The discovery of extrasolar planets has provided strong validation for the nebular theory, but it also forced scientists to recognize that the theory was incomplete. Which idea was added to the theory after the discovery that some massive planets orbit very close to their stars?

Planets can migrate inward or outward from the orbits on which they originally formed

Explanation:

The most widely accepted theory of planetary formation, known as the nebular hypothesis, maintains that 3.2 billion years ago, the Solar System formed from the gravitational collapse of a giant molecular cloud which was light years across. Several stars, including the Sun, formed within the collapsing cloud. The theory became incomplete because scientists found that the planets could move from the orbit where they were originally formed.