Exoplanet research has made huge progress lately, with more and more worlds outside the Solar System being found. And now, we are getting closer to understanding how some of these planetary systems form.

In a study published in Nature, an international team of researchers has found conclusive evidence that giant planets migrate in young star systems. The system they studied, known as Kepler-223 and located 4,450 light-years, has a unique configuration, with four planets bigger than Earth but smaller than Neptune, so-called sub-Neptunes, orbiting very close to their star in seven to 19 days.

The four planets, which all have masses between three to nine times that of Earth’s, have orbital periods in the ratio of 3:4:6:8. The ratio is so precise that it creates an incredibly stable system, and the planets regularly align themselves in a cosmic ballet. Astronomers had seen extrasolar systems with two or three planets in resonance, but never four.

The team constructed a computer simulation to explain how this peculiar arrangement came to form, using Kepler 223 as a testing ground for the idea that gassy planets form further away from the star and then migrate inward across the protoplanetary disk.

"We think that two planets migrate through this disk, get stuck and then keep migrating together; find a third planet, get stuck, migrate together; find a fourth planet and get stuck," lead author Sean Mills explained in a statement.

^{This animation illustrates the Kepler-223 planetary system. Each time the innermost planet (Kepler-223b) orbits the system’s star three times, the second-closest planet (Kepler-223c) orbits precisely four times. W. Rebel}

The Kepler observatory has discovered many star systems with multiple super-Earths and sub-Neptunes orbiting close to their host star, something we don’t observe in our own Solar System. How these planets come to be in this position is a bit of a mystery though, with the migration theory being touted alongside others.

The researchers think that all these sorts of planets moved inwards due to the influence of resonance, and later became destabilized due to either larger planets or swarms of planetesimals, remnants from the initial formation material, crossing their orbits.

"Our work essentially tests a model for planet formation for a type of planet we don't have in our Solar System," added Mills. "That's why there's a big debate about how they form, how they got there, and why don't we have one."

While Kepler 223 doesn’t look much like the Solar System, the teams believe that our Solar System’s giants – Jupiter, Saturn, Uranus, and Neptune – were once moving around in a resonance configuration, and only interactions with the numerous smaller objects lead them to their present-day orbits.

The variety of the worlds and systems that have been showcased by Kepler observations indicates the complexity of planetary formation, but we are getting closer and closer to figuring out how it all fits together.