The Solar System

The solar system started as a swirling cloud of gas and dust that collapsed under gravity. As it collapsed it spun faster and flattened into a disk, the same way pizza dough spreads out when you spin it. The sun formed at the center, and the planets formed from clumps of material still moving together in that flat disk, in the same direction. That shared origin is why all eight planets orbit in nearly the same plane and the same direction around the sun today.

Distance from the sun controls orbital period two ways at once. A farther planet has a much longer path to travel around its orbit, and it moves more slowly along that path because the sun's gravity pulls on it less strongly out there. Both effects stretch out the year, so period increases sharply with distance. That is why, among Mercury, Earth, Jupiter and Neptune, Neptune has the longest year without any calculation: it sits farthest out, so both the longer path and the slower speed stretch its year far beyond Earth's.

This connects directly to seasons, a separate idea that trips students up. Earth's orbit is nearly circular, so distance from the sun barely changes month to month. Seasons come instead from Earth's axial tilt, about 23.5 degrees, which points the northern hemisphere toward the sun in July and away in January. That tilt changes sunlight angle and day length far more than the small orbital distance shift does. Distance still matters elsewhere: it sets how much sunlight a planet gets to begin with, before an atmosphere changes the story.