Saturn

On Monday evening, Jupiter and Saturn appeared closer to each other in the sky than they have for hundreds of years in what has become known as the Great Conjunction.

The last time Jupiter and Saturn came this close was 1623, but that conjunction was too near the sun to be seen on Earth. The most recent visible to humans was 800 years ago.

"This is the closest we've come, so far, to identifying a place with some of the ingredients needed for a habitable environment," NASA says. There are signs of a promising reaction under the surface.