Scientific American, April 2000, had an article on “Brown Dwarfs”, also known as “failed stars”. These accumulations of matter from dust and gas clouds are not massive enough or hot enough to sustain nuclear fusion (hydrogen to helium) in their cores, and so do not shine brightly like stars, but emit only weak light from the energy liberated by their gravitational contraction, as well as some from deuterium fusion. .

Yet they are bigger than the solar planet Jupiter, our biggest, which also generates some internal heat. A minimum-size brown dwarf has a mass of about 13 Jupiters, while a minimum-size star would have a mass of about 75 Jupiters. Our Sun is 1000 times bigger than Jupiter.

There now seems to be a continuous series of sizes in celestial objects, from an asteroid (which may have an irregular shape like a big rock, not even be spherical), to a satellite like our Moon, to a small stony planet like Earth or Mars, to a big gaseous planet like Jupiter, to a small brown dwarf like Gliese 229B, to a larger brown dwarf like Teide 1, to a red dwarf star like Gliese 229A, to a yellow star like the Sun, to a big blue star which lasts only a few million years, because of its fast production of energy.

As the picture accompanying the article shows, these objects differ in mass, radius, surface temperature, age, the presence or absence of nuclear reactions, presence or absence of lithium (only brown dwarfs have it, not stars), and whether they mix from surface to centre by convection or have a layered structure like Jupiter and smaller planets.

“Planets versus Brown Dwarfs”:

Is there a fundamental difference between the largest planets and the smallest brown dwarfs? The classical view is that planets form in a different way than brown dwarfs or stars do. Gas-giant planets are thought to build up from planetesimals — small rocky or icy bodies-amid a disk of gas and dust surrounding a star. Within a few million years these solid cores attract huge envelopes of gas.This model is based on our own solar system and predicts that all planets should be found in circular orbits around stars and that gas giant planets should travel in relatively distant orbits.

These expectations have been shattered by the discovery of the first extrasolar giant planets. Most of these bodies have been found in close orbits, and most travel in eccentric ovals rather than in circles. Some theorists have even predicted the existence of lone planets, thrown out of their stellar systems by orbital interactions with sibling planets. This makes it very hard for observers to distinguish planets from brown dwarfs on the basis of how or where they formed or what their current location and motion is. We can find brown dwarfs by themselves or as orbital companions to stars or even other brown dwarfs. The same may be true for giant planets.

An alternative view is gaining adherents: to distinguish between planets and brown dwarfs based on whether the object has ever managed to produce any nuclear fusion reactions. In this view, the dividing line is set at about 13 Jupiter-masses. Above that mass, deuterium fusion occurs in the object. The fact that brown dwarfs seem to be less common than planets-at least as companions to more massive stars-suggests that the two types of objects may form by different mechanisms. A mass-based distinction, however, is much easier to observe. -G.B.