WHAT IS A PLANET? It seems such a simple question, but the answer keeps getting more and more confused. On the one hand, the line between planets and lesser bodies is notoriously hazy.

Just last year astronomers identified a body larger than Pluto in the outermost solar system, rekindling the old debate over whether Pluto really qualifies as a planet and, if it does, why large asteroids do not. Even newspapers and museums have jumped into the fray. Less well known, though, is the muddle at the upper end of the planetary scale: the blurring of the divide between planets and stars.

The distinction used to be straightforward: a star shines by its own light, whereas a planet merely reflects the light of the star it orbits.

In the more rigorous parlance of physics, stars are massive enough to undergo stable hydrogen fusion in their interiors over a sustained period, making them self-luminous. They form out of collapsing clouds of interstellar gas. Planets, on the other hand, are too puny and cold to initiate fusion.

They are thought to congeal out of the debris floating around newborn stars; in short, they are leftover scraps of the star formation process.

In the past decade, however, astronomers have discovered panoply of bodies that smear these boundaries. Straddling the mass range between planets and stars, sharing many characteristics with both but classifiable as neither, they yield fundamental insights into stellar and planetary formation and properties. These are the brown dwarfs.

They span a mass range of 12 to 75 jupiters: too light to attain the high central temperatures required to fuse ordinary hydrogen nuclei but heavy enough to fuse deuterium, a less common isotope of hydrogen.

Newly formed brown dwarfs shine like feeble stars but quickly exhaust their deuterium supply and star to cool down like planets.

The laws of physics imply that the size of young brown dwarfs (like that of stars) represents a balance between the inward pull of gravity and the outward push of thermal gas pressure. But older ones (like very massive planets) settle into an equilibrium between gravity and the quantum pressure exerted by densely packed electrons.
The atmospheres of young brown dwarfs should manifest the exotic meteorological phenomena, such as clouds, dust settling and precipitation, usually associated with planets.· It was summarized of Magazine “Scientific American”, January 2006