单选题 .  ①Astronomers have had difficulty accounting for certain planets discovered outside our solar system. ②They are called hot Jupiters because each is similar in mass to Jupiter, the largest solar-system planet, but orbits its parent star at a fraction of the distance at which Earth, let alone Jupiter, orbits the Sun. ③In the standard, solar-system-based theory of planetary formation, such as a massive planet could not form so close to a star. ④So most attempts to explain a hot Jupiter's existence envision it forming farther away, then migrating inward. ⑤According to one hypothesis, the planet's gravitational field tugs on the protoplanetary disk of dust and gas from which it formed. ⑥The disk exerts its own gravitational tug, and this interplay of forces robs the planet of momentum in its orbital path, forcing it to spiral in toward the star. ⑦According to another hypothesis, the planet's gravitational field is so strong that it creates a groove in the disk, partitioning it into inner and outer regions; the resulting gravitational interactions between the planet and these regions cause the planet to lose orbital momentum and spiral inward. ⑧Another question remains: what prevents the planet from continuing its spiral until it collides with the star?29.  The author of the passage mentions "Earth" primarily in order to ______