Our galaxy is 10 billion years old. The Earth is only 4.5 billion years old. We are a firefly flicker in the great astrobiology of the galaxy.

What fraction of the twinkling lights you see at night have Earths?

A major puzzle for which nobody has an answer is this: is there some size at which the planets change their nature from water-rich planets like Neptune, to rocky planets like the Earth? We have found two planets that are the size of the Earth in radius, but they are very close to their host star, so water on the surface would evaporate away.