In a protoplanetary disk, material orbits around which central object?

• A. Proto-Sun
• B. Earth
• C. Protoplasm
• D. A distant star

Answer: (A)

A protoplanetary disk forms around a young star as it condenses from a collapsing cloud. The central object is the protostar, the newborn Sun in a solar-type system. Gravity from this central protostar pulls the surrounding gas and dust into orbital motion, creating the disk that orbits around it. The material stays in orbit because of angular momentum: it’s moving sideways as it’s pulled inward, so it doesn’t fall straight in but rather winds into a flattened disk. Over time, this disk provides the material from which planets can form. Earth is a body that would form inside such a disk and then orbit a star, not serve as the central object of the disk. Protoplasm is unrelated to celestial mechanics, and a distant star would have its own system rather than be the center of this disk in question.