Or tides will prevent a moon from forming inside this distance Almost all rings are inside the Roche limit. Comet orbits suggest a huge reservoir of objects far away. However, they could not have formed there, wasn't enough material out there. Comets and water-bearing asteroids may have been early sources of water and building blocks of life . Major extinction of life 65 million years ago.
Iridium is much more abundant in iron meteorites.
Life began 3.8 billion years ago. This is just the beginning, the heavy ones are the easiest to find.