An extinct radionuclide is one that scientists believe was formed by primordial processes, such as stellar nucleogenesis in the supernova(s) that contributed radioisotopes to the early solar system, about 4.6 billion years ago. Generally, radioisotopes with a decay half-life shorter than about 100 million years are not found in nature, except for those generated continuously by a natural process, such as cosmic rays, or a decay chain of much longer lived isotopes, such as uranium or thorium. Other short-lived isotopes are thus seen only as extinct radionuclides, whose former existence is inferred now from a superabundance of their stable decay products.
Examples of extinct radionuclides include iodine-129 (the first to be noted in 1960, and inferred from excess xenon-129 concentrations in meteorites, in the xenon-iodine dating system), aluminium-26 (also inferred from extra magnesium-26 found in meteorites), and iron-60.
A partial list of radionuclides not found in nature, but for which decay products exist:
| Isotope | Halflife (Myr) |
|---|---|
| Samarium-146 | 103 |
| Niobium-92 | 34.7 |
| Curium-247 | 16 |
| Lead-205 | 15 |
| Hafnium-182 | 9 |
| Palladium-107 | 7 |
| Caesium-135 | 3 |
| Technetium-97 | 3 |
| Gadolinium-150 | 2 |
| Zirconium-93 | 2 |
| Technetium-98 | 2 |
| Dysprosium-154 | 1 |
Some notable isotopes with shorter lives still being produced on Earth include:
Radioactives with half-lives shorter than one million years are also produced: for example, carbon-14 by cosmic ray production in the atmosphere (half-life 5730 years).