How do we know that Neptune is made up of water, methane, and ammonia – above a small, rocky core…or that Saturn’s surface gravity is 10.44 m/s…or high wind speeds on Uranus?
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Lots of fancy scientific methods — the primary one being **spectroscopy.** This means that **we analyze the light in the planet’s atmosphere.**
Every element has a unique atomic structure, and thus reflects/absorbs light differently. Scientists can use these light readings and a bunch of math to determine the makeup of the planet.
As for the cores of planets, we infer based on what we learn about the atmosphere plus things like the planet’s mass, seismic activity — anything we can determine from flybys and telescopes.
Basically: **scientists know a LOT about various materials that could, theoretically, make up a planet.** They know about how those materials react to light, and how much they weigh, and how they interact with things like seismic activity.
Surface gravity is pretty straightforward, because the math for the gravity of a spherical object is straightforward. If you can time the orbit of a planet’s moon and measure the orbit’s size visually, you can work backwards to find the planet’s mass. Then you just plug that mass and the planet’s radius into the equation for gravitational force, and you’ve got the planet’s surface gravity.
For the chemical composition of a planet, we’ve got *spectroscopy.* Each chemical element tends to absorb and emit several very specific frequencies of light, so if you graph out a spectrum of exactly which colors an object reflects or absorbs, you can identify a “fingerprint” for each element just by looking at it with a specialized camera.
Once you know the radius of a planet and its mass, you just divide its volume by its mass to work out the density. Then you can say “This planet is not very dense, it must be mostly light elements like hydrogen,” or “This planet is denser than the materials we see on the surface, there must be a much denser core inside, made of rock (or metal if the density is high enough).”
For Uranus’ wind speeds, we can simply watch through Earth-based telescopes as cloud features circle around the planet. If you know the size of the planet and the time it takes for a feature to travel around it, then you know its speed.
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