Is There Such a Thing as a Planet Sound?

Image of the Week for Voyager Squashes View of Solar System
This artists rendering depicts Voyager 2 spacecraft as it studied the outer limits of the heliosphere--a magnetic bubble around the solar system that is created by the solar wind. Data from Voyager was used to make samples of what it might sound like if you could hear the passage through the boundary. NASA

Can a planet make a sound? It's an interesting question that gives us insight into the nature of sound waves. In a sense, a planets do emit radiation which can be used to make sounds we can hear. How does that work?

The Physics of Sound Waves

Everything in the universe gives off radiation that—if our ears or eyes were sensitive to it—we could "hear" or "see". The spectrum of light that we actually perceive is very small, compared to the very large spectrum of available light, ranging from gamma-rays to radio waves. Signals that can be converted to sound make up only one part of that spectrum.

The way people and animals hear sound is that sound waves travel through the air and eventually reach the ear. Inside, they bounce against the eardrum, which begins to vibrate. Those vibrations pass through small bones in the ear and cause small hairs to vibrate. The hairs act like tiny antennae and convert the vibrations into electrical signals that race to the brain through the nerves. The brain then interprets that as sound and what the timbre and pitch of the sound are.

What About Sound in Space?

Everyone has heard the line used to advertise the 1979 movie "Alien", "In space, no one can hear you scream." It's actually quite true as it relates to sound in space. For any sounds to be heard while someone is "in" space, there have to be molecules to vibrate. On our planet, air molecules vibrate and transmit sound to our ears. In space, there are few if any molecules to deliver sound waves to the ears of people in space. (Plus, if someone is in space, they're likely to be wearing a helmet and a spacesuit and still wouldn't hear anything "outside" because there's no air to transmit it.)

That doesn't mean there aren't vibrations moving through space, only that there are no molecules to pick them up. However, those emissions can be used to create "false" sounds (that is, not the real "sound" a planet or other object might make). How does that work?

As one example, people have captured emissions given off when charged particles from the Sun encounter our planet's magnetic field. The signals are at really high frequencies that our ears can't perceive. But, the signals can be slowed down enough to allow us to hear them. They sound eerie and weird, but those whistlers and cracks and pops and hums are just some of the many "songs" of Earth. Or, to be more specific, from Earth's magnetic field. 

In the 1990s, NASA explored the idea that emissions from other planets could be captured and processed so people could hear them. The resulting "music" is a collection of eerie, spooky sounds. There is a good sampling of them on NASA's Youtube Site.  These are literally artificial depictions of real events. It's very similar to making a recording of a cat meowing, for example, and slowing it down to hear all the variations in the cat's voice.

Are We Really "Hearing" a Planet Sound?

Not exactly. The planets don't sing pretty music when spaceships fly by. But, they do give off all those emissions that Voyager, New Horizons, Cassini, Galileo and other probes can sample, gather, and transmit back to Earth. The music gets created as the scientists process the data to make it so that we can hear it. 

However, each planet does have its own unique "song". That's because each one has different frequencies that are emitted (due to different amounts of charged particles flying around and because of the various magnetic field strengths in our solar system). Every planet sound will be different, and so will the space around it. 

Astronomers have also converted data from spacecraft crossing the "boundary" of the solar system (called the heliopause) and turned that into sound as well. It's not associated with any planet but does show that signals can come from many places in space. Turning them into songs we can hear is a way of experiencing the universe with more than one sense. 

It All Began With Voyager

The creation of "planetary sound" started when the Voyager 2 spacecraft swept past Jupiter, Saturn and Uranus from 1979-89 The probe picked up electromagnetic disturbances and charged particle fluxes, not actual sound. Charged particles (either bouncing off the planets from the Sun or produced by the planets themselves) travel in the space, usually kept in check by the planets' magnetospheres. Also, radio waves (again either reflected waves or produced by processes on the planets themselves) get trapped by the immense strength of a planet's magnetic field. The electromagnetic waves and charged particles were measured by the probe and the data from those measurements were then sent back to Earth for analysis.

One interesting example was the so-called "Saturn kilometric radiation". It's a low-frequency radio emission, so it's actually lower than we can hear. It is produced as electrons move along magnetic field lines, and they're somehow related to auroral activity at the poles. At the time of the Voyager 2 flyby of Saturn, the scientists working with the planetary radio astronomy instrument detected this radiation, speeded it up and made a "song" that people could hear. 

How Do Data Collections Become Sound?

In these days, when most people understand that data is simply a collection of ones and zeroes, the idea of turning data into music isn't such a wild idea. After all, the music we listen to on streaming services or our iPhones or personal players is all simply encoded data. Our music players reassemble the data back into sound waves that we can hear. 

In the Voyager 2 data, none of the measurements themselves were of actual sound waves. However, many of the electromagnetic wave and particle oscillation frequencies could be translated into sound in the same way that our personal music players take data and turn it into sound. All NASA had to do was to take the data accumulated by the ​Voyager probe and convert it into sound waves. That's where the "songs" of distant planets originate; as data from a spacecraft.