Brown dwarf stars are true oddballs. Indeed, their very existence presents astronomers with an enticing mystery to solve. This is because they challenge any attempt to derive a neat distinction between “failed stars” and giant planets. Many astronomers think that these puny, relatively cool denizens of the Cosmos are born just like their larger true stellar kin, within the phantom-like, swirling, whirling billows of one of the many giant cold, dark and beautiful molecular clouds that haunt our Milky Way Galaxy. These gigantic dark clouds are lit with the glittering fires of myriad newborn stars–and a star is born when a particularly dense blob, embedded within one of the ruffling folds of a molecular cloud, collapses under the merciless pull of its own gravity.
Thus, dazzling, brilliant baby stars (protostars) are nested within a dense contracting blob, that is composed mostly of gas with a small amount of dust. At the time of stellar birth, the temperature in the heart of the dense blob soars to the searing-hot point that causes hydrogen atoms to begin to fuse together to form helium atoms. Hydrogen is both the most abundant and lightest atomic element in the Universe, and helium is the second-lightest. All stars, regardless of their mass, are composed mostly of hydrogen, and both hydrogen and helium were formed in the wild exponential inflation of the Big Bang birth of the Universe about 13.8 billion years ago.
In dramatic contrast, gas giant planets–similar to Jupiter–muddy the issue. At the heftier end of the mass-range, which is 60 to 90 times the mass of Jupiter, the volume of a brown dwarf is determined primarily by electron-degeneracy pressure–as it also is for white dwarf stars. White dwarf stars are the relic cores left behind by small Sun-like stars after they have burned their necessary supply of nuclear-fusing fuel, and have gone with relative peace and gentleness into “that good night”. In contrast, at the lighter end of the mass-range–which is approximately 10 times the mass of Jupiter–the volume of a brown dwarf is controlled precisely the same way that it is for a planet. The complicating factor is that the radii of brown dwarfs vary by only 10-15% above the mass-range for their giant planet cousins. This presents astronomers with the difficult task of distinguishing giant planets from brown dwarfs.
Because puny brown dwarfs never gain sufficient weight to engage in the process of nuclear fusion, those that occupy the lighter end of the mass-range (less than 13 Jupiter-masses) cannot grow hot enough to even fuse deuterium. Meanwhile, those brown dwarfs that occupy the heftier end of the mass-range (greater than 60 Jupiter-masses) cool off so rapidly–after a mere 10 million years, or so–that they can no longer sustain nuclear-fusion. Ten million years is a very brief period of time in the strange “life” of a “failed star.”
Astronomers use X-ray and infrared spectra to find elusive brown dwarfs. First, there is a type of brown dwarf that spews out X-rays. Second, all “warm” brown dwarfs continue to softly glow in the red and infrared part of the electromagnetic spectrum during their entire “life” as a “failed star”. Hence, by using X-ray and infrared spectra, astronomers can find elusive brown dwarfs where they lay hidden. Alas, astronomers can only use this technique up to a point, and can no longer use it when these substellar runts have managed to cool off to temperatures that are more characteristic of a planet than a “failed star”.
Therefore, the primary difficulty in distinguishing brown dwarfs from giant planets is that the two groups share certain characteristics. Like our Sun and other stars, Jupiter and other gas-giant planets are mainly composed of hydrogen and helium. Three of the gaseous giant planets in our Sun’s family–Jupiter, Saturn, and Uranus–spew out much more heat than they get from our Star. All four gaseous giant planets in our Solar System–now including the outermost planet, Neptune–are accompanied by tiny “solar systems” all their own, made up of an entourage of orbiting moons.
Like their more successful “true star” kin, brown dwarfs can either be born as solitary objects or in close proximity to others of their kind. Some brown dwarfs circle other stars and, like planets, may also have eccentric orbits.
When a star is born, within its parent cloud made up of gas and dust, the disk of swirling, circling material (protoplanetary accretion disk) may eventually give rise to a family of orbiting planets. During the initial stages of star-birth, jets composed of material are shot screeching outward from the poles. However, jets of this type are not emitted to herald the birth of a planet.