An overview of substellar objects

Abstract

In this paper are reviewed various types of substellar objects both with regard to the properties they have in common and to their major differences. In size substellar objects in the solar system range from dust grains up to the planet Jupiter. Over all but the largest of these objects there is a smooth continuum of sizes and masses. All-important in determining compositions are the ability of an object to retain volatiles and the temperatures at which these volatiles can condense. Thus the distance at which water can condense into ice helps determine the compositions of the smaller bodies. The giant planets Jupiter and Saturn are dominated by high pressure forms of hydrogen, while Uranus and Neptune are dominated by high pressure forms of water. At the low end of the scale a sharp distinction is usually drawn between asteroids and comets, but there is a substantial overlap in masses and sizes for these objects, and some intermediate cases are now recognized. Brown dwarfs fill in a gap of roughly two orders of magnitude in the mass spectrum between the most massive planets and the least massive stars. There has been much progress in understanding the theory of brown dwarfs in recent years, and their bulk properties are now fairly well known. Brown dwarfs are assumed to have homogeneous compositions resembling those of typical stars and to be convective throughout. For solar composition ( Z = 0.02), objects below 0.075 M ⊙ cannot maintain sustained hydrogen fusion, and so they never reach the main sequence. For objects whose masses exceed 0.015 M ⊙ deuterium fusion briefly becomes important, but this isotope is much too rare to provide more than ephemeral energy. Contraction dominates energy generation in brown dwarfs, but for masses near the VLM/BD (very low mass star/brown dwarf) interface, hydrogen fusion (to 3He, but not to 4He) will be competitive with contraction for a long time. Despite the wide range of masses brown dwarfs are nearly all of the same size, about that of Jupiter. For an object of solar composition at the VLM/BD interface values of T e ∼ 2000 K, r ∼ 0.08 R ⊙, and L ∼ 10 −4L ⊙ are typical. For low-metal abundances masses and luminosities at the VLM/BD interface are significantly higher than is the case with Z = 0.02. The spectra of brown dwarfs are not well understood. Atmospheric opacities are greatly complicated by absorptions of H 2, H 2O, CO, metallic hydrides, dust particles, and other substances. Brown dwarfs have proven to be very difficult to observe. The status of various candidate objects is discussed, but an object that all agree on as being unambiguously a brown dwarf has yet to be discovered. An additional complicating factor is that young brown dwarfs closely resemble “mature” VLM stars in nearly all of their observed properties. Substantially more progress has been made in the observation of planetary disks than is the case with brown dwarfs. Two different types of disk systems can be distinguished. Protoplanetary disks have a large gas content and are associated with pre-MS stars. Their ages are generally less than 10 7 years. Dust debris disks are found about older MS stars. The IRAS survey discovered over 100 such stars with appreciable IR excesses, and three of these, Vega, Fomalhaut, and β Pictoris, are spatially resolved. Several planetary disks show evidence of gaps at about the point where water would condense into solid form. This may indicate the presence of planets in these regions. The paper concludes with a brief discussion of possible planets about pulsars and with a discussion of new methods of detecting other planetary systems.