Earth lies in the middle of the Sun’s habitable zone. That’s the distance from the Sun where conditions are most comfortable for life. And astronomers are concentrating their hunt for life in other star systems in their habitable zones.

There’s an idea that galaxies have habitable zones as well. The zones would have a good mix of chemical elements, not too much radiation, and a low risk of exploding stars.

Stars consist mainly of hydrogen and helium. But to make planets like Earth, you need a good supply of heavier elements — things like oxygen, silicon, and iron. Those elements are forged in the hearts of stars, then blown out into space when the stars die. So to have good conditions for life, you need a region where lots of dead stars have “seeded” the galaxy with heavy elements.

But you don’t want to be too close to stars that die in massive explosions. They can damage a planet’s atmosphere, exposing life to high levels of radiation. So you want to be away from a galaxy’s core, where stars are packed close together — including ones that might explode.

The cores of most galaxies also harbor supermassive black holes. As they pull in stars and gas they produce lots of radiation, saturating the space around them.

So in our home galaxy, the Milky Way, you want to be away from the core, but not too far away. And that’s just where Earth is — about half way from the center to the edge — in the galactic habitable zone.

Script by Damond Benningfield