Gravity is the force that governs the structure of the cosmos, dictating how matter attracts other matter. Every object in the solar system exerts this fundamental force. The gravitational pull experienced on a celestial body’s surface determines the weight of an object standing on it. Understanding which world has the weakest grip requires examining the physics of gravity and how it is calculated. This exploration reveals a surprising near-tie among the major planets and an absolute winner among the smaller bodies.
Determining Surface Gravity
Surface gravity, often denoted as g, is the acceleration an object experiences at the body’s surface. It is the standard measure used to compare the gravitational pull of different worlds. This value is fundamentally determined by a body’s mass and its radius. Surface gravity is directly proportional to the object’s total mass, meaning more massive objects have a stronger gravitational pull.
The calculation also involves an inverse square relationship with the radius. This means the gravitational pull weakens rapidly as the distance from the center of mass increases. The balance between high mass pulling inward and large radius pushing the surface outward dictates the final gravitational environment. Earth’s surface gravity is the standard baseline, set at 1g, which corresponds to an acceleration of approximately 9.8 meters per second squared.
Comparing celestial objects requires looking at their overall composition and density. Density is the amount of mass packed into a specific volume. A large, puffy planet made of low-density gas might have a lower surface gravity than a smaller, denser rocky planet. This complex interplay explains why simply looking at a body’s size is not enough to determine its surface gravity.
The Lowest Gravity Among the Eight Planets
When considering the eight major planets, the one with the least surface gravity is Mars. The Red Planet’s surface gravity measures approximately 0.377 times that of Earth, meaning an object would weigh slightly less than 38% of its Earth weight. This low value is a direct result of Mars’ relatively low mass, which is only about 10.7% of Earth’s total mass.
The planet that comes closest to challenging Mars for the lowest gravity title is Mercury, the solar system’s smallest major planet. Mercury’s surface gravity is measured at approximately 0.378g, a value so close to Mars that the two are often quoted as having nearly identical gravitational pulls. This near-tie is a remarkable consequence of Mercury’s extremely high density, which is second only to Earth among the rocky planets.
Despite being significantly smaller than Mars, Mercury is packed with a massive iron core, which concentrates a large amount of mass into a small volume. This high mass-to-volume ratio gives Mercury a disproportionately strong gravitational pull for its size. Conversely, Mars is less dense than Mercury, and although it is physically larger, its mass is not as concentrated, resulting in the marginally lowest surface gravity among the eight planets.
The Solar System’s Absolute Lightweights
Once the scope is broadened beyond the eight major planets to include dwarf planets, moons, and asteroids, the minimum surface gravity drops dramatically. The most famous example of a low-gravity environment is the dwarf planet Pluto, which has a surface gravity of only about 0.063g, or roughly 6.3% of Earth’s gravity. An average person standing on Pluto would weigh only a few pounds, and a simple jump could launch a person over twenty feet into the thin, tenuous atmosphere.
Pluto’s extremely low gravitational pull is due to its low mass, which is less than one-sixth the mass of Earth’s Moon. This low gravity means that any atmosphere it possesses is extremely extended, and its escape velocity is quite low. Despite Pluto being the most prominent of the low-gravity worlds, it is far from the absolute lightweight champion.
The true lightweights of the solar system are small, irregularly shaped moons and asteroids. For instance, Phobos, one of the two moons orbiting Mars, has a surface gravity of just 0.00058g. This gravitational pull is so weak that an object could be launched into orbit by simply walking quickly. The extremely low gravity of these small bodies makes them fascinating destinations, as a person could jump nearly a kilometer high without ever reaching escape velocity. These smaller, less massive objects represent the minimum in surface gravity within the solar system.