On Earth, life exists in abundance and is so widespread that it can be seen from space. Besides the blue of our oceans, there is another colour which dominates the continents: green. Forests and grasslands are primarily green on Earth but that may not be the case elsewhere in our galaxy.
The green of our plants comes from chlorophyll, a chemical present in all plant leaves which is responsible for soaking up sunlight to help convert water and carbon dioxide into oxygen and glucose in a process known as photosynthesis. Chlorophyll is green on Earth because it has evolved over billions of years under a yellow dwarf star.
Our sun emits particles of light called photons in many different colours but, as our sun is on the lower end of the temperature scale, the majority of the photons are low energy, at the green-red end of the spectrum. The ozone in our atmosphere scatters blue and green light so the red photons are the most abundant when they reach the surface.
For this reason, chlorophyll has evolved to absorb these low energy, redder, photons. Although blue light is less abundant in our Solar System, these photons have very high energy and are highly valuable for photosynthesis.
This leaves the green photons which have neither the necessary energy nor abundance to be of any use to the plant. These are discarded by the chlorophyll and reflected back at us making the plant appear green. On another planet around a different star, this situation may be very different.
Forests and grasslands are primarily green on Earth but that may not be the case elsewhere in our galaxy.
The colour of chlorophyll is determined primarily by how much light is available and the colour of that light. Stars come in a broad spectrum of colours from the cool red dwarfs to the hot blue giants.
The number of photons of each colour would vary depending on the temperature of the star. For instance, a hot white star like Sirius, the brightest star in our sky, emits most of its light towards the blue end of the spectrum. If this light reaches the surface, chlorophyll may have evolved to absorb the high energy photons and reflect the low energy photons creating a bright red forest.
Alternatively, if there is too much light reaching the leaves and they risk overheating, most of it could be reflected creating a white tree.
At the other end of the spectrum, around a cool red dwarf star, light is a precious resource that can't just be thrown away. Leaves may need to absorb all available light and would appear completely black.
Why does this matter? When searching for alien life, there are certain indicators to look for, such as a particular molecule in the atmosphere. These are known as biosignatures and we have already found some planets which could possibly support life.
If we sent a space probe to one of these planets, what would it see? Although we are many decades away from this, the properties of the host star and the planet's atmosphere can provide vital clues as to what a second Earth could look like, and it may not be green forests.
- Alex Wallace is a PhD student at Mount Stromlo working on the detection of young planets in other solar systems (known as exoplanets) through direct imaging.