In geochemistry, geophysics and nuclear physics, primordial nuclides are nuclides found on Earth that has existed before the Earth was formed. It is also called primordial isotopes. Primordial nuclides are found in the interstellar medium. They were formed in the Big Bang or after the Big Bang, by nucleosynthesis in stars and supernovae nucleosynthesis, by cosmic ray spallation, and from other processes. Only 286 primordial nuclides are known.