By Chris Impey, University of Arizona<\/span><\/p>\n Planets orbit their parent stars while separated by enormous distances. In our solar system, planets are like grains of sand in a region the size of a football field. The time that planets take to orbit their suns has no specific relationship to each other. <\/p>\n But sometimes, their orbits display striking patterns. For example, astronomers studying six planets orbiting a star 100 light-years away have just found that they orbit their star with an almost rhythmic beat, in perfect synchrony. Each pair of planets completes their orbits in times that are the ratios of whole numbers, allowing the planets to align and exert a gravitational push and pull on the other during their orbit.<\/p>\n This type of gravitational alignment is called orbital resonance, and it\u2019s like a harmony between distant planets.<\/p>\n I\u2019m an astronomer who studies and writes about cosmology. Researchers have discovered over 5,500 exoplanets in the past 30 years, and their extraordinary diversity continues to surprise astronomers.<\/p>\n EarthSky lunar calendars are back in stock! And we\u2019re guaranteed to sell out, so get one while you can. Your support means the world to us and allows us to keep going. Purchase here..<\/p>\n Greek mathematician Pythagoras discovered the principles of musical harmony 2,500 years ago by analyzing the sounds of blacksmiths\u2019 hammers and plucked strings. <\/p>\n He believed mathematics was at the heart of the natural world. He proposed that the sun, moon and planets each emit unique hums based on their orbital properties. Pythagoras thought this \u201cmusic of the spheres\u201d would be imperceptible to the human ear.<\/p>\n Four hundred years ago, Johannes Kepler picked up this idea. He proposed that musical intervals and harmonies described the motions of the six known planets at the time. <\/p>\n To Kepler, the solar system had two basses, Jupiter and Saturn; a tenor, Mars; two altos, Venus and Earth; and a soprano, Mercury. These roles reflected how long it took each planet to orbit the sun, lower speeds for the outer planets and higher speeds for the inner planets. <\/p>\n He called the book he wrote on these mathematical relationships The Harmony of the World. While these ideas have some similarities to the concept of orbital resonance, planets don\u2019t actually make sounds, since sound can\u2019t travel through the vacuum of space.<\/p>\n 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 one complete circuit of the star. So, for example, two planets orbiting a star would be in a 2:1 resonance when one planet takes twice as long as the other to orbit the star. Resonance is seen in only 5% of planetary systems.<\/p>\n In the solar system, Neptune and Pluto are in a 3:2 resonance. There\u2019s also a triple resonance, 4:2:1, among Jupiter\u2019s three moons Ganymede, Europa 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 orbital periods that are the ratio of whole numbers. <\/p>\n Musical intervals describe the relationship between two musical notes. In the musical analogy, important musical intervals based on ratios of frequencies are the fourth, 4:3, the fifth, 3:2, and the octave, 2:1. Anyone who plays the guitar or the piano might recognize these intervals.<\/p>\nWhat is orbital resonance?<\/h3>\n
Harmony of the spheres<\/h3>\n
Orbital resonance<\/h3>\n
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<\/figcaption><\/figure>\nThe relation to music<\/h3>\n