Theory — The Giant Planets, Rings, and Moons

The four giant planets are utterly unlike the rocky terrestrial worlds. Jupiter and Saturn are gas giants, mostly hydrogen and helium; Uranus and Neptune are ice giants, richer in water, ammonia, and methane. All four are far larger and more massive than Earth and have no solid surface to stand on.

1. Banded atmospheres and storms

Rapid rotation stretches their clouds into light zones and dark belts running parallel to the equator. Long-lived storms, such as Jupiter’s Great Red Spot, swirl between the bands.

2. Rings and moons

Every giant planet has a ring system, made of countless icy and rocky particles orbiting in a thin disk; Saturn’s are the brightest. Each also hosts a family of moons, from tiny shepherd moons to worlds larger than Mercury, such as Jupiter’s Ganymede and Saturn’s Titan.

3. Weighing a planet with a moon

A moon’s orbit is governed by the planet’s gravity, so the moon’s orbital size and period reveal the planet’s mass. Newton’s form of Kepler’s third law, written in convenient units, is:

Mass from a moon’s orbit M = a3 / P2
  M in solar masses, a in AU, P in years
Measure a moon’s orbit and you have weighed the planet.

Because the moon’s own mass is tiny compared with the planet’s, it can be neglected, so this simple form works well for planet-plus-moon systems.

4. Why this matters

The same method, applied to a star with an orbiting planet or a companion star, is how astronomers measure stellar masses. Weighing a giant planet from one of its moons is the same physics on a smaller stage.

Apparatus

Studying the giants uses tools for imaging the disk, spreading its light, and timing moon orbits. In the simulation these are modelled, but the readings match what each instrument would give.

Banded planet disk
An imaged disk showing the light zones and dark belts of a giant planet.
Ring system
A thin disk of icy particles orbiting a giant planet, seen nearly edge-on.
Galilean moon
A large moon whose orbit is timed to weigh the planet it circles.
Telescope
Used to image the giant planets and track the positions of their moons over time.
Spectrometer
Reveals the hydrogen, helium, and methane in a giant planet’s atmosphere from its spectrum.
Orbit timer
A clock used with successive images to measure a moon’s orbital period.

Instructions

Work through both tabs. Calculate first by hand, then press the button to compare.

Part A — Planet mass from a moon’s orbit

Part B — Rings and moons

Simulation

Giant-Planet BenchCalculate first, then check against the simulation.

Team Questions

1. Jupiter and Saturn are classified as:
2. A moon’s orbit lets us find a planet’s:
3. The light and dark bands on a giant planet are caused mainly by:

Example Report

Worked example: the mass of Jupiter from Ganymede

Ganymede orbits Jupiter with a semi-major axis a = 7.15 × 10-3 AU and period P = 1.96 × 10-2 yr.

M = a3 / P2 = (7.15 × 10-3)3 / (1.96 × 10-2)2 ≈ 3.66 × 10-7 / 3.84 × 10-4 ≈ 9.5 × 10-4 solar masses.

Jupiter’s mass is about 9.5 × 10-4 solar masses (roughly 318 Earth masses), so the calculated and accepted values agree. Weighing the planet from its moon is the calculate-then-compare core of the lab.

Practice Questions

1. Uranus and Neptune are called ice giants because they contain more:
2. Which planet has the brightest, most prominent rings?
3. In M = a^3 / P^2 with solar masses, AU, and years, a and P are the moon’s: