SHADOWS & SOUNDS OF EXOPLANETS
the science behind the album

Watch David’s talk with the Science Museum of Virginia:

Octave of Light is inspired by the search for life on exoplanets, and the clues that might reveal life’s presence: chemical signs like water vapor, methane, and oxygen.

Over the past year, I’ve been working with Roy Gould of the Harvard-Smithsonian Center for Astrophysics, whose book A Universe In Creation inspired many of the pieces on the album.

Roy argues that we live in a universe finely tuned for life: from the diversity of chemical building blocks in nature, to the abundance of fertile planets for life to take root.

- will we find life on an exoplanet in the next decade?

transit method

Most exoplanets are detected as shadows moving across their parent stars via the transit method. Sensitive telescopes detect minute fluctuations in the light of a distant star as a planet passes across its surface:

As if this wasn’t impressive enough, a telescope equipped with a spectrograph can spit the light into its component colors and deduce which ones are missing. By taking the negative, we then get the color of the planet itself!

The spectrograms, courtesy of Roy Gould, come out looking like this: 

These graphs tell us a lot, as specific wavelengths (x-axis) match to specific chemicals in the planet’s atmosphere. That big dip at 0.58 corresponds to sodium! Some elements show up as a single dip, and others give rise to an intricate pattern spanning a wide range of wavelengths.

the limits of our eyes

Although these lines denote ‘colors’, sadly our eyes can’t see them. Not only are they out of visible range (being mostly in the infrared), but they are also spread too widely to fit within the frequency range of our eyes. Humans can see colors between 430–770 THz (almost one doubling of signal) so even if we shifted the infrared frequencies up, we would still see only a tiny slice of what is there. We would need around 4 times the bandwidth (i.e. four frequency doublings) to perceive the spectrum of planet WASP 39-b shown above.

As a composer I began wondering, if we can’t see exoplanets

- could we hear them instead?

Compared with the feeble 1 octave (one doubling of frequency) of our eyes, our ears can hear a massive 8-10 octaves: 20-20,000 Hz!

This is certainly enough room to fit the data,

and so I got to work…

sonification

Light can easily be translated into sound because - like sound - light is a wave. Waves have a wavelength (literally, how long is each wave), and the proportions of one wavelength to another can be preserved even when sped up or slowed down by multiplying the entire series by the same number.

As light wavelengths are sooo short, they needed to be multiplied by a big number to be within audible range. I chose 2^37 - that’s 37 octaves lower than the original frequency!

However, because it’s a multiple of 2, the pitch classes are preserved (although rounded to equal temperament). A G in the sonifications is still a ‘G’ in the data - just a very high one!

Here is the first sonification I made:

This spectrum eventually Track 1 on the album - Water Romanza.

At the beginning, you can hear soprano Beth Sterling singing these notes as the cantus firmus of the piece, then joined by violinist Amelia Sie with the romanza melody.

complex molecules

This is how we started the trailer! We all know methane as a flammable gas, but its complex molecule (and therefore complex chord) allows it to be clearly spotted in a spectrogram. Methane can arise on a planet without living things, but -

- finding methane and oxygen together strongly suggests life’s presence

This is because oxygen is so reactive that it quickly combines with methane to produce carbon dioxide and water":

CH4 + O2 = CO2 + H2O

Finding both together means they are being continuously produced - perhaps by living things!

sounds of exoplanets

Exoplanet spectrograms are even more complex, making for richer spectra and even more complex chords. The amazing thing is that we can break them down into their component elements to see what is there.

The last three ‘notes’ of WASP 39-b match up to notes 3-5 of the Water Vapor spectrum above. Therefore, we can see - and hear - that the planet’s atmosphere contains water vapor!

Not only this, but the individual dips at approx. 0.58 and 0.768 reveal the presence of sodium and potassium respectively - audible as Bb and F.

Throughout the album, you'll learn a musical language of molecules, elements and exoplanets - woven into songs for soprano, piano, violin and electronics.