The reason is, basically, our 'clocks'.
Establishing chronologies of past events or determining ages of objects require having clocks that 'tick' at different paces - nuclear clocks used for dating are based on the rate of decay of an atomic nucleus expressed by a half-life, which is the time it takes for half of a number of nuclei to decay, a property of each nuclear species. Radiocarbon dating is the most famous. It was invented in Chicago in the late 1940s and can date artifacts back to prehistoric times because the half-life of radiocarbon (carbon-14) is a few thousand years.
But the evaluation of ages of the history of earth or of the solar system requires extremely "slow-paced" chronometers consisting of nuclear clocks with much longer half-lives. Nucleus samarium-146 (146Sm) belongs to a family of nuclear species which were "live" in our sun and its solar system when they were born. Events thereafter, and within a few hundred million years, are dated by the amount of 146Sm that was left in various mineral archives until its eventual "extinction." Samarium-146 was recently examined by an international team and they believe now that formation of our solar system may have occurred over a shorter period of time than previously thought.
146Sm has become the main tool for establishing the time evolution of the solar system over its first few hundred million years. This by itself owes to a delicate geochemical property of the element samarium, a rare element in nature. It is a sensitive probe for the separation, or differentiation, of the silicate portion of earth and of other planetary bodies.
The main result of the work of the international scientists, detailed in a recent article in Science, is a new determination of the half-life of 146Sm, previously adopted as 103 million years, to a much shorter value of 68 million years. The shorter half-life value, like a clock ticking faster, has the effect of shrinking the assessed chronology of events in the early solar system and in planetary differentiation into a shorter time span.
The new time scale, interestingly, is now consistent with a recent and precise dating made on a lunar rock and is in better agreement with the dating obtained with other chronometers. The measurement of the half-life of 146Sm, performed over several years by the collaborators, involved the use of the ATLAS particle accelerator at Argonne National Laboratory in Illinois.
The Sikhote-Alin meteorite from eastern Siberia, "offers us a snapshot of the original composition of the entire solar system before the planets formed," said Michael Zolensky, a scientist at NASA's Johnson Space Center (JSC). "It tells us what the initial materials were like that went into making up the Earth, the Moon and the Sun.“ It is a carbonaceous chondrite, which accounts for only 2 percent of all meteorites, but they are rich in organic materials and this was dated at 4.5-billion-years-old. But if the half-life is 34% different, it may also be 34% younger and our solar system may have taken a billion years longer to form.