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| Space on Apr 21, 2014

How do astronomers know the mass of Jupiter?

Learn to find the mass of our solar system’s largest planet, Jupiter, using its orbiting moons.

Jupiter is the largest planet in our solar system and boasts of more than twice the mass (heaviness) of all the other solar system planets, dwarf planets, moons and asteroids combined. But how do astronomers even begin to know Jupiter’s mass? If a planet has an observable moon (or moons), astronomers can figure out that planet’s mass. Jupiter has four major moons – Io, Europa, Ganymede and Callisto – that’ve been watched and studied with great intensity, ever since Galileo first discovered them through an early telescope in the year 1610. Follow the links below to learn more about finding the mass of Jupiter, using its moons.

How can an orbiting moon reveal its planet’s mass?

Computing Jupiter’s mass with Jupiter’s moon Callisto.

Computing Jupiter’s mass with Jupiter’s moon Io.

Refining our answers.

The Galilean moons, in their order going outward, from Left to right: Io, Europa, Ganymede and Callisto

The Galilean moons, in their order going outward, from Left to right: Io, Europa, Ganymede and Callisto

Io orbits Jupiter in approximately 1.8 days, while Europa does likewise in 3.6 days and Ganymede in 7.2 days. Callisto (not shown), the outermost moon, has an orbital period of about 16.7 days.

Io orbits Jupiter in approximately 1.8 days. Europa orbits in 3.6 days. Ganymede orbits in 7.2 days. Callisto (not shown), the outermost moon, takes about 16.7 days to complete a single orbit around Jupiter.

How can an orbiting moon reveal its planet’s mass? The more massive the planet, the more swiftly its moons revolve around it. Because Jupiter’s moons move in orbit around Jupiter so very swiftly, astronomers know right off the bat that Jupiter is an exceedingly massive world. Jupiter’s moons – Io, Europa, Ganymede and Callisto – lie more distant from Jupiter than our moon does from Earth. Yet these moons orbit Jupiter in far less time than our moon orbits Earth.

If Earth were as massive as Jupiter, our moon’s orbital period would be some 1.5 days, instead of its present 27.322 days!

By orbital period, we mean the period of time that it takes the moon to go full circle in front of the constellations of the Zodiac. This time period is known as the sidereal month.

Computing Jupiter’s mass with Jupiter’s moon Callisto. Yes, we can compute Jupiter’s mass, relative to the mass of Earth, with Jupiter’s moon Callisto. All we need to know is Callisto’s mean distance from Jupiter, or semi-major axis, in Lunar Distances (LD), and Callisto’s orbital period relative to the moon’s orbital period (sidereal month).

One Lunar Distance (LD) = 384,400 kilometers (238,855 miles)

Moon’s orbital period = 27.322 days.

Callisto’s mean distance from Jupiter is 1,882,700 kilometers (1,169,856 miles) and its orbital period is 16.689 days. Converting Callisto’s mean distance and orbital period into lunar figures:

a = Callisto’s mean distance = 4.898 lunar

p = Callisto’s orbital period = 0.611 lunar

We plug these numbers into the equation below. Voila! We have Jupiter’s mass in Earth masses.

Mass of Jupiter = a3/p2
Mass of Jupiter = a x a x a/p x p
Mass of Jupiter = 4.898 x 4.898 x 4.898/0.611 x 0.611
Mass of Jupiter = 314.756 Earth-masses

Computing Jupiter’s mass with Jupiter’s moon Io. We can double-check our answer, using Jupiter’s moon Io, whereby a = Io’s mean distance = 1.097 Lunar Distances (LD), and p = Io’s orbital period = 0.0648 lunar.

Mass of Jupiter = a3/p2
Mass of Jupiter = a x a x a/p x p
Mass of Jupiter = 1.097 x 1.097 x 1.097/0.0648 x 0.0648
Mass of Jupiter = 314.3909 Earth-masses

Computing Jupiter’s mass with either Jupiter’s moon Callisto or Jupiter’s moon Io gives us pretty much the same answer.

By the way, Earth’s mass = 5.97 x 1024 kilograms

Jupiter's moons as seen through the telescope. Image credit: Jan Sandberg

Jupiter’s moons as seen through the telescope. Image credit: Jan Sandberg

Refining our answers. Technically speaking, we actually figured Jupiter’s mass relative to the Earth-moon system, not the Earth itself.

However, since the Earth comprises about 98.78% (0.9878) of the mass in the Earth-moon system, our answer represents a good approximation of Jupiter’s mass in earthly masses. We’re also presuming that virtually all the mass in the vast Jovian system of Jupiter and its 67 known moons is contained within the planet itself.

Image credit: NASA

Image credit: NASA

Should we wish to refine our answer, we can multiply 1/0.9878 x 314.3909, which gives us a little over 318 Earth-masses. NASA’s fact sheet says Jupiter’s mass equals 317.83 Earth masses, so our revised answer is close to correct.

Bottom line: This post shows you how to find the mass of the solar system’s largest planet, Jupiter, using its orbiting moons. The same principle can be used to find the masses of other planets in distant space, or of stars in multiple star systems.