Planets & Brown Dwarfs
P.Butler, G.Marcy, D.Fischer, S.Vogt
last Update: September 2003
At Lick Observatory, we surveyed 107 G and K stars. Of these, 7 stars have "planetary" companions, with M sini < 10 Mjup. However none of our 107 stars exhibits a companion that has Msini = 10 - 80 Mjup.

Brown dwarfs are rare around Solar-Type stars. At most 1% of Solar-type stars has a brown dwarf within 5 AU. Thus, planets are distinguished from brown dwarfs by virtue of their relative occurrence.

The histogram of masses (Msini) for all known companions to  Main Sequence stars (FGKM type), within the mass range 0 - 15 Jupiter masses. The most massive companions are easiest to detect and virtually no companions are missed above 10 Jupiter masses, if they orbit within 3 AU. Approximately 1000 stars are represented in this survey which is a nearly complete study of FGK main sequence stars in the Hipparcos catalog of stars.

Apparently, the number of companions begins to rise at 8 Jupiter masses, and continues to rise toward the lowest detectable masses, ~0.3 Jupiters, where detectabiliy becomes poor.

At most 1% of stars harbor brown-dwarf companions within several AU. Their masses are spread out from 10 - 70 Mjup, as demonstrated beautifully by the data of Michel Mayor, Antoine Duquennoy, and Didier Queloz at Geneva Observatory and Observatoire de Haute Provence.


That our Solar System has its largest planet, Jupiter, in a circular orbit promotes the stability of circular orbits among the other 8 planets. If our Jupiter were in an eccentric orbit, the Earth and Mars would likely be gravitationally scattered out of the Solar System. Thus our existence, and the existence of life in the habitable zone, depends on both Jupiter and Earth being in mutually stable, circular orbits. It is probably no accident that our Solar System contains circular orbits.

Eccentric orbits may occur relatively commonly for extrasolar planets. Just one eccentric giant planet orbiting a star can spell dynamical doom for terrestrial planets, and may bode ill for slowly evolving creatures. The claim that all planetary orbits must be like ours may well be a circular argument.

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