Because they formed (conglomerated) from an accretion disk around the Sun. Being a disk, it is in one plane. Why a disk? The sphere of dust surrounding the S

Because they formed (conglomerated) from an accretion disk around the Sun. Being a disk, it is in one plane. Why a disk? The sphere of dust surrounding the Sun is drawn inwards and conservation of angular momentum flattens it out to eventually form a disk. Simple explanation: Accretion disk: http://www.daviddarling.info/encyclopedia/A/accretiond.html http://www.daviddarling.info/encyclopedia/A/accretiond.html Planetary formation: http://www.daviddarling.info/encyclopedia/P/plansysform.html http://www.daviddarling.info/encyclopedia/P/plansysform.html Wikipedia (detailed maths): http://en.wikipedia.org/wiki/Accretion_disc http://en.wikipedia.org/wiki/Accretion_disc