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An introduction to exoplanets
An introduction to exoplanets

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4.1  Biosignatures

Signs of life that we can detect in a planet’s atmosphere are called ‘biosignatures’. These are gases that would be unlikely to occur without the presence of life. Earth’s main biosignature is a very important gas for all of us: oxygen.

An atmosphere with as much oxygen as Earth’s – 20 per cent of the total – simply shouldn’t exist under normal circumstances. This is because oxygen is an extremely reactive gas. That means that it readily combines with other chemicals to form something completely different. So, generally, oxygen doesn’t hang around for long.

We experience oxygen’s tendency to react with other chemicals every day. When you light a fire, you are adding enough heat to prompt the chemicals in the fuel to combine with the oxygen in the air. After a while, rust forms on iron products left uncoated – this is caused by oxygen in the air reacting with the iron.

Activity 2  Reactions with oxygen

Timing: Allow about 10 minutes

For this activity you’ll need an apple and a sharp knife. Always take care when using sharp knives!

Cut the apple in half and leave it exposed to the air for a few minutes. You should start to see the apple change colour and turn brown fairly quickly. This is another example of oxygen in the air reacting with other chemicals – in this case, chemicals in the apple. If you like, leave the apple for a bit longer and return to it later: the brown colour will have increased as the chemical reaction has continued – it will look pretty unappetising!

It’s possible to prevent apples from going brown by coating them in lemon juice. Citric acid, which is present in all citrus fruit, prevents the reaction with oxygen from happening.

Photosynthesis and oxygen

Given that oxygen so readily reacts with other chemicals, it would be expected to eventually disappear from our atmosphere. But it isn’t disappearing. There has been around the same amount of oxygen in our atmosphere for millions of years. The reason is that oxygen is being continuously resupplied to the atmosphere by plants. Photosynthesis is a chemical process that occurs in the cells of plants – they collect light from the Sun and combine it with carbon dioxide gas in the air to form sugars, their energy source. A by-product of photosynthesis is oxygen.


The oxygen we breathe comprises an oxygen molecule in the form of two oxygen atoms stuck together. Another gas present in Earth’s atmosphere is ozone. This is a gas comprising three atoms of oxygen stuck together, and it can only exist in large quantities if there’s a lot of oxygen present. Earth has a layer of ozone gas beginning around 10 km above its surface; it’s very useful to us as it absorbs harmful ultraviolet light from the Sun. The ozone layer is a bit like the Earth’s natural sunscreen.

If there’s enough oxygen and ozone in a planet’s atmosphere, it might be possible to detect these gases in transmission spectra. Therefore, if one of the TRAPPIST planets has an atmosphere exactly like the Earth’s, we would be able to detect the ozone in its atmosphere, using a space telescope that is due to launch in 2021. You’’l learn more about this later.