ScienceShot: Planets 'Sing' in Three-Part Harmony

Here's music of the spheres: Astronomers have found three planets orbiting a nearby star in resonance, which means their gravity has locked them into orbital periods that are simple multiples of one another. The planets line up every 124 days. Several two-planet resonances have long been known—for example, between Neptune and Pluto and between two planets orbiting a distant pulsar—but this is the first three-planet resonance ever seen. The planets orbit Gliese 876, a red dwarf 15 light-years away in the constellation Aquarius (an unrelated red dwarf system is pictured). Researchers discovered the star's first two resonant planets about a decade ago; their masses are similar to those of Jupiter and Saturn. Now, as astronomers will report in an upcoming issue of The Astrophysical Journal, a third and smaller giant—similar in mass to Uranus—orbits beyond the others. The three planets are in a 4:2:1 resonance: the innermost giant completes four orbits in the time the middle one completes two and the newfound outermost world completes one. The resonance may date back to the planets' births and thus may yield insights into the formation of giant planets around other stars, including our sun.

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