In particle physics, an exotic baryon is a baryon (a strongly interacting fermion) composed of more than three quarks. Currently, only one type of exotic baryon, the pentaquark , is known. The pentaquark consists of four quarks and one antiquark. The composition of exotic baryons is constrained by the fact that the total number of quarks and antiquarks must be odd (if it were even, the particle would be a boson, not a fermion, and hence by definition a meson), and that all baryons must be "colorless", which can only be arranged if the quarks either come in a triplet, with one quark of each color (i.e. a red quark, a green quark, and a blue quark are all present), or in a quark-antiquark pair (e.g. a red quark and an anti-red antiquark). The pentaquark can therefore be viewed as a triplet of quarks plus a quark-antiquark pair, and thus resembles a bound state of an ordinary baryon and a meson. The possible existence of exotic baryons has been contemplated by physicists since the early 1970s, as it is an immediate consequence of quantum chromodynamics (the quantum field theory which describes hadrons), but the experimental discovery of the first exotic baryon did not occur until 2003.