Famed late NASA astronomer Carl Sagan first hypothesized that the reason early Earth stayed warm although the sun shone dimly had to do with a greenhouse effect involving a gas mixture different from that in Earth's atmosphere today.
Artist's depiction of what exoplanet Kepler 22b might look like. It was discovered by the Kepler satellite telescope. Kepler 22b likely receives a similar amount of light and heat from its star as our Earth does from our sun.
First author Kazumi Ozaki (r.) takes geological samples from layers of rock formed during the Great Permian Extinction. He is shown here with other researchers in Japan doing work unrelated to this study.