Sometimes collapsing clouds of gas and dust don't quite manage to make it as stars. These objects, known as brown dwarfs, have many of the elements of their more famous siblings but lack the mass needed to jumpstart nuclear fusion in their core. Because brown dwarfs never burn fusion at their core, scientists sometimes refer to them as "failed stars."
Brown dwarfs start out just like their main-sequence siblings. A cloud of dust and gas collapses, gravity piling the components in tightly and forming a young protostar at its center.
For main sequence stars, the gravity pushes inward until hydrogen fusion is jump-started in their core. But brown dwarfs never reach this crucial stage. Instead, before the temperatures get hot enough for hydrogen fusion to start, the close-packed material reaches a stable state.
Characteristics, classifications, and observations
Brown dwarfs come in a variety of masses and temperatures. They range from 13 to 90 times the mass of Jupiter, around a tenth the mass of the sun.
Stars are classified by their spectral type, or the energy they radiate. Brown dwarfs are similarly classified.
M stars are the coolest of the successful stars in the universe, as well as the most plentiful. Most M stars are red dwarfs, but a few are brown dwarfs. L dwarfs and T dwarfs are both identified by the elements seen in their spectrum.
Because brown dwarfs give off so little light and energy, they can be challenging to locate. They were originally theoretical objects, unseen until the late 1980s. As astronomical instruments grew (and continue to grow) more sensitive, more brown dwarfs have been detected, though they remain a challenge. [Video: Getting WISE to Brown Dwarfs]
Brown dwarfs were originally called "black dwarfs." Now, that term is used for the final stage of the stellar evolution of a main sequence star, a white dwarf that has completely radiated away all of its heat.
Why not a planet?
Because brown dwarfs have so little mass, it can be easy to confuse them with massive planets, especially with the growing catalog of gas giants. Similarly, their lack of fusion can raise similar concerns.
One way to tell the difference is that brown dwarfs, like all stars, create their own light. Brown dwarfs glow in the red and infrared spectrum until they sufficiently cool down, emitting X-rays and infrared light that scientists can measure.
Even so, the dividing line between a cool brown dwarf and a planet can be quite small. Some brown dwarfs are cool enough to maintain atmospheres much like gas giants. Brown dwarfs can host planets, but gas giants can have moons in orbit. So how do you definitively determine whether a free-floating object is a planet or a very cool brown dwarf? [Could Rocky Planets Form Around Brown Dwarfs? (Video)]
Ultimately, the International Astronomical Union considers any object high enough to fuse deuterium a brown dwarf, while objects with less than that 13-Jupiter mass are considered planets.