ElI5: how do they know what other planets are made off, how can the be so sure? Jupiter could be made of a completely unknown gas?
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To understand this, first you have to understand how light interacts with atoms. Atoms are made of protons, neutrons, and electrons. Protons and neutrons are in the center, and the electrons are on the outside. The electrons exist in something called an “orbital,” which is hard to explain but you can think of it as certain shapes and distances that they are more likely to be found in. Those orbitals have certain energy levels, and we number them like 1,2,3.. and so on. Light is made of something called photons. These are like tiny little packets of energy. When a photon hits an electron, the electron will absorb it and move up to a higher-energy orbital. When an electron moves down to a lower-energy orbital, it will emit a photon.
It turns out that the energy of a photon is also related to another property called its _wavelength_. The wavelength describes how much a photon is “spread out” in space as it travels. Shorter wavelengths mean less spread, and also higher energy. We can use a device called a spectrometer to measure the wavelength of photons.
If you collect light from a star or from a planet using a telescope, and then shine that light through a spectrometer, you find that there are lines at very specific wavelengths. For a star, these will be bright lines, and they are there because they come from the photons that were emitted by electrons falling down from higher energy levels into lower energy levels. For a planet, they will be dark lines, and they come from the photons that got _absorbed_ by gases in the planet’s atmosphere when light from a star shone on them and was absorbed by electrons increasing their energy levels.
Since we can measure the wavelengths of these photons, we can figure out what the energy level must have been of the photons that were emitted or absorbed. The last piece of the puzzle is to figure out what kinds of atoms those photons came from. It turns out that every different atom has different separations in energy levels between the electron orbitals. These are like fingerprints for elements. If you know what the pattern of wavelengths is for each type of atom, you can look at a spectrum of light with lots of different patterns on it and figure out what kinds of atoms must have made it. You can even figure out how much of each type of atom there are, based on the brightness of the lines.
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