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Most sun-like stars rotate relatively quickly early in their history, spinning once every few days before slowing down as they age. But sodium and potassium on the moon show that our sun had a lazy start, which may explain why we are here today.

The moon has surprisingly fewer volatile elements and compounds 鈥� those that turn into gas and blow away relatively easily 鈥� than Earth. Prabal Saxena at NASA鈥檚 Goddard Space Flight Center in Maryland and his colleagues investigated whether solar activity in the first billion years or so after the moon formed could explain the discrepancy.

Saxena and his team used data from the Kepler Space Telescope on other sun-like stars to build three models of the young Earth and moon, each with the sun rotating at a different rate. The faster the young sun rotated, the more often it would have experienced flares and coronal mass ejections (CMEs), blasting huge plumes of plasma into space and battering the inner solar system.

They found that for the fast- and even medium-rotating models, there were too many CMEs. Even if the sun rotated just about once every week, there could be tens of CMEs a day, enough to erode away all of the moon鈥檚 potassium and much of its sodium 鈥� volatile elements that we know still exist there today.

They found that if the sun was a slow rotator, turning just once every eight to ten days, Earth and the moon would experience just about one powerful CME every few days. 鈥淚t was still really chaotic in that first 500 million to billion years,鈥� says Saxena. 鈥淚t鈥檚 like deciding between going to either a heavy metal concert or a hard rock concert 鈥� it鈥檚 different but still loud.鈥� That would have left behind enough sodium and potassium on the moon to match observations.

In modern times, the sun turns only once every 27 or so days, which is good for us because a faster rotation and more CMEs would be incredibly bad for both volatiles like water and the living organisms that needs them. The fact that the sun seems to have started slow, allowing Earth to hold on to its volatiles, could help explain why life arose here in the first place, Saxena says.