Theia

A colossal interplanetary collision doesn't sound like a good thing.

But when Theia collided with Earth 4.5 billions years ago, it resulted in a satellite that is anomalously big in comparison to its parent planet. There is nothing else like it in the solar system, where satellites are relatively small bodies that either accredited slowly from orbiting debris or were captured in passing. Elsewhere it seems a similar story. Giant collisions in other solar systems would produce abundant dust visible to the infrared Spitzer Space Telescope, but although a few such dusty systems have been found, collisions big enough to produce something like the moon seem to happen in only 5 to 10 per cent of solar systems - with the number of instances where this has actually happened considerably smaller even than that.

Why does this matter? Because the moon's size provides a steadying gravitational hand that helps to stabilise the tilt, or "obliquity", of Earth's axis. That prevents wild changes in the pattern of solar heating on the planet's surface that could lead to extreme climate swings, including frequent periods where the whole planet freezes over. That's a big deal for us. Conditions might be bad for complex land-based life if there were no moon and obliquity varied significantly.

more space info to come, and pictures.