We know that there is sound on planets and moons in the solar system – places where there’s a medium through which sound waves can be transmitted, such as an atmosphere or an ocean. But what about empty space? You may have been told definitively that space is silent, maybe by your teacher or through the marketing of the movie Alien – “In space no one can hear you scream”. The common explanation for this is that space is a vacuum and so there’s no medium for sound to travel through.
But that isn’t exactly right. Space is never completely empty – there are a few particles and sound waves floating around. In fact, sound waves in the space around the Earth are very important to our continued technological existence. They also they sound pretty weird!
Fundamentally, sound waves are oscillations in pressure which travel through the medium that they’re in. In most cases, this is a series of compressions, where molecules are closer together, and rarefactions, where they are further apart – caused by the molecules themselves moving backwards and forwards. Here on the ground there is quite a lot of air around – each square centimetre of it contains 300,000,000,000,000,000,000 molecules. In contrast, in interplanetary space on average you’ll find just five protons (which make up the atomic nucleus with neutrons) in the same volume – almost completely empty in comparison … but not quite.
Notice how I say protons, because space (like 99.9% of the entire universe) isn’t filled with gas but with plasma: a different state of matter made of charged particles. These charged particles mean that plasma can have some different properties, for instance they can generate and be affected by electric and magnetic fields. These kinds of interactions can give rise to the plasma-equivalent of sound waves: magnetosonic waves. These too are pressure waves, but with some added magnetism.