When our sun first got going, some 4.5 billion years ago, it wasn't the same blazing star we know today--its warmth and brightness grew gradually as more and more of its fuel ignited. So, for Earth's first two billion years, our planet was bathed in a light 25 percent dimmer than it receives today.

If the sun dropped back down to that magnitude today, our planet would plunge into an ice age dramatic enough to bury the continents in miles-thick ice sheets and freeze the oceans solid. But according to the geological evidence, ancient Earth was not frozen: It was covered in vast liquid oceans and dotted over with arcs of island chains that sprouted up from undersea volcanoes and then wore back down again in the rain.

Scientists have been working to resolve this troubling paradox for decades: how, they have asked, could a faint young sun have kept Earth out of an ice age for two billion years, when several ice ages have come and gone in more recent times, under a much brighter star?