Planets orbit their parent stars while separated by enormous distances. In our solar system, planets are like grains of sand in an area the size of a football field. The time it takes for planets to revolve around their sun has no specific relationship with each other.
But sometimes their orbits show striking patterns. For example, astronomers study six planets orbiting a star At 100 light years away, they have just discovered that they are orbiting their star in an almost rhythmic rhythm, in perfect synchronization. Each pair of planets completes their orbits in times that are the ratios of whole numbers, allowing the planets to align during their orbit and exert a gravitational pull on the other.
This type is called gravity alignment orbital resonanceand it is like a harmony between distant planets.
Related: TRAPPIST-1: A guide to the seven-Earth-sized exoplanet system
I’m a astronomer who studies and writes about cosmology. Researchers have discovered more than 5,600 exoplanets over the past thirty years, and their extraordinary diversity continues to surprise astronomers.
Harmony of the spheres
Greek mathematician Pythagoras discovered the principles of musical harmony 2,500 years ago by analyzing the sounds of blacksmith hammers and plucked strings.
He believed that mathematics is at the heart of the natural world and proposed so the sunMoon and planets each emit a unique buzz based on their orbital properties. He thought that this ‘music of the spheres’ would be imperceptible to the human ear.
Four hundred years ago, Johannes Kepler picked up this idea. He proposed that musical intervals and harmonies described the movements of the six then known planets.
Nasty Keplerthe solar system had two basses, Jupiter And Saturn; a tenor, Mars; two contraltos, Venus And Soil; and a soprano, Mercury. These roles reflected how long it took for each planet to orbit the sun, slower speeds for the outer planets and higher speeds for the inner planets.
He called the book he wrote about these mathematical relations ‘The harmony of the world.” Although these ideas have some similarities with the concept of orbital resonance, planets have actually stopped making sounds since then sound cannot travel through the vacuum of space.
Resonance happens when planets or moons have orbital periods that are ratios of whole numbers. The orbital period is the time taken for a planet to make a complete circuit of the star. For example, two planets orbiting a star would be in a 2:1 resonance if one planet takes twice as long as the other to orbit the star. Resonance can only be seen in 5% of planetary systems.
In the solar system Neptune And Pluto are in a 3:2 resonance. There also is a triple resonance4:2:1, among the three moons of Jupiter: Ganymede, Europe And Io. In the time it takes Ganymede to orbit Jupiter, Europa orbits twice and Io orbits four times. Resonances occur naturally when planets happen to have an orbital period that is the ratio of whole numbers.
Musical intervals describe the relationship between two musical notes. In the musical analogy, important musical intervals based on frequency ratios are the fourth, 4:3, the fifth, 3:2, and the octave, 2:1. Anyone who has the guitar or piano might recognize these intervals.
Orbital resonances can change how gravity has an influence two bodies, causing them to speed up, slow down, stabilize on their orbits, and sometimes disrupt their orbits.
Think of pressing one child on a swing. A planet and a swing both have a natural frequency. Give the child a push that matches the swinging movement and he or she will get a push in the back. They also get a boost if you push them every other time they are in that position, or every third time. But push them at random times, sometimes with the swing motion and sometimes against it, and they get no boost.
For planets, the boost may keep them on their orbits, but it is much more likely to disrupt their orbits.
Resonance of exoplanets
Exoplanets, or planets outside the solar system, exhibit striking examples of resonance not only between two objects, but also between resonant ‘chains’ involving three or more objects.
The star Gliese 876 has three planets with an orbital period of 4:2:1, just like Jupiter’s three moons. Kepler 223 has four planets with ratios of 8:6:4:3.
The red dwarf Kepler 80 has five planets with ratios of 9:6:4:3:2, and TOI 178 has six planets, five of which are in a resonant chain with ratios of 18:9:6:4:3.
TRAPPIST-1 is the record holder. It has seven Earth-like planetsof which two might be habitablewith track ratios of 24:15:9:6:4:3:2.
The latest example of a resonant chain is the HD110067 system. It is located about 100 light-years away and has six sub-Neptune planets, a common type exoplanet, with track ratios of 54:36:24:16:12:9. The discovery is interesting because most resonance chains are unstable and disappear over time.
Despite these examples, resonant chains are rare only 1% of all planetary systems exhibit them. Astronomers think that planets form in resonance, but small gravitational thrusts occur as they pass through stars and wandering planets erase the resonance over time. With HD 110067, the resonance chain has survived for billions of years, providing a rare and pristine view of the system as it was when it formed.
Sonification of the track
Astronomers use a technique called sonification to translate complex visual data into sound. It gives people another way to experience the beautiful images of the Hubble Space Telescopeand it has been applied to X-ray data and gravitational waves.
For exoplanets, sonification can convey the mathematical relationships of their orbits. Astronomers at the European Southern Observatory created what they ‘music of the spheres” for the TOI 178 system by associating a pentatonic scale sound with each of the five planets.
a similar musical translation is done for the TRAPPIST-1 system, in which the orbital frequencies are scaled up by a factor of 212 million to bring them within audible range.
Astronomers have done that too created a sonification for the HD 110067 system. People may disagree on whether these renditions sound like real music, but it is inspiring to see Pythagoras’ ideas realized after 2,500 years.