The Sun is composed of 70% hydrogen, 28% helium, and 2%t heavier elements. 96% of Venus’ atmosphere is made up of Carbon Dioxide (CO2). Titan—a moon of Jupiter—has an atmosphere of 98% nitrogen. Have you ever wondered how we know all this very specific data about things so far away?
To understand the explanation, we must understand some basic chemistry (if you are already familiar with the structures of atoms and how they bond, feel free to skip the next few paragraphs).
In our universe, physical objects are composed of matter. In turn, matter is composed of molecules, which are composed of atoms, which are composed of subatomic particles. Actually, there are things even smaller than subatomic particles, but a description of those is not pertinent to this subject.
In the atom, we have protons, neutrons, and electrons. To understand how we know what molecules exist in outer space, we need only concern ourselves with the proton and the electron (at least in this simplified explanation).
The number of protons in an atom determines what type of element it is. A hydrogen atom has one proton, and a helium atom has two protons. The list goes on until we reach ununoctium, which has 118 protons, and does not exist in nature (by existing in nature, we mean that it does not occur unless made in a laboratory. Since human beings are part of nature, and we can make ununoctium, philosophically it must be considered to be part of nature. To claim we have made a supernatural element would be very strange).
The atom’s nucleus (center) is where its protons (and its neutrons) sit, and the atom’s electrons surround this nucleus. The electrons from one atom will interact with the electrons from another atom and bond together, creating a molecule. For example, in water (H2O), two hydrogens will share each of their electrons with an oxygen atom’s electrons to create a water molecule.
Now that we understand atoms and molecules, we can look at the subject of this post: how we identify elements and molecules in space.
It’s quite fascinating that the molecules in an object that is 588 million kilometers away (365 million miles)—such as Titan—can be identified here on Earth. How do we do it? The answer is light. But, we don’t see the atoms and molecules just by looking at them. They are far too small to just be glanced at and identified. What we do is look at the absorption and emission lines of the light coming to us. Let’s find out what this means.
Think of the structure of the atom, and how it is surrounded by its electrons. Without leaving the atom’s structure, its electrons can jump around various states or energy levels while still being part of the atom. The electrons reside in different energy levels that correspond to the amount of energy that the individual electron possess. The higher the energy level an electron sits in, the more energy it has. Why do electrons have different energy levels? Why doesn’t every electron have the same amount of energy? The answer again is light.
Light itself can behave as both a particle and a wave, and light has energy. Right now, let’s just look at light as a particle and we’ll come back to also describe it as a wave.
A particle of light is called a photon. When a photon hits an electron with the right amount of energy, the electron will absorb the photon and its energy, thus causing it to jump into a higher energy level. Electrons can also jump down an energy level and emit a photon, which causes them to lose energy.
The interesting thing about energy levels is that they are very precise. Each level has a certain value of energy that it corresponds to. For example, there are energy levels called the first, second, and third energy levels (and so on), but there are no levels in between those. There is no second and a half energy level or anything like that. When an electron goes up in an energy level, it gains/absorbs a precise amount of energy, and when it goes down in an energy level, it loses/emits a precise amount of energy.
So, what does all this mean? What does it have to do with how we identify molecules and elements very far away from Earth? Well, each molecule has its own system of energy levels for its electrons to hop around in. This also means each molecule has its own specific amounts of energy that correspond to the difference in energy levels. When an electron jumps around in an oxygen molecule, it absorbs and emits different amounts of energy than a carbon dioxide molecule will. If there is a way to look at the energy in the light coming from space, there is a way to identify the molecules.
This technique is called spectroscopy, and what it does is look at the absorption and emission lines of light. Take a look at the picture below. It is a picture of the continuous light spectrum in the optical regime (this means visible light. There are many other types of light that the human eye cannot see, such as radio and x-ray). Remember how I said light behaves as both a particle and a wave? Well, without getting too technical, let’s think about light as a wave too. The longer the wavelength of the light, the redder it is. The shorter the wavelength, the bluer it is. As you can see, the wavelength changes across the spectrum. Each wavelength also corresponds to an amount of energy the particle of light (photon) is carrying.
I bet you can see where this is going. So, if electrons present within molecules and atoms absorb and emit photons at specific energies and wavelengths, is there a way to see this when we analyze the light coming from space? Indeed there is. Look at the picture below. It is an illustration of absorption lines. When light passes through the atmosphere of another planet, the molecules that make up that planet’s atmosphere will absorb the light at exact wavelengths characteristic of that molecule. From there, we can identify the molecules present in the object we are looking at.
If there is a cloud of gas in between us and a star (space is full of gas clouds and dust), then we can look at what wavelengths it absorbs to identify the composition of the cloud. If the cloud isn’t directly between us and the star, we can still identify what molecules compose the cloud. After the electrons absorb the photons from the star, they will re-emit them after some time, but in a different direction. If they are redirected towards us, we can still identify the molecules present.
Emission lines were actually how we compiled a list of what molecules gave off light at what energy levels. We could pump light into a sample molecule and record what wavelengths it re-emitted the light at. Once we had the numbers, we could look at absorption spectra. The missing wavelengths corresponded to the molecules that were absorbing the light.
This is one of the fundamental techniques in astronomy, and spectroscopy doesn’t just occur in the optical light spectrum. We can look at light all across the electromagnetic (light) spectrum. This includes, infrared, radio, and x-ray. Even if we can never reach the far out realms of our galaxy, the light can reach us and tell us its story.