Artist's illustration of the K-138 system, which was discovered by citizen scientists. Credit: NASA / JPL-Caltech / R. Hurt (IPAC)

Citizen scientists have discovered a five-planet system orbiting a distant star known as K2-138, astronomers reported Thursday at the annual meeting of the American Astronomical Society and in a preprint posted online.

Why it’s cool: Like the planets around the star Trappist-1, K2-138's orbiters are resonant. That means the length of the planet’s years relate to each other in whole-number ratios. K2-138 is like Trappist-1 in another way: it's one of the few resonant systems whose orbital patterns can create a song when assigned tones.

How it works: K2-138’s orbits exist in a ratio where each planet’s year is roughly 50% longer than the next planet in toward’s the star’s - a ratio of 2:3. Some of the planets in Trappist-1 share this ratio, while others resonate at 4:5. Musically, the ratio of 2:3 is a “perfect fifth.”

How they found it: The system was first detected by the K2 project, an extension of the Kepler mission. The K2 project has detected over 287,309 stars so researchers crowdsource citizen scientists in the Exoplanet Explorers project to look for the dips of light caused by a planet crossing in front of a star.

What they found: In the end, they confirmed five planets, larger than Earth but smaller than Neptune. The innermost planet takes 2.35 earth days to rotate the star. Then, from innermost to outermost: 3.56, 5.40, 8.26, and 12.76 days.

Start of something new: This is the first time citizen scientists have found a multiplanet system. This means the Exoplanet Explorer project works, and can be used to hunt for more multi-planet systems.

1 weird thing: K2-128's planets are packed extremely close to their star. It’s a configuration never before seen, and one not currently accounted for in current models of solar system formation. That means our galaxy is more diverse than previously thought.

There might be more: In a press conference at the 231st annual meeting of the American Astronomical Society, study author Jessie Christiansen of CalTech said there are hints of a sixth, currently unconfirmed outer planet. Tantalizingly, if that planet exists, its year is roughly 42 days long, which means that if the resonance pattern holds true, there could be two other planets with 19 and 27 day-long orbits awaiting detection. If so, K2-138 would displace Trappist-1 as the longest resonance chain detected in any solar system.

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