Mass is a fundamental property of every object in the universe. It tells us how strongly an object pulls on other things through gravity, how it responds to forces, and how it evolves over time. For celestial bodies—planets, moons, asteroids, comets, stars, and even entire planetary systems—knowing the mass is key to answering basic questions: Does a planet have enough gravity to hold an atmosphere? Is an asteroid a loose rubble pile or a dense metal object? How massive is a star, and what does that tell us about its lifetime and the types of planets it might host?
Unlike everyday objects, we can’t put a planet or a star on a scale. Instead,
scientists infer mass by watching how gravity acts. The core idea—used for more
than three centuries—is simple but powerful: gravity controls orbits. If we can
measure how fast something orbits and how far away it is, we can compute how
much mass is doing the pulling. Over time, this principle has been refined and
expanded with new tools: precise telescopes, spacecraft tracking, laser
ranging, and sensitive radio links. This article explains the main ideas in
clear terms, moving from the historical roots to the most modern techniques
applied across the solar system and beyond.
Historical
Methods
In the past, scientists had limited tools and knowledge, yet they
still managed to estimate the masses of celestial bodies using clever methods.
One of the earliest techniques involved studying the motion of moons or nearby
objects. For example, in the 17th century, Johannes Kepler described how
planets move around the Sun. Later, Isaac Newton applied his law of universal
gravitation to planetary and lunar motion, allowing scientists to link the mass
of a body to the movement of its satellites.
A well-known example is the study of Jupiter. Galileo discovered
four of Jupiter’s largest moons in 1610. By carefully tracking their orbital
periods and distances, scientists could apply Newton’s equations to calculate
the gravitational pull of Jupiter, and therefore its mass. This was one of the
earliest accurate mass estimates for a celestial body.
Similar logic worked for Saturn using its moons and rings. Later,
small mismatches—called perturbations—in planetary orbits provided new clues.
For example, oddities in Uranus’s path led astronomers in the 1840s to predict
and then observe Neptune. From the size of the perturbations and the orbit
Neptune eventually followed, its mass could be estimated.
The
core idea: Orbit reveals Mass
If a small object orbits a larger one, the time it takes to complete
an orbit (the orbital period) and the size of its orbit (often described by the
semi‑major axis, which is essentially the average orbital radius for most
purposes) are tied to the total mass of the system. In simple terms: heavier
central objects make things orbit faster at a given distance. This idea is
captured by a relationship sometimes called Newton’s version of Kepler’s third
law. In words, it says that the square of the orbital period depends on the
cube of the orbital size divided by the mass that is pulling.
When one body is much more massive than the other (for example, the
Sun compared with Earth, or Jupiter compared with a small moon), the period and
size of the orbit mostly reveal the mass of the larger body.
Modern
Methods
Today, astronomers have advanced technology, including spacecraft,
telescopes, and computer models. These tools provide much more precise
measurements of the masses of celestial bodies. One of the most reliable
techniques is to track spacecraft as they fly past or orbit a body. By
measuring the tiny changes in the spacecraft’s speed and direction, scientists
can calculate the gravitational pull of the body and determine its mass.
Another modern method involves studying moons, rings, and nearby
objects. Just as early astronomers did with Jupiter’s moons, scientists today
apply Newton’s and Kepler’s laws with far greater accuracy. For example, data
from spacecraft like Voyager, Cassini, and Juno have provided detailed
measurements of the moons and rings of giant planets, leading to very accurate
mass estimates.
Estimating
the Mass of Distant Celestial Bodies
Beyond our Solar System, scientists also estimate the masses of
celestial bodies such as exoplanets orbiting other stars. Since these objects
are too far away for spacecraft to visit, astronomers use indirect methods. One
of the most common is the radial velocity technique, which measures the small
wobble of a star caused by the gravity of an orbiting planet. This wobble
allows scientists to estimate the planet’s mass. Another method is transit
timing variations, which looks at how multiple planets in the same system
affect each other’s orbits.
In addition, the masses of stars, asteroids, and dwarf planets are
estimated through their gravitational influence on nearby bodies or spacecraft.
Binary star systems are particularly useful, since the motion of two stars
around each other directly reveals their masses.
Conclusion
From Galileo’s early telescope observations to modern space
missions, scientists have continuously improved the ways they estimate the mass
of celestial bodies. Historical methods relied on careful observation and
mathematical reasoning, while today’s techniques use advanced spacecraft and
precise measurements. Whether studying planets, moons, asteroids, stars, or
exoplanets around distant stars, estimating mass remains a key step in
understanding how celestial bodies form, evolve, and interact with their
environments. However, the key to estimating mass is gravity and motion.