This resource page was written by Albert, one of our work experience students!
Image credit: EETG & Engine House VFX
As we explained in the information page on exoplanet detection, we can work out that there is an exoplanet orbiting a star by looking at the light that is blocked out by the planet, each time it passes between the star and our telescopes.
This method doesn’t just tell us that there is a planet present – it also helps us to identify properties of the planet too. The duration of the transit and the amount of light blocked can help us to determine the size and orbit of the planet as well as its distance from the host star, which allows us to work out its temperature. Furthermore, the light that we receive from the star once it has passed through the planet’s atmosphere can tell us about the planet’s climate.
Astrophysicists use a technique called spectroscopy to do this. It involves looking at the incoming light and splitting it up into a spectrum – just like how a glass prism splits white light into a rainbow. If an exoplanet moves in front of its star, the star’s light passes through the different layers of the exoplanet’s atmosphere and some of the light is absorbed. We can then observe how the light has been changed by passing through the atmosphere of the exoplanet. Every chemical element, like carbon or oxygen, or molecule like water or carbon dioxide, absorbs and emits their own unique spectrum of light. By looking at the wavelengths (colours) of the light that have been absorbed by the exoplanet atmosphere, we can learn which chemicals the atmosphere contains, how much of each chemical is present and even measure wind speeds!
When combined with the other properties such as the exoplanet’s size and temperature, astrophysicists can use computer models to get a better picture of the atmospheric conditions on the planet – and, perhaps, if it could support life. A major part of studying exoplanets is focused on finding potentially habitable worlds. Through our understanding of life on Earth, we are able to identify biosignatures: properties of a planet that are specifically attributed to the presence of life. An example of a biosignature would be the presence of oxygen or methane, which is produced on Earth by living things (although they can also be produced by other sources).
Another important factor that is considered when searching for habitable exoplanets, is whether the planet is the correct distance from its star to allow for a temperature at which liquid water can exist. This “correct distance” is called the habitable zone. Life as we know it on Earth needs liquid water to survive, so when an exoplanet is within the habitable zone it may have liquid water (depending on its composition). Being in a habitable zone, therefore, means there is a greater chance it could host alien life.