... Although they all formed from similar material, it is a mystery why the planets closest to the Sun Mercury, Venus and Earth are so different. Astronomers hope to learn more about how they were created by studying asteroids, which are pieces of rock that were surplus to requirements in planet building. A team of astronomers has been observing an asteroid called Lutetia that is 100 kilometres wide. ... Meteorites are much smaller than asteroids and range in size from marbles to basketballs.) ...