The idea of journeying through the cosmos often sparks curiosity about how our most fundamental physical experience—our weight—would change. Unlike many properties we possess, a person’s weight is not a fixed number, but a dynamic measurement that is entirely dependent on location. Traveling to another celestial body would dramatically alter this measurement, offering the possibility of feeling far lighter or much heavier than we do on Earth. This variation is a direct consequence of the physical laws governing the universe. The answer lies not just in the size of a world, but in a delicate balance of its mass and diameter.
The Difference Between Weight and Mass
To understand why your weight changes from planet to planet, it is necessary to first distinguish it from your mass. Mass is a measure of the total amount of matter contained within your body. This quantity is an intrinsic property that remains constant regardless of where you are in the universe. If you have a specific mass on Earth, you will have that exact same mass on Mars, Jupiter, or floating in the vacuum of space.
Weight, in contrast, is not a measure of matter but a measure of force. It is the force exerted on your mass by the gravitational pull of the body you are standing on. Because weight is directly dependent on the strength of the local gravitational field, it changes dramatically from one planet to the next. Scientifically, mass is constant, while weight is entirely variable.
The Physics Governing Planetary Gravity
The force that determines your weight, known as surface gravity, is governed by two principal factors: the planet’s total mass and its radius. According to the law of universal gravitation, the gravitational pull is directly proportional to the mass of the planet. A world with more material will exert a stronger tug on any object near its surface.
However, a planet’s size, or radius, acts in opposition to this effect. The gravitational force weakens rapidly with distance, following an inverse-square relationship. This means that a person standing on the surface of a planet with a larger radius is farther from the center of that planet’s total mass, which significantly reduces the felt gravitational pull.
Therefore, a planet’s surface gravity, and consequently your weight, is proportional to its mass divided by the square of its radius. This relationship explains why density also plays a significant role. A small planet made of extremely dense material, like iron, could potentially exert a stronger surface gravity than a much larger planet composed of very low-density gases.
Identifying the Planet with the Least Weight
When considering the eight major planets in our solar system, the one on which you would weigh the least is Mercury. The closest planet to the Sun is also the smallest and least massive, which results in a remarkably low surface gravity. Mercury’s gravitational pull is approximately 38% of Earth’s, meaning you would weigh less than half of your current weight there.
Mars, the other small terrestrial planet, is a very close second, with a surface gravity also calculated to be about 38% of Earth’s. The reason Mercury often edges out Mars for the lowest weight title is its slightly smaller mass, despite its higher density. If a person weighs 150 pounds on Earth, they would weigh only about 57 pounds standing on the surface of either Mercury or Mars.
This low surface gravity on both planets is a direct consequence of the physics principles at work. Although both planets have solid surfaces, their extremely low total mass is not enough to generate a strong gravitational field.
How Much Would You Weigh Elsewhere
Moving beyond the lowest-gravity worlds, the variation in weight across the solar system is surprisingly wide. On Venus, a planet often called Earth’s twin due to its similar size, you would weigh about 91% of your Earth weight. This is very close to home because Venus has a mass and radius nearly identical to ours, making its surface gravity only slightly less.
The gas giants present a more complex scenario because they lack a solid surface to stand on. Scientists calculate the surface gravity at the point where atmospheric pressure equals Earth’s sea-level pressure. Despite being the largest planet by far, Jupiter’s low density means its surface gravity is only about 2.34 times that of Earth, meaning a 150-pound person would weigh around 351 pounds.
The other massive gas giants actually have surface gravities close to or even less than Earth’s. On Saturn, you would weigh approximately 93% of your Earth weight, while on Uranus, your weight would drop to about 92% of your Earth weight. Neptune, the densest of the gas giants, would make you feel slightly heavier, with a surface gravity about 1.12 times that of Earth.