Gravity is the universal force of attraction between any two objects that possess mass. All planets have gravity because it is a fundamental property of matter. Every planet is an immense collection of matter, and this mass generates a gravitational field that pulls everything toward its center. This article explores the mechanism behind this force and explains why the gravitational pull experienced on the surface varies widely from one planet to the next.
The Fundamental Cause of Planetary Gravity
The force of gravity originates from the existence of mass, formalized by Sir Isaac Newton’s law of universal gravitation. This law states that every particle of matter attracts every other particle with a force proportional to the product of their masses. Planets are massive structures, and their gravity is the cumulative pull of all the material they contain, from the core to the surface layers.
The gravitational attraction between two objects, such as a planet and a person on its surface, is directly linked to the planet’s mass. A more massive planet exerts a stronger gravitational force. Gravity is a universal force that holds the planet together and governs the orbits of moons and the structure of the solar system. This attraction ensures that any object near a planet will accelerate toward the planet’s center of mass unless moving fast enough to escape.
Factors That Determine Planetary Gravity
While all planets have gravity, the surface gravity—the force a person would feel—differs substantially based on two factors: the planet’s mass and its radius. The relationship between these factors is governed by the inverse square law, which states that gravitational force weakens rapidly as the distance from the center of mass increases. Therefore, the distance from the center of the planet to its surface is just as important as its total mass in determining the surface experience.
A planet with a large mass will have a strong gravitational pull, but if it is also very large, a person on the surface is farther from the center, reducing the perceived surface gravity. Earth’s surface gravity is used as the standard, labeled as one g (1g). Mars, which is less massive and smaller than Earth, has a weaker pull, measuring only about 0.38g.
Conversely, a gas giant like Jupiter is massive, roughly 300 times the mass of Earth. Despite its immense size, which places the cloud tops far from the center, Jupiter’s mass results in a surface gravity of approximately 2.4 times that of Earth (2.4g) at its upper atmosphere. Jupiter’s gravity is powerful enough to influence the shape and orbits of its many moons.
Gravity on Non-Planetary Objects
Gravity is not exclusive to the eight recognized planets; any celestial body that possesses mass exerts a gravitational pull. This includes moons, asteroids, and dwarf planets like Pluto and Ceres. These bodies possess gravity for the same reason as the planets.
However, because these objects are significantly smaller and less massive than planets, their gravitational fields are weaker. For instance, Earth’s moon has a surface gravity of only about 0.165g, allowing astronauts to leap across its surface. The gravity on an asteroid can be so weak that a person could easily push off and escape into space.
The weak gravitational pull on smaller bodies impacts their environment, particularly their atmosphere. A low escape velocity, directly related to low gravity, makes it difficult for these objects to retain light gases over long periods. This is why bodies like the Moon and many asteroids are airless, while the dwarf planet Pluto has only a thin, temporary atmosphere.