The cloud coverage isn’t uniform though, and the way de Wit and his colleagues figured that out is the truly impressive part of their work. As exoplanets move around their stars, they go through phases, just like the Moon: they appear entirely dark to us when they’re between Earth and the star; half-illuminated when they’re off to one side or the other; and show up as “full planet” when they’re just about to go behind the star. And thanks to Kepler’s extraordinary sensitivity, astronomers managed to measure those subtle changes in the planet’s brightness as well. “It’s the first time we’ve reached such high precision,” says de Wit.

With that precision, says lead author Brice-Olivier Demory, also of MIT, “we could see that something strange was going on.” When Kepler 7b was next to the star on one side, the researchers observed, it was significantly brighter than when was off to the other side. The only plausible explanation: one hemisphere is much cloudier and more reflective than the other.

The scientists aren’t entirely sure why this should be, but they offer one possibility. Just as the Moon is tidally locked to the Earth and always presents the same side to our view, Kepler 7b always shows the same hemisphere to its star. It would be as if our sun hung permanently over the Atlantic Ocean: the eastern half of the Western Hemisphere would always be brightly lit, as would the western half of the Eastern Hemisphere.