarfs: the former is defined as having a mass less than 13 Jupiter masses (MJ) and is, hence, unable to undergo deuterium fusion. Coincidentally, this is in the general mass regime of ~10 to 20 MJ thought to be at the upper end of the planet formation spectrum and at the bottom of the range for starlike formation by direct fragmentation.


The Radial-Velocity Technique

The indirect detection of substellar companion objects to stars has produced impressive results during the 1990s, with the discovery of about 20 objects below the stellar-mass limit. Most of these new discoveries are a direct consequence of major improvements in the precision with which it is possible to measure slight variations in a star's radial velocity (i.e., the speed with which the star approaches or recedes from us). In the mid- 1980s, the best routine velocity precision was about 500 m/s; now the state of the art is 3 m/s.6

The stellar radial velocity will be observed to vary as the star and its brown dwarf (or planetary) companion each execute symmetric reflex orbits around the center of mass (or balance point) of the system. Thus, from the detection of the stellar orbital motion, researchers can infer the presence of the unseen companion object and compute a lower limit on its mass. There are now about a half dozen research groups in the world that are capable of routinely measuring radial velocities with precisions in the range of 3 to 15 m/s. Because the Sun's reflex orbital velocity caused by Jupiter is about 12 m/s, high precision is needed to detect Jupiter-mass objects. The objects detected around other stars so far range from about 0.5 MJ7 all the way up to the stellar-mass limit of about 80 MJ (0.08 solar masses).

Researchers do not yet have enough information to know which of these objects are planets or brown dwarfs. Some argue that the distribution of orbital eccentricities as a function of orbital period implies that virtually all of the substellar companions are brown dwarfs (some, by virtue of the sin i ambiguity, are in fact stars, as noted below). Others contend that this distribution is the result of early dynamical evolution of the system, rather than the mechanism of formation.

Data reported at International Astronomical Union Colloquium 170 in June 1998 may shed some light on this issue. Nearly a decade of high-precision radial-velocity observations of some 200 stars by several groups have revealed seven companion objects with M sin i less than 5 MJ, one object with M sin i between 5 and 10 Jupiter masses, and no objects with M sin i between 10 and 80 MJ. These results imply that 10- to 80-MJ SMOs are found much less frequently within 5 AU of their parent star than are SMOs with mass less than 5 MJ. A lower


G.W. Marcy and R.B. Butler, “Detection of Extrasolar Giant Planets,” Annual Reviews of Astronomy and Astrophysics, 36: 57, 1998.


This is the lower limit to the mass due to an ambiguity in the inclination of the planet's orbit relative to our line of sight.

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