As a star forms from a cloud of contracting gas, the temperature in its center becomes so large that hydrogen begins to fuse into helium — releasing an enormous amount of energy which causes the star to begin shining under its own power. A planet forms from small particles of dust left over from the formation of a star. These particles collide and stick together. There is never enough temperature to cause particles to fuse and release energy. In other words, a planet is not hot enough or heavy enough to produce its own light.
Brown dwarfs are objects which have a size between that of a giant planet like Jupiter and that of a small star. In fact, most astronomers would classify any object with between 15 times the mass of Jupiter and 75 times the mass of Jupiter to be a brown dwarf. Given that range of masses, the object would not have been able to sustain the fusion of hydrogen like a regular star; thus, many scientists have dubbed brown dwarfs as “failed stars”.
Brown dwarfs are very dim and cool compared with stars. The best hope for finding brown dwarfs is in using infrared telescopes, which can detect the heat from these objects even though they are too cool to radiate visible light. Many brown dwarfs have also been discovered embedded in large clouds of gas and dust. Since infrared radiation can penetrate through the dusty regions of space, brown dwarfs can be discovered by infrared telescopes, even deep within thick clouds.
The discovery of objects like brown dwarfs will also give astronomers a better idea about the fate of our universe. The motion of the stars and galaxies are influenced by material which has not yet been detected. Much of this invisible dark matter, which astronomers call “missing mass”, could be made up of brown-dwarfs. Our universe is currently expanding, due to the Big Bang. If there is enough mass, it is thought that the expansion of the universe will eventually slow down and then the universe will start collapsing. This scenario could mean that the universe goes through an endless cycle of expansions and contractions, with a new Big Bang occurring every time the universe ends its collapse. If there is not enough mass for the universe to collapse, then it will expand forever. We will only know the fate of the universe when we can accurately estimate how much mass the universe has in it. The detection missing mass objects, such as brown dwarfs will likely be a key to answering this question.
Brown Dwarfs were only a theoretical concept when the Spitzer Space Telescope was first proposed. Since the mid-1990s, various infrared telescopes and surveys have identified a few hundred of these objects. Spitzer will devote much of its time to the discovery and characterization of brown dwarfs. It is expected that Spitzer will study thousands of these objects, including those only slightly larger than Jupiter. This will provide astronomers with enough data on brown dwarfs for good quality statisical studies.