### Passages: Origin of Hot Jupiters
Astronomers have proposed three models to explain the existence of "hot Jupiters"—giant planets with orbital periods of less than 10 days that orbit very close to their host stars.
Model 1 (In-situ Formation)
Giant planets form at their current close-in locations ( from the host star). Protoplanetary disks under certain conditions can concentrate high densities of rocky and icy grains in the inner disk. This local concentration allows a solid core of approximately Earth masses () to accumulate rapidly. Once the core forms, it quickly accretes gas from the surrounding disk before the disk dissipates (typically within 10 million years).
Model 2 (Disk Migration)
Giant planets cannot form close to their host stars because the high temperatures and intense stellar winds prevent the accumulation of volatile gases. Instead, they form in the outer disk () where volatile materials are abundant. As the planet orbits, it exerts gravitational forces on the surrounding gaseous disk, creating spiral density waves. These waves exert a net torque on the planet, causing its orbit to shrink. The planet spirals inward toward the star (Type II migration) over 1 to 5 million years, maintaining a circular orbit that remains aligned with the star's equator.
Model 3 (High-Eccentricity Tidal Migration)
Giant planets form in the cold outer disk (). Gravitational perturbations from a distant companion star or another massive planet disrupt the giant planet's orbit, forcing it into a highly eccentric (non-circular) and highly inclined orbit. During periastron passage (closest approach to the host star), the star's strong gravity raises tidal bulges on the planet. The tidal friction converts orbital energy into thermal energy within the planet, causing the orbit to gradually shrink and circularize over hundreds of millions of years, long after the protoplanetary gas disk has dissipated.
### Matching Task
Match each new experimental finding on the left to its correct implication for the models on the right.
- Spectroscopic measurements of a young protoplanetary disk showing that the total mass of solids within of the host star is less than of the mass needed to form a core.Contradicts Model 1 by demonstrating that the raw materials required for rapid core growth are not present in the inner disk.
- Observation of a young hot Jupiter (age ) in a circular, coplanar orbit around a star whose gaseous disk is still actively accreting.Supports Model 2 (and contradicts Model 3) because disk-driven migration occurs rapidly while gas is present, producing circular, aligned orbits, whereas tidal migration requires hundreds of millions of years.
- Detection of a hot Jupiter in a highly inclined, retrograde orbit around a star that has a stellar companion on a wide, inclined orbit.Supports Model 3 because the high orbital misalignment and presence of a companion star are key signatures of gravitational perturbations and subsequent tidal circularization.