Are protons found freely in nature?

Not bound to nucleons or electrons, the free proton is a stable particle that hasn’t been known to break down spontaneously to other particles. Free protons are found naturally in many situations in which energies or temperatures are high enough to separate them from electrons, for which they have some attraction.

They exist in plasmas in which temperatures are too high to allow them to combine with electrons. Free protons of high energy and velocity make up 90% of cosmic rays, which grow in vacuum for interstellar distances. Free protons come from atomic nuclei in some rare types of radioactive decay. They also result (along with electrons and antineutrinos) from the radioactive decay of free neutrons, which are unstable.


The spontaneous decay of free protons has never been observed. Therefore, according to the Standard Model, free protons are considered stable particles. However, some grand unified theories (GUTs) of particle physics predict that proton decay should take place with lifetimes and experimental searches have established lower bounds on the mean lifetime of a proton for various assumed decay products.