Most schoolchildren learn that everything in the universe is a solid, a liquid or a gas. However, those lessons usually lack the fourth and by far the most common state of matter: plasma. Not only should plasma be added to the list, but the order should be reversed to put it in first place; as a matter of fact, beyond the Earth's atmosphere, the dominant form of matter is plasma, and “empty” space has been found to be quite “alive” with a constant flow of plasma, making up over 99% of the visible universem and perhaps most of that which is not visible.
Neither solid, nor liquid, nor gas, plasma most closely resembles the latter; however, unlike gases, whose components are electrically neutral, plasma is composed of approximately equal numbers of positively charged ions and negatively charged electrons. It is so energetic, or “hot”, that in space it consists solely of ions and electrons; it is only when plasma is cooled that atoms or molecules that are so predominant in forming gases, liquids, and solids we are so accustomed to on Earth, are possible. Accordingly, in space, plasma remains electrically charged, carrying electric currents, and more influenced by electromagnetic forces than by gravitational forces.
The space surrounding the Sun, its corona and beyond, is a plasma. Indeed, much of all space is occupied by plasma, mostly in the dark current mode; planets and their moons each carry an electric charge as they travel through this plasma. The plasma sea in which the solar system floats extends out to what is known as the heliopause*, where there is probably a double layer separating our Sun's plasma from the lower voltage plasma that fills our arm of the Milky Way.
Plasma emits light because of “spontaneous emission”, which basically means that the ion, atom or molecule, have a higher level of energy than stable ones, so it becomes unstable and the particles emit photons. Through this release, the particles return to stability, only to start again; each time a particle goes from the same higher state to lower, the same photon will be emitted.
The term “Plasma” was borrowed from blood plasma in order to describe its almost life-like and self-organizing properties. Although plasma is the dominant constituent of the universe as a whole, most people are ignorant of the plasma concept. In daily life on Earth, perhaps the plasma to which people are most commonly exposed is the one that produces the cool efficient glow from fluorescent lights. The most familiar examples of electrical plasmas here on Earth are neon signs and lighting, television screens, and electric arc welding machines. Fire and lightning are also forms of plasma.
Glossary
*Heliopause is a bubble of charged particles in the space surrounding the Solar System, "blown" into the interstellar medium (the hydrogen and helium gas that permeates the galaxy) by the solar wind.
References
thunderbolts.info
plasmas.org
egglescliffe.org.uk
plasmacosmology.net
pluto.space.swri.edu