Ecole Normale Supérieure, Lyon, France

A geochemist wonders about the Solar System's true age.

Scientists have long looked at the constituent elements of meteorites to find out how old the Sun and its planets are. The most perfect example of the oldest meteorites — those that formed at the same time as the planets — broke up and fell as a large shower near Pueblito de Allende in Mexico in 1969. This was named the Allende chondrite, and was recently the subject of a study by Jim Connelly of the University of Texas at Austin and his colleagues.

Meteorites are often dated by measuring how much aluminium-26 they contain. This isotope decays at a rate that allows researchers to tell when one meteorite is older than another, but too fast to work out these rocks' absolute ages. For the relative ages to be accurate, however, aluminium-26 must have been spread evenly among the protoplanetary debris from which meteorites were born. If it was not, this isotope would reflect where they formed as well as when they formed, and meteorite chronologies would be higgledy piggledy.

But true ages can be calculated from lead isotopes. Until recently, lead had not been measured in both of two common parts of the oldest meteorites — chrondules and calcium–aluminium-rich inclusions — for any one rock, and there was no way of telling whether different rocks formed in the same bit of the nascent Solar System. But the lead isotopes in both chrondules and calcium–aluminium-rich inclusions can be counted in Allende.

Connelly and his team have confirmed an age difference between the chondrules and calcium–aluminium-rich inclusions that had been inferred from aluminium-26 measurements. This means that the relative meteorite chronologies are correct, and that aluminium-26 was indeed evenly distributed when the Sun ignited (J. N. Connelly et al. Astrophys. J. 675, L121–L124; 2008). If this is right, then the Solar System must be 4,567.5 million years old.