What does the nebular hypothesis propose?

• A. The solar system formed from a spinning cloud of gas and dust that flattened into a disk; planets formed within that disk
• B. The solar system formed from a single planet breaking apart
• C. The sun formed after all planets
• D. Planets formed from interstellar comets only

Answer: (A)

The nebular hypothesis says the solar system began as a spinning cloud of gas and dust that collapsed under gravity and flattened into a rotating disk. The center became the Sun while material in the surrounding disk clumped together through collisions and gravity to form planetesimals, which grew into the planets. This explains why planets orbit in roughly the same plane and in the same direction, and why they formed from the same disk of material as the Sun.

This idea fits observed features like the orderly, disk-shaped layout of the planets and the composition differences between rocky planets near the Sun and gas giants farther out. It also avoids implausible notions like a single planet breaking apart to create the system, or the Sun forming after all the planets, or planets forming only from interstellar comets.