http://www.newsweek.com/meteroite-comet-building-blocks-solar-system-formation-1396417

A fragment of cometary building block has been found inside a meteorite that broke away from an asteroid. The rare discovery provides a critical insight into the formation of the solar system over 4.5 billion years ago, and how it evolved into what we see today.

When the Sun first formed, it is believed to have a cloud of gas and dust. Gravitational forces clumped much of this together to form the planets. The rest made up the moons, dwarf planets, asteroids and comets. The difference between the latter two relates to composition—asteroids tend to be made of metal and rock, while comets are made up of ice, dust and rocky material. Comets are normally found further away from the Sun, in the colder parts of the solar system.