Every planet has gravity, and this fundamental force is a universal law of nature. Gravity is an attractive force that exists between any two objects that possess mass. This invisible pull keeps our feet on the ground and holds Earth in its orbit around the Sun.
The Universal Answer Why All Planets Have Gravity
Every object in the universe, from the smallest asteroid to the largest star, exerts a gravitational influence because it has mass. The force of attraction between two objects is directly proportional to the amount of mass they contain. Planets are immense accumulations of mass, which guarantees the existence of their gravitational fields.
The force also depends on the distance separating objects, but a planet’s enormous mass ensures its gravitational field is the dominant force in its immediate vicinity. This field pulls all of a planet’s material inward, which is why large celestial bodies naturally form into a spherical shape. Without this inward-pulling force, the collection of dust and gas that forms a planet could never coalesce into a cohesive body.
Determining the Strength of Planetary Gravity
While all planets have gravity, the strength of that pull varies significantly between them. A planet’s surface gravity is determined by two primary factors: its total mass and its radius, which is the distance from the center to the surface. The greater a planet’s mass, the stronger the total gravitational force it generates.
The radius is equally important because the gravitational force weakens rapidly as the distance from the planet’s center increases. The force decreases by the square of the distance, meaning doubling the distance reduces the gravity to one-quarter of its original strength. This relationship explains why a large, low-density gas giant can have a surface gravity not much stronger than Earth’s. Although the gas giant has enormous mass, its effective “surface” is much farther from the center, which dilutes the pull.
A planet with a large radius but the same mass as Earth would have a weaker surface gravity because the surface is farther from the center of mass. Scientists calculate surface gravity using the mass and the radius, which provides the acceleration an object would experience at that planet’s surface. Both mass and radius must be considered together to accurately predict the gravitational field on a planet’s surface.
A Gravity Tour of the Solar System
The varying mass and radius of the planets result in a wide range of surface gravities across the solar system. Compared to Earth’s gravity, which is often used as a baseline, Mars offers a much lighter experience. With a surface gravity that is only about 38% of Earth’s, a person weighing 150 pounds on Earth would weigh approximately 57 pounds on Mars.
Conversely, the massive gas giant Jupiter presents a far more intense gravitational field. Jupiter’s surface gravity is roughly 2.4 times that of Earth, meaning the same 150-pound person would weigh about 360 pounds there. This difference highlights how a planet’s mass can dramatically increase the gravitational pull despite the large radius of the gas giant.
Even the Moon, which is a satellite and not a planet, provides a useful comparison with its surface gravity at only about 17% of Earth’s. Such low gravity allows astronauts to bound across its surface with ease, demonstrating the practical effect of a smaller, less massive body.