Can fire exist outside of our planet?

Earth isn’t the only watery planet in the known universe, but it is the only fiery planet, at least in the solar system. The sun is mostly hydrogen undergoing nuclear fusion, not fire. And on other planets magma from volcanoes and lightning are also not fire. To get fire, it took billions of years of photosynthesis, which means fire can’t exist without life. And fire and life have been shaping each other ever since.

Fire is a process.  It's what happens when a fuel reacts with an oxidizer and releases heat and light. It is the visible effect of the process of combustion – a special type of chemical reaction. It occurs between oxygen in the air and some sort of fuel. The products from the chemical reaction are completely different from the starting material.

The fuel must be heated to its ignition temperature for combustion to occur. The reaction will keep going as long as there is enough heat, fuel and oxygen. This is known as the fire triangle.

The fire triangle ( Source: Google Images)

Combustion is when fuel reacts with oxygen to release heat energy. Combustion can be slow or fast depending on the amount of oxygen available. Combustion that results in a flame is very fast and is called burning. Combustion can only occur between gases.

The most common fuel on Earth is organic matter, like wood or paper, and the related fossil fuels, and the most common oxidizer is oxygen, which makes up about 21% of our atmosphere.

But oxygen is not the only oxidizer in the universe.

There are other elements that can accept electrons from fuels, like fluorine or chlorine.

These elements are very reactive and can burn metals or other substances that oxygen can't.

For example, some rockets use fluorine-based compounds as oxidizers to boost their thrust.

So, technically, fire can happen on other planets, but only if there is a fuel and an oxidizer present.

And that's where things get tricky.

Most planets in our solar system don't have enough oxygen to support combustion.

Some planets, like Jupiter and Saturn, are mostly made of hydrogen and helium, which are fuels, not oxidizers.

Other planets, like Mars, have very thin atmosphere with very little oxygen.

The only planet that has a lot of oxygen is Earth, and that's because of life.

Life on Earth has been producing oxygen for eons through photosynthesis, which is another kind of chemical reaction that uses light to split water molecules into hydrogen and oxygen.

The oxygen then accumulates in the atmosphere and allows fire to happen.

So, fire is not only a product of chemistry, but also of biology, bio-chemistry to be precise.

It can't exist without life, and life has been shaped by fire for millions of years.

Fuels can be solids, liquids or gases. During the chemical reaction that produces fire, fuel is heated to such an extent that (if not already a gas) it releases gases from its surface.

Only gases can react in combustion. Gases are made up of molecules (groups of atoms). When these gases are hot enough, the molecules in the gases break apart and fragments of molecules rejoin with oxygen from the air to make new product molecules – water molecules (H₂O) and carbon dioxide molecules (CO₂) – and other products if burning is not complete.

The heat generated by the reaction is what sustains the fire. The heat of the flame will keep remaining fuel at ignition temperature. The flame ignites gases being emitted, and the fire spreads. As long as there is enough fuel and oxygen, the fire keeps burning.

Fuel + oxygen (from the air) = combustion products (mainly CO₂ + H₂O) + heat energy.

In complete combustion, the burning fuel will produce only water and carbon dioxide (no smoke or other products). The flame is typically blue. For this to happen, there needs to be enough oxygen to combine completely with the fuel gas.

If there is not enough oxygen available during a chemical reaction, incomplete combustion occurs, and products such as carbon (C) and carbon monoxide. Carbon monoxide consists of one carbon atom and one oxygen atom. It is an odorless, colourless, tasteless, flammable gas.  (CO) as well as water and carbon dioxide are produced. Less heat energy is released during incomplete combustion than complete combustion.

In incomplete combustion, the burning flame is typically yellow or orange and there is smoke.

Fire has occurred here on Earth for millions of years and was harnessed by humans more than 100,000 years ago. 

So if living matter is present and if enough oxygen is present, there are chances of fire existing on exo-planets -   the planets that orbit around other stars located outside our solar system, the extra-solar planets. 

Will we see extraterrestrial fire in the future? The chances of detection in our solar system are small, but with the effort being put into detecting another Earth, we could see an exoplanet with the right ingredients for fire (free oxygen, heat, and combustible material – the “fire triangle”) in this century.

Fires can exist in space, when right conditions are provided. Recent tests aboard the International Space Station have shown that fire in space can be less predictable and potentially more lethal than it is on Earth. “There have been experiments,” according to NASA experts, “where astronauts observed fires that they didn’t think could exist, but did.”

Here on Earth, when a flame burns, it heats the surrounding atmosphere, causing the air to expand and become less dense. The pull of gravity draws colder, denser air down to the base of the flame, displacing the hot air, which rises. This convection process feeds fresh oxygen to the fire, which burns until it runs out of fuel. The upward flow of air is what gives a flame its teardrop shape and causes it to flicker.

But odd things happen in space, where gravity loses its grip on solids, liquids and gases. Without gravity, hot air expands but doesn’t move upward. The flame persists because of the diffusion of oxygen, with random oxygen molecules drifting into the fire. Absent the upward flow of hot air, fires in microgravity are dome-shaped or spherical—and sluggish, thanks to meager oxygen flow. If you ignite a piece of paper in microgravity, the fire will just slowly creep along from one end to the other. Astronauts are all very excited to do their experiments because space fires really do look quite alien.

Such fires might appear eerily tranquil to people accustomed to the capricious nature of earthly flames. But a flame in microgravity can be more tenacious, capable of surviving on less oxygen and burning for longer periods of time.

The left one is fire on Earth while the right one is fire on a space station