Planets

Definition

Planets are large, spherical celestial bodies that orbit around stars, such as the Sun, in the universe. They are distinguished from other objects in the solar system, like stars, moons, and asteroids, by their size, composition, and the way they move through space.

5 Must Know Facts For Your Next Test

1. Planets are classified into two main categories: terrestrial planets (rocky, Earth-like planets) and gas giant planets (large, gaseous planets).
2. The eight major planets in our solar system are Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, and Neptune.
3. Planets are kept in their orbits around the Sun due to the force of gravity, as described by Newton's Law of Universal Gravitation.
4. The elliptical shape of planetary orbits is a consequence of the balance between the centrifugal force and the gravitational force acting on the planets.
5. The motion of planets around the Sun is governed by Kepler's Laws of Planetary Motion, which describe the relationships between a planet's orbital period, distance from the Sun, and velocity.

Review Questions

• Explain how the force of gravity keeps planets in their orbits around the Sun.
  • According to Newton's Law of Universal Gravitation, the Sun's gravitational pull on the planets is the force that keeps them in their orbits. This gravitational force is directed towards the Sun and is proportional to the masses of the Sun and the planet, as well as the inverse square of the distance between them. The balance between this gravitational force and the centrifugal force experienced by the planet as it moves in its elliptical orbit results in the planet's stable, continuous motion around the Sun.
• Describe how Kepler's Laws of Planetary Motion relate to the elliptical shape of planetary orbits.
  • Kepler's First Law states that the orbit of every planet is an ellipse with the Sun at one of the two foci. This means that the planets do not orbit the Sun in perfect circles, but in slightly flattened, elliptical paths. Kepler's Second Law further describes how a planet's orbital velocity varies as it moves around the Sun, with the planet moving faster when it is closer to the Sun and slower when it is farther away. These relationships between a planet's distance from the Sun and its orbital velocity are a direct consequence of the elliptical shape of the planet's orbit, as dictated by Newton's Law of Universal Gravitation.
• Analyze how the different classifications of planets (terrestrial and gas giants) relate to their composition and the role of gravity in their formation and structure.
  • The two main categories of planets, terrestrial and gas giants, differ significantly in their composition and the role of gravity in their formation and structure. Terrestrial planets, like Earth, are relatively small, dense, and rocky, with a solid surface. Their high density is a result of the strong gravitational forces that compressed the materials during their formation, allowing them to retain a solid, rocky structure. In contrast, gas giant planets, such as Jupiter and Saturn, are much larger, less dense, and composed primarily of gases like hydrogen and helium. The immense gravitational forces present in the gas giants' formation allowed them to accumulate large quantities of these lighter gases, which were then held in place by the planets' powerful gravitational fields. The differences in the planets' compositions and structures are directly related to the varying degrees of gravitational influence that shaped their development.

Related terms

Solar System: The solar system is the gravitationally bound system of the Sun and the objects that orbit it, including planets, dwarf planets, moons, asteroids, comets, and other small bodies.

Gravity: Gravity is the force of attraction between any two objects with mass, which keeps planets in orbit around the Sun and causes the motion of all celestial bodies.

Elliptical Orbit: The elliptical orbit is the path a planet takes as it revolves around the Sun, which is an ellipse (a flattened circle) rather than a perfect circle.