The solar system is 4.5682 billion years old, according to a new study published in the scientific journal Nature Geoscience. This is up from about 4.567 billion years, the previous estimate, and is based on a new analysis of metal deposits left by meteorite impacts.
The new study has been published by a pair of Arizona State University School of Earth and Space Exploration scientists, chemistry professor Meenakshi Wadhwa and postdoctoral researcher Audrey Bouvier. Bouvier and Wadwha looked at samples left by the meteorite Northwest Africa 2364, which was found in Morocco several years ago and is now held at the Northern Arizona University. The authors of the new study analyzed its lead-isotope ratio as a way of estimating the meteorite’s age – and turned up an unexpected result.
Lead-isotope dating is comparable to other forms of dating based on the radioactive decay of mildly radioactive isotopes – the same sort of natural process used in the much more popularly recognized technique of carbon dating for artifacts and fossils on Earth. In this case, lead is measured as a product created by the radioactive decay of the heavy metal uranium. Uranium-238 isotope decays to create lead-206, which has a half-life of 4.5 billion years (similar to the age of the solar system), whereas uranium-235 isotope decays to create lead-207, which has a comparably much shorter half-life of “just” 700 million years. In chemical parlance, a half-life is the amount of time it takes for half of a substance to undergo radioactive decay. In other words, after 700 million years, half of the quantity of a sample of lead-207 will have decayed into lighter elements.
Obviously, the radioactive decay of lead is much too short to be of much use in studying life on Earth – but it is much more useful for dating the evolution of the solar system over a far longer timespan. What surprised Bouvier and Wadhwa was that, when they performed this analysis on the new African meteorite, their results indicated that the lead content of the meteorite was at least several hundred thousand years and possibly two million years older than that in previously discovered and analyzed meteorites.
The current version of the nebular hypothesis, the most commonly accepted theoretical explanation for the natural origins of the solar system, states that the Sun first formed through the collapse of a large molecular cloud, and then rocky objects gradually coalesced around the infant Sun through accretion, or collisions between progressively larger and larger objects. Eventually, this led to the emergence of planets, although billions of smaller objects were left floating throughout the solar system, forming the meteorites which regularly strike Earth as well as numerous comets and asteroids.
However, while the nebular hypothesis provides a general framework for how the solar system might have evolved, it is less clear on how much time might have been required to complete the transformation from gas cloud to star system. One way to create those time estimates is to analyze bodies believed to have formed at various points in the solar system. Meteorites, for instance, formed early on and did not undergo further collisions. The age indicated by the lead isotope decay in Northwest Africa 2364 is slightly greater than that in previous meteorites, and is therefore an indication that they (and the rest of the solar system) began to form slightly longer ago than previously believed.
The fact that the solar system’s estimated age has been adjusted by just two million years, out of about 4.6 billion, is an indication of how precise the current understanding of solar and planetary evolution has become. It is possible that further meteorite analyses will result in further adjustments. However, only a small percentage of the total number of small objects in the solar system have ever collided (or will ever collide) with Earth, meaning that using the oldest known meteorite as a basis for estimating the age of the solar system will always produce best-guess estimates subject to yet further revision.
