Planetary Sciences News

Set to Music, Exoplanets Reveal Insights on Their Formation

Sonification hints at how some Kepler planets may have been configured in the late stages of their development.


In 1619, German astronomer Johannes Kepler published his Harmonices Mundi (Harmony of the World), a text that investigated how mathematics could help the planets of the solar system create celestial music based on their orbital resonances. Three hundred years later, an astronomer using discoveries from NASA’s Kepler mission has arranged thousands of exoplanets in their own grand sonata.

As telescopes have become more automated, astronomical data have evolved from a trickle to a roaring river. During its primary and extended missions, Kepler identified nearly 5,000 confirmed and candidate exoplanets. The first candidates came in small doses, allowing astronomers to get to know them well. But later observations came in giant batches that were more challenging to parse.

“There’s too many of them to look at individually,” said Jason Steffen, an astronomer at the University of Nevada, Las Vegas. “Now that there are so many, they are falling through the cracks.”

Steffen, who studied some music theory and said he knows “just enough to be dangerous,” realized that the Kepler worlds could produce scientific insights when set to music. At the same time, he came across a YouTube video claiming that sonification—setting data to sound—doesn’t produce much useful science.

“I took that as a challenge,” Steffen said. “Planetary systems have good insight that can be gained when you sonify the data.”

Steffen produced a YouTube video of the thousands of exoplanets that combines information about their orbits with sound.

The volume of the system is set by its largest planet, with louder chords corresponding to larger worlds. The lowest note is set by the orbital period of the largest world, with a lower note lining up with a longer orbital period. Finally, the music steps through different combinations of adjacent sets.

“Music is really multidimensional,” said Matt Russo, an astromusician who set the data for the seven-planet TRAPPIST-1 system to music. Music allows researchers to present different pitches and notes to create multiple layers, he said.

“There are some cool analogies one can draw, and maybe some real physical insights that can be had from this kind of sonification,” said Daniel Fabrycky, an exoplanet researcher at the University of Chicago.

Fabrycky, who studies the orbits of planets, was one of the colleagues Steffen sent his final composition to. “Systems that sound better—have more pleasing chords to the ear—might have formed in a more gentle way than ones that sound dissonant,” Fabrycky said.

“Sound Is in a Better Position Than Sight”

After a star is born, the leftover disk of gas and dust gives rise to planets. Some planets travel in resonance, orbits that correspond to how often they travel around their star. Neptune and Pluto, for instance, are in a 3:2 resonance; Neptune travels three around the Sun for every two orbits made by Pluto.

According to Fabrycky, planets don’t start off in resonance. Instead, gravity and interactions with the disk can cause the planets to move. “After both planets form, they can scoot towards each other and capture into that resonance,” Fabrycky said.

Orbital resonances can reveal insights about the later stages of planet formation, when the worlds are still embedded in the system’s protoplanetary disk, Fabrycky said. Exoplanets were first identified in the early 1990s, but astronomers didn’t spot the first resonant exoplanets until the early 2000s, and multiple exoplanets in resonance weren’t seen for another decade. Today, we know of fewer than 10 exoplanet systems where three or more worlds are locked into resonances.

An animation of the four planets of the Kepler-223 system
An artist’s animation shows the four planets of Kepler-223, which move in resonance with one another. Each time the innermost planet orbits the star three times, the second planet orbits exactly four times. Credit: W. Rebel

Steffen’s sonification project may help to reveal more.

“For resonance structure, it seems sound is in a better position than sight to do the job of picking out frequencies in particular,” Fabrycky said.

In fact, Steffen’s sonification project has already raised Fabrycky’s interest in one planetary system. KOI-4032 has four confirmed worlds and a fifth candidate planet. To date, nothing has been published about the system, but the pleasing notes in Steffen’s simulation (at 1:52 in the video above) triggered Fabrycky’s interest.

That’s just the sort of thing Steffen hoped would happen. “There are certain systems that pop out that just haven’t gotten any attention that might be worth a deeper investigation,” he said.

With NASA’s Transiting Exoplanet Survey Satellite already hunting for new worlds and missions like the European Space Agency’s Planetary Transits and Oscillations of stars preparing for a 2026 launch, the treasure trove of exoplanets will only continue to expand. Sonification can help to sort through the data in a meaningful way, making sure the Kepler worlds don’t get lost in the shuffle.

“It would be a great present if we could give this back to Kepler,” Russo said. “He was looking for musical patterns in our solar system. To show him that we’ve found musical patterns in the form of orbital resonances in other systems—I would like to see his face if he could hear that.”

—Nola Taylor Redd (@NolaTRedd), Freelance Science Journalist

Correction, 16 October 2019: This article has been updated to correct the orbital resonance of Neptune and Pluto.

Citation: Redd, N. T. (2019), Set to music, exoplanets reveal insights on their formation, Eos, 100, Published on 16 October 2019.
Text © 2019. The authors. CC BY-NC-ND 3.0
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