How can we understand the existence of the recently discovered, "hot Jupiters" (gas giant planets close to a star)?

The best explanation is that they participated in a planetary migration inward from their site of birth.

• the disk gas slowly rains in on the star, and this motion drags the planets along inwards
• the gravitational force of the planets themselves create spiral density patterns in the disk gas which in turn cause the planets to lose angular momentum and fall inward

Planets would continue this "death march" inwards until the disk gas is dispersed (the final result depends on the timing of events).

Computer simulations are used to study how the planets grow by accretion from the disk and how the spiral structures change their orbits.