Brown Dwarfs (4) ...

The basic interior structures expected for stars, brown dwarfs, and gas giant planets is summarized schematically in the following figure. Generally stars initiate thermonuclear reactions but brown dwarfs and planets do not. Thus, lithium is destroyed in stars. The presence of methane is also an indication that the temperatures are too low for the object to be a star. Gas giant planets can also contain lithium and methane, but their upper interiors tend to be dominated by hydrogen in molecular form and helium.

From Stars to Brown Dwarfs to Planets
The following figure summarizes the size and surface temperature trend from middle main sequence stars like the Sun through the lowest mass stars (red dwarfs) through brown dwarfs and finally to planets.

Brown dwarfs can have surface temperatures comparable to that of the lowest mass stars, but atmospheric compositions similar to large planets. The difficult challenge is to distinguish them from these other two kinds of objects at interstellar distances. A number of brown dwarf candidates have now been identified, but in many cases there is uncertainty about whether they are brown dwarf companions of stars, or orbiting planets.

Definitions

A subcommittee of the International Astronomy Union has struggled with the issue of definitions for planets and brown dwarfs. It has suggested provisionally that the dividing line be set by whether the object has sufficient internal temperature and pressure to fuse deuterium into heavier elements. Practically, this places an upper limit of about 13 Jupiter masses for the most massive giant planet.

Brown Dwarfs and Extrasolar Giant Planets
The discovery of many extrasolar giant planets and brown dwarfs raises issues concerning the very definition of a planet. For example, it is possible that some stars may be orbited by both giant planets and brown dwarfs. In addition, although we may formally consider a planet to be an object that condenses from a solar nebula, we are uncertain of the exact mechanism by which this happens. Conventional models of star formation hold that hydrogen-rich objects form from direct collapse of molecular clouds having masses much greater than about 1/1000 solar mass (the approximate masses of Jupiter and Saturn; we exclude Uranus and Neptune from the discussion because they have much larger concentrations of heavier elments than Jupiter and Saturn).
A Unified Picture
Accumulating evidence suggests that there exists a set of closely-related objects from about 0.001 solar masses (the lightest giant planets) up to about 0.075 solar masses (the lightest stars). These objects share generally the following characteristics:

1. They are fully convective throughout essentially their entire volumes.
2. They have liquid metallic hydrogen (electron-degenerate) interiors.
3. They have cool atmospheres rich in molecular compounds that regulate the escape of primordial heat from the deep interior.

This suggests that it should be possible to describe both giant planets and brown dwarfs within a unified theory. There are efforts currently underway to do so.

New Spectral Classes
One development related to a unified description of giant planets and brown dwarfs is that new spectral classes have been proposed for hydrogen-rich objects (extrasolar giant planets, Jupiter, Saturn, brown dwarfs, ...) lying below the main sequence. These lie below the M class in the normal spectral sequence and are called L and T. The luminosities of the objects being classified generally are between 100,000 and a billion times less than that of the Sun. The study of the atmospheric spectrum of these objects is important to understanding their nature. For example, it is important to determine whether refractory elements like silicon are distributed uniformly through the object or concentrated in their cores. This would help answer the question of whether the object formed from stellar collapse of hydrogen gas or from planet-like accretion on refractory element cores, which would help distinguish brown dwarfs from giant planets.