The fiery demise of a fifth rocky planet in our Solar System might have led to a flurry of asteroid impacts that pockmarked the Moon and Earth billions of years ago.

The Late Heavy Bombardment (LHB) is a relatively brief period, about 3.9 billion years ago, when wayward space projectiles heavily pelted the Moon and inner planets. Craters on the Moon better match asteroids from the Asteroid Belt, located beyond the orbit of Mars.

Maybe it's the legendary (mytholological) Iburu. Or was that supposed to be the tenth planet?

I heard somewhere long ago, that while the asteroid belt was somewhere that a planet had room to form, the biggest problem would have been the tidal forces of Jupiter, which would have made any planet unstable, with Mars and Jupiter gathering what didn't remain in the orbit the asteroid belt has now. With the way computers and speeding up there may come a time when we can put in what data we have today, and run the program backwards.
The asteroid belt seems like it is to the Sun, what the rings are to Saturn, only bigger.