The good news is that energy is saved. The bad news is that disorder increases.
Where does the solar system come into all of this?

Order and disorder. Two seemingly abstract concepts. Or should that be a single concept? They are opposites, after all. But scratch under the surface and they have very concrete foundations. Foundations on which the entire universe stands.

If energy is the engine, then order —or rather the journey from order to disorder and vice versa— is the steering wheel, the accelerator of the universe and all its transformations.

Two of the fundamental laws of scientific knowledge are based on energy and disorder: the first and second laws of thermodynamics. The first affirms that the energy in a system is always saved. For example, the energy saved in the chemical bonds in the fuel in your car turns into heat. This becomes mechanical energy, it makes the car’s wheels go round and it turns into heat again, then disperses.

If the energy is saved, why can’t we do anything with fuel anymore once it’s burnt? That’s where the second law of thermodynamics comes in.

It’s very simple, systems tend spontaneously towards disorder.
And that's not all. To bring back a bit of order, that energy needs spending.

To measure the level of disorder in a system, they came up with a new quantifier: entropy.
The higher the entropy in a system, the more chaotic it is. And the lower the entropy, the more ordered the system.

All transformations tend to happen spontaneously, evolving in the direction of greater disorder. That’s why energy sources, be they fossil or renewable, are born out of the exploitation of transformations from ordered to disordered states. A water mill, or a modern turbine in a hydroelectric plant, produce energy by carrying a little bit of the water conveniently located on mountain tops to the sea. Nuclear plants, thermal power stations that rely on fossil fuels and wind turbines produce energy by tidily turning the stuff they store into, respectively, uranium atoms, carbon-carbon and carbon-hydrogen chemical bonds in fossil fuels and the pressure difference between large atmospheric masses (the high or low pressure gradient prompts the wind), in new and more ordered amounts, from which you cannot get any more energy. 

In light of this tendency to disorder, how has the planet Earth managed to resist destruction and even allow all the species that live on it to grow and develop?

The evolution of plants and animals and the very progress of us humans are only possible because biological systems, like social ones, have been kept in order and developed in an organised way, thanks to the energy from, you guessed it, the Sun.

To capture the energy in solar rays, vegetable cells learnt to exploit sunlight, do photosynthesis and make use of highly disordered material, like the carbon dioxide in the atmosphere and the water on Earth and in the air. They reassembled those ingredients to get highly ordered material, like sugars, cellulose and lignin, but also fruit, leaves and the seeds plants need to reproduce and evolve. Herbivorous animals, meanwhile, discovered how to feed off the order stored in plants. The carnivores did something similar, learning to feed off the energy stored in herbivores!

So, what about the second law of thermodynamics?

How do we ascertain the level of disorder in the Earth’s system and whether it tends spontaneously to increase or decrease? We should clarify something first. By system we mean a closed one in which no energy comes in or out.

On Earth the entire ecosystem fights growing entropy, looking to build ever more ordered structures. But if we look at the whole closed system, which must include both the Sun and Earth, entropy is on the rise at quite some speed.

Every little increase in order on Earth leads to the transformation of an enormous amount of energy —neatly stored on the Sun inside hydrogen— into disordered energy, in the form of solar rays, helium and other particles that form in nuclear reactions on the Sun.

Every second, the biggest fusion-powered nuclear power station in the solar system consumes 5 million tonnes of hydrogen and shoots out electromagnetic waves all over the place. A small fraction of them are visible light and almost all of them are lost in space. A tiny quantity breaks the mould, though, and hits Earth. Everything around us is born and evolves thanks to this tiny fraction and the enormous consumption that it results in (entropy).

So, the Sun is the guarantee of order on Earth. Luckly it’s been going strong for about 4.5 billion years and still has enough nuclear fuel left for another 4 billion.

Nothing to worry about for the time being, then.