Final answer:
Jovian-sized exoplanets close to their stars can be explained by the concept of planet migration, where gravitational interactions within a protoplanetary disk or with other planets caused their inward movement from where they originally formed. Option D is correct.
Step-by-step explanation:
How might we explain the presence of jovian-sized exoplanets at distances normally occupied by terrestrial planets? The most widely accepted explanation is planet migration. Present theory suggests that giant planets, such as the so-called 'hot Jupiters,' formed at a distance where water ice could condense, which is several astronomical units away from the star.
However, these planets did not stay at their formation distances. Gravitational interactions within a protoplanetary disk or later with sibling planets could have caused these planets to migrate inward to orbits closer to their stars, where we observe them now.
This understanding of planetary migration is supported by evidence that Jupiter may have migrated inward in our own solar system, influencing the orbits of other planets, including potentially causing the inward migration of Uranus and Neptune from their original positions.
Determining the presence of these massive planets closer to their stars than Mercury is to our Sun has led to a revision in our understanding of planet formation and the dynamic nature of planetary systems. Other explanations, such as them being brown dwarfs captured by parent stars or massive terrestrial planets, are less likely due to the conditions required for their formation.