The early Earth didn't have many chemicals needed for life on its surface, but they were present in asteroids and comets. Astronomers think they were delivered by countless objects crashing into the planet early in its history. But there's also a constant rain of organic-rich cosmic dust entering the Earth's atmosphere, and some can even pile up into sedimentary deposits left behind by glaciers. Did cosmic dust help fertilize prebiotic chemistry?

SETI researchers have directed their search toward interesting star systems where extraterrestrial civilizations could send signals toward Earth. According to a new paper, sufficiently advanced civilizations will communicate across the galaxy, sending signals between stars, rogue planets, and interstellar objects. They'll probably be using pulsars as a navigation system to coordinate their activities at a vast scale. To confirm their existence, we just have to listen in.