How can you measure the weight of a planet?

-Karthik Gurumurthy

The weight (or the mass) of a planet is determined by its gravitational effect on other bodies. Newton’s Law of Gravitation states that every bit of matter in the universe attracts every other with a gravitational force that is proportional to its mass. For objects of the size we encounter in everyday life, this force is so miniscule that we don’t notice it. However, for objects the size of planets or stars, it is of great importance.

In order to use gravity to find the mass of a planet, we must somehow measure the strength of its tug on another object. If the planet in question has a moon (a natural satellite), then nature has already done the work for us.By observing the time it takes for the satellite to orbit its primary planet, we can utilize Newton’s equations to infer what the mass of the Planet must be.

For planets without observable natural satellites, we must be more clever. Although Mercury and Venus, for example, do not have moons, they do exert a small pull on each other and on the other planets in the solar system. As a reult, the planets follow paths that are subtly different than they would without this disturbing effect. Although the math is a lot more complicated and the uncertainties are greater, astronomers can use these small deviations to determine how massive the moonless planets are.