The Kepler-223 system holds four Neptune-like worlds circling their star in a precise, musical counterpoint to one another. All four planets orbit in resonance, meaning gravity has created a rhythmic relationship between each planet’s orbit. Imagine if every time Earth orbited the sun once, Venus orbited the sun twice (in real life, Earth and Venus are not in resonance). This unusual phenomenon was first discovered with Jupiter's moons, and may mirror an early version of our solar system in which Jupiter, Saturn, Uranus and Neptune once were in a similar configuration. Kepler-223 is the first system with four planets discovered in resonance. The two innermost planets, Kepler-223 b and c, are in 4:3 resonance, the second and third in 3:2, and the third and fourth planets are in 4:3. Each planet likely has a solid core, with an envelope of gas and a tight orbit, classifying them as "sub-Neptunes." Though these worlds are much larger than Earth, they orbit nearer their star than Mercury does our sun.