Don: I'm worried, Yaël.

 

Yaël: What about?

 

D: Well, this might sound kinda paranoid, but what if the Earth collided with another  planet?

 

Y: You're really worried about that?

 

D: We do a science show, and everyday we're learning about strange things that nobody  knew about before, so yeah, it's crossed my mind.

 

Y: OK. Well, since the planets of our solar system have been in their fixed orbits for  billions of years, it seems unlikely that we'd suddenly collide with Venus or Mars.

 

D: I know, but scientists at UCLA and Tennessee State University noticed a strange ring  of debris and dust orbiting a star. They thought at first that the star was young---only a few million years old---but then realized that it was billions of years old, like our sun.

 

Y: OK, so?

 

D: So given the star's age and the distance of all that dust and debris from the star, that  stuff should have coalesced into planets by now or been blown away into space. So  the theory is that the debris was caused when two planets collided and blew each other to bits.

 

Y: Wow. So if that's true, then it means that it's possible for planets in a stable solar system to somehow get off track and run into each other. Scary thought.

 

D: Yep. Actually, many scientists think that when the earth was about a million years  old, it collided with a body about the size of Mars, and that the resulting debris came  to form our moon.

 

Y: Interesting. So if we're ever struck by another planet, maybe Earth will get another  moon.

 

D: Maybe, but nobody would be around to see it.