What are planets made of?
How do we know?

D. Indumathi, The Institute of Mathematical Sciences, Chennai

How do we know what the Earth and other planets are made of? We can look down at Earth's crust and look up at Earth's atmosphere. We can make measurements on Earth's mantle and core by using natural probes such as waves created during earthquakes that circle the globe several times before they fade away. So we can learn a great deal about the Earth. Even though what we have learned about the Earth is already amazing, it is even more amazing that we have learned so much about the planets so far away. How do we know so much about them?

Earth's structure
We know that the Earth has many layers, the most important being the core, mantle and crust. The different layers of Earth have different materials and different densities. Most earthquakes happen within 100 km of the Earth's surface. Earthquakes generate waves of different kinds: S and P waves. As these waves spread out in all directions from the centre of the earthquake, they pass through these different regions. Just like light waves, these waves also experience reflection and refraction as they cross different layers. From this, we can find out the size of these layers and their density.
Also, the seismic waves cannot pass through part of the core of the Earth. From the total mass of the Earth (known from the gravity that it exerts), the average density of Earth can be found out since its size (volume) is known and density is mass per unit volume. From knowing the mass of the mantle and crust, we can find out the mass that is in the core that cannot be probed by seismic waves. From these observations, it was found that the core is really very dense, about 3-4 times as dense as the crust. Also, while the core is made of (mostly) iron with a little nickel, etc., the inner core is solid while the outer core is molten and liquid. This is very important for life on Earth! Why?
This outer core is not only liquid and hot but is also spinning because of the rotation of the Earth. This is responsible for Earth's magnetic field since iron is a magnet and causes a magnetosphere or region around Earth that has a magnetic field. This magnetosphere protects the Earth from solar wind and the most harmful components of solar radiation coming from our Sun. This is necessary for life to emerge and evolve on Earth for, without it, the atmosphere would have also been stripped from the Earth long before now, just as has happened with Mars. In fact, of all the planets, only Venus and Mars lack a magnetosphere.
This model of the Earth also helps us to understand the composition of other planets since they are similar. See the picture of the model of Jupiter (cut-out of the globe) showing the dense small core of heavier elements with the rest of the planet being made of metallic possibly liquid hydrogen and some helium. This transforms smoothly into its atmosphere which is gaseous hydrogen! But we cannot go to all these planets and study the rocks and atmosphere there, so we need other techniques.

Spectroscopy
The science of learning about the objects in the sky is called astronomy. We can use telescopes to study the visible universe. This is because many objects such as stars emit light. However, stars also emit light that human eyes cannot see. This is because electromagnetic radiation occurs at different wavelengths (see Box).

BOX on
Electromagnetic Radiation
Newton showed that white sunlight can be broken down into different colours by a prism. Light has a wave-like property and each colour of light has a different wavelength. Red has the largest and violet the smallest wavelength in the VIBGYOR set of rainbow colours-- violet, indigo, blue, green, yellow, orange and red. But these are the colours that the human eye can see. Light is also emitted in wave-lengths that we cannot see. There is light with wavelength larger than red, that is called infrared light. Even longer wavelength is called radiowaves (the wavelength or frequency in which radio waves are emitted).
Light with wavelength smaller than violet is called ultraviolet light. Even shorter wavelength (higher frequency) light are labelled as gamma rays and X-rays. You all are familiar with X-rays for sure. These are just a different form of light that the human eye cannot see.
END OF BOX

We know that stars are hot and so they emit light. Different materials emit different colours (or more generally wavelengths) of light. For instance, neon lights are used commonly in night lights and have a typical orange-red colour. This is because of the property of the gas itself: the atoms of neon both emit and absorb this colour of light. This means that when the gas is heated, it emits this colour of light. On the contrary, when light of this colour (actually, a given wavelength) falls on neon gas, it gets absorbed. If light of any other colour falls on neon, it simply passes through. When light falls on matter it can also get scattered and diffused.

Emission spectra
Suppose there is a star far away containing hydrogen gas. When light from it reaches us, it contains many wavelengths including the wavelength corresponding to hydrogen. This means that in the spectrum of this star, there will be all colours including a sharp line at this wavelength. This emission spectrum from the star tells us that the star contains hydrogen. The picture shows the simple emission spectrum of hydrogen and the more complicated one of iron below.

Absorption Spectra
Suppose the starlight passes through an interstellar cloud or a nebula which may not itself produce light. If the cloud contains hydrogen, then it will absorb this particular wavelength of light so that this will not reach us. So in the spectrum, instead of a bright light at the wavelength of hydrogen, we will see a black line (signifying no light in this wavelength). Such black lines are called absorption lines because that particular wavelength of light has been absorbed by the cloud. The picture shows the continuous spectrum of light from the Sun. We know that the white light from the Sun is composed of all possible colours of the rainbow and indeed this is what the spectrum shows us. The sharp black lines in this spectrum tell us the atoms that are there in the Sun which have absorbed light from specific wavelengths. For instance, hydrogen and helium are present in the Sun; in fact, Helium was first discovered this way.
So the emission and absorption spectra tell us a good deal about the light from the star and in turn what matter there is in the star and interstellar material. 
Planets
Planets and asteroids shine only by reflecting the light of their parent star. The reflected light contains absorption bands due to minerals in the rocks present in terrestrial planets. In the case of gas giants, the signature is that of the elements and molecules present in their atmospheres.
Stars, galaxies, planets can all be observed by their emissions in many different wavelengths. Each kind of light (or electromagnetic radiation as it is scientifically called) tells us something about what these objects are made of. Many instruments are loaded on satellites so that the Earth's atmosphere does not spoil or interfere with the signal from the far away star or planet.

Other objects in the sky
Apart from galaxies, stars, and planets, new objects such as quasars have also been studied using spectroscopy. The spectra of these objects showed new features that need a whole new article to explain! In short, it led to the realisation that the Universe as a whole is not static but rapidly expanding. New and sophisticated instruments are now being designed and built to probe the farthest reaches of our Universe.

--Images from Wikipedia