Final answer:
Planets are spherical primarily due to the force of gravity, which causes matter to assume the shape with the least gravitational potential energy over time. The role of thermal energy is secondary, where internal heat from gravitational contraction can affect the planet's structure but is not the key reason for its spherical shape.
Step-by-step explanation:
The reason why planets are spherical has to do with the force of gravity, which pulls objects toward their center of mass. Over time, gravity acts to mold planets into a spherical shape because this shape has the least gravitational potential energy for a given amount of matter. This process occurs during the formation of planets when they are still molten or semi-fluid and continues as they cool and solidify. The choice that best answers this question is: c) Planets are spherical because of the force of gravity, which pulls objects into a spherical shape over time.
Regarding the shape of planets and thermal energy, giant planets also generate some heat internally due to gravitational contraction and the presence of residual heat left from their formation. This internal heat can cause expansion in some layers and contribute to their spherical shape; however, gravity is the main cause for the spherical shape of the planets.
Answering the GRASP CHECK question, b. Earth appears to be the center of the solar system because, in the reference frame of Earth, the sun, moon, and planets all appear to move across the sky as if they were circling Earth. This is a geocentric perspective, which was historically believed before the heliocentric model was widely accepted.