Answer: increase in pressure.
Explanation:
CREATION OF SOLAR SYSTEM:
The most accepted explanation of how our Sun, the planets, and the Universe began is the NEBULAR HYPOTHESIS. The nebular theory states that the Sun, the planets, and all other objects in the Solar System formed from nebulous material billions of years ago.
With respect to The nebular hypothesis, like about 4.57 billion years ago, the cloud to collapse. And the products of the collapse are the pockets of dust and gas. Both the the pockets of dust and gas began to collect into denser region.
The more the denser region gathered more matter, INCREASING IN PRESSURE caused it to HEAT UP. Large amount of the material ended up in a ball at the center while small amount of the matter flattened out into disk.
