Brown Dwarfs: The Missing Link Between Stars and Planets

Abstract

It is well known that the lightest stars are about hundred times heavier than Jupiter. Before the discovery of planets outside the solar system, Jupiter was known to be the heaviest object that was not a star. This huge gap in the hierarchy of mass was quite a puzzle. So, quite naturally, astronomers were curious to know if there was any celestial object lighter than the lightest star but heavier than Jupiter. Is there a missing link between the stars and the planets? In the year 1963, Shiv S. Kumar, an Indian origin American astronomer, theoretically predicted that such objects might exist. It was also realized that such kind of objects, if existed, should be extremely faint. Kumar named them “Black Dwarfs.” But the name “Black Dwarf” was already assigned to the end product of a kind of objects called White Dwarfs. White Dwarfs are the final state of a Sun-like star. After burning for several billion years, when the nuclear fuel, e.g., hydrogen, gets depleted at the core of a star, helium starts burning at the core and hydrogen starts burning outside the core, at the shell of the star. This generates so much heat energy that gravitation cannot hold the star and so the star starts expanding. When our Sun will undergo such a stage, it will expand so far out that even the Earth will be engulfed by it. Such an expanded star is known as Red Giant star. All stars, irrespective of how heavy they are, pass through this stage at the end of their life. Once helium gets depleted and the nuclear burning stops, radiation force becomes weak and the core becomes heavier. As a consequence the star starts contracting due to the gravitational pull of the matter at the core. For a star like our Sun, the contraction stops at a state called White Dwarf. A White Dwarf is as small as the Earth but as heavy as the Sun. If the mass of the stellar core is more than 1.4 times the mass of the Sun, the celebrated Chandrasekhar limit, the contraction stops at a form known as neutron star which has a radius of just 10 km—smaller than the size of even a town or a city. On the other hand, if the star is eight to ten times heavier than the Sun, nothing can stop the gravitational collapse and the object becomes the so called “black hole.” White Dwarfs emit light through the release of gravitational potential energy generated due to contraction. When a White Dwarf cools down completely and becomes invisible, it is called Black Dwarf. However, it takes such a long time for a star to become Black Dwarf that we do not yet find any of them. Actually, the time taken for a star to become Black Dwarf is longer than the present age of the universe. So, in 1975, astronomer Jill Tarter renamed the objects that were theoretically predicted by Kumar as Brown Dwarfs. It was predicted that these objects would be bright in infrared light which is beyond the visible range of light. It would emit radiation at a wavelength longer than that of red light. So, in order to make an analogy with visible colors, the name Brown Dwarf was given. However, such kind of objects was elusive for more than three decades. Quite a few astronomers claimed to have discovered it, but careful analysis refuted their claims and most of the time a low-temperature M-dwarf star was mistaken as a Brown Dwarf. Finally, in the year 1995, the discovery of a genuine and unimpeachable Brown Dwarf orbiting around a Red Dwarf star Gliese 229 was confirmed by a team of astronomers from Caltech and Johns Hopkins University. This object is located at about 19 light years away from us. In the same year, the discovery of an isolated Brown Dwarf in the Pleiades open cluster was also reported by a group of Spanish astronomers. This object known as Teide 1 after the Teide Observatory is located at a distance of about 400 light years from us. Incidentally, in the same year, the discovery of the first planets outside the solar system was also confirmed.

As one great furnace flamed, yet from those flames

No light, but rather darkness visible

John Milton

(In Paradise Lost)