Variously described as dirty snowballs or snowy dirtballs, comets are prime candidates for seeding planets, including Earth, with water and organic material.

• 彗星以前一直被描述成由大量灰尘或积雪形成的不同球状物质所组成，彗星上存在水和有机物质，据信是包括地球在内的许多星体的诞生源头。