Plutonium-244
Plutonium-244 (Pu) is an isotope of plutonium that has a half-life of 80 million years. This is longer than any of the other isotopes of plutonium and longer than any other actinide isotope except for the three naturally abundant ones: uranium-235 (704 million years), uranium-238 (4.468 billion years), and thorium-232 (14.05 billion years). Although studies are in conflict, given the mathematics of the decay of plutonium-244, an exceedingly small amount should still be present in the Earth's composition, making plutonium a likely although unproven candidate as the shortest lived primordial element. Accurate measurements, beginning in the early  1970s, have detected primordial plutonium-244, making it the shortest-lived primordial nuclide. The amount of Pu in the pre-Solar nebula (4.57×10 years ago) was estimated as 0.8% the amount of U. As the age of the Earth is about 57 half-lives of Pu, the amount of plutonium-244 left should be very small; Hoffman "et al." estimated its content in the rare-earth mineral bastnasite as  = 1.0×10 g/g, which corresponded to the content in the Earth crust as low as 3×10 g/g (i.e. the total mass of plutonium-244 in Earth's crust is about 9 g ). Since plutonium-244 cannot be easily produced by natural neutron capture in the low neutron activity environment of uranium ores (see below), its presence cannot plausibly be explained by any other means than creation by r-process nucleosynthesis in supernovas. Plutonium-244 thus should be the second shortest-lived (after samarium-146) and the heaviest primordial isotope yet detected or theoretically predicted. Trace amounts of Pu (that arrived on Earth within the last 10 million years) were found in rock from the Pacific ocean by a Japanese oil exploration company. The detection of primordial Pu in 1971 is not confirmed by  recent, more sensitive measurements using the method of accelerator mass spectrometry. In this study, no traces of plutonium-244 in the samples of bastnasite (taken from the same mine as in the early study) were observed, so only an upper limit on the Pu content was obtained: < 0.15×10 g/g, which is 370 (or less) atoms per gram of the sample, at least 7 times lower than the abundance measured by Hoffman "et al." Live interstellar plutonium-244 has been detected in meteorite dust in marine sediments, although the levels detected are much lower than would be expected from current modelling  of the in-fall from the interstellar medium. It is important to recall, however, that in order to be a primordial nuclide – one constituting the amalgam orbiting the Sun that ultimately coalesced into the Earth – that plutonium-244 must have comprised some of the solar nebula, rather than having been replenished by extrasolar meteoritic dust. The presence of plutonium-244 in meteoritic composition without evidence the meteor originated from the formational disc of the Solar System supports the hypothesis that Pu was abundant enough to have been a part of that disc, if an extrasolar meteor contained it in some other gravitationally  supported system, but such a meteor cannot prove the hypothesis. Only the unlikely discovery of live Pu within the Earth's composition could do that. Unlike plutonium-238, plutonium-239, plutonium-240, plutonium-241, and plutonium-242, plutonium-244 is not produced in quantity by the nuclear fuel cycle, because further neutron capture on plutonium-242 produces plutonium-243 which has a short half-life (5 hours) and quickly beta decays to americium-243 before having much opportunity to further capture neutrons in any but very high neutron flux environments. In theory, a nuclear weapon explosion can produce some plutonium-244 by rapid successive neutron capture, which might be  assigned to nuclear fallout of such a weapon. To date, however, no such transmutation of plutonium or uranium has been found in sufficient quantity to be identified, especially given surface- or atmospheric-level testing of nuclear ordnance was eschewed by the only two world powers to hitherto have the capability to execute such tests, by mutual treaty in the Partial Nuclear Test Ban Treaty of 1963.