Why do the planets orbit the Sun in the same plane?

We think that this is a reflection of the way in which the cloud that collapsed to form our Sun was rotating. Computer models of such collapsing, rotating clouds show that they rapidly form into a flattened disk with a central bulge. In the bulge, the star will form, and if the disk is not too massive, it will either dissipate, or grow a retinue of planets. The planets will orbit the new star in the same plane as the original disk of gas out of which they accreted. Observations of distant, young stars only a few million years old, also show that they are surrounded by rotating 'placental' disks of gas. We think this is the evidence we have been looking for that our theory for how solar systems form is right on track.