An exotic atom is an atom that has a particle replaced by a particle of the same charge. For example, Positronium, an exotic atom, contains an electron and a positron. The positron (which is the antiparticle of the electron) replaces a proton which would normally be found in that atom. Most exotic atoms are hard to discover, because they decay very quickly. Positronium on average has a half-life (how long until half of the object is gone via decay) of 0.125 nanoseconds. There are a few types of exotic atom.