Rogue planets are homeless worlds. They have neither sunrises nor sunsets, because unlike the planets we’re more familiar with, these lonely worlds aren’t tethered to a star. Instead, they travel in solitary arcs around the Milky Way’s core.

Earlier this week, Cosmos: A Spacetime Odyssey, introduced many of its viewers to the concept of these lonely planets.

“The galaxy has billions of them, adrift in perpetual night. They’re orphans, cast away from their mother stars during the chaotic birth of their native solar systems,” Neil DeGrasse Tyson says, as a planet emerges from the darkness. “Rogue planets are molten at the core, but frozen at the surface. There may be oceans of liquid water in the zone between those extremes.