How did our solar system reach its current configuration? One of the leading candidates to explain things like the sparseness of the Asteroid Belt and the small size of Mars is the grand tack, in which Jupiter originally migrated inward toward the Sun until its interactions with Saturn pulled them both back outward.

The idea that giant planets may go for a wander around their star's orbital neighborhood has picked up some support from many of the exosolar systems we've discovered. We've spotted tightly packed systems of large planets when there probably wasn't enough material in the region to form all of them, suggesting that they formed somewhere else and then migrated into place.

But this idea raises some questions. What stops the planets from their wanderings, keeps them from smashing into each other, and prevents them from falling into their host star? A phenomenon known as orbital resonance may be the answer, and researchers argue that it explains the presence of four exoplanets all with orbits of less than 20 days.

The exosolar system is named Kepler-223 and has been known about for some time. But it has been a bit difficult to study given that the signals of the transiting planets are difficult to pick out from the fluctuations of its host star (which is 6 billion years old and Sun-like). Nevertheless, enough Kepler data is available for their signals to clearly stand out from the noise. The planets have orbital periods of 7.4, 9.8, 14.8, and 19.7 days.

The authors find that the orbits of all four appear to be in resonance, meaning that the ratio of their orbital periods reduce down to the ratio of two small integers. In Kepler-233, those ratios are 3:4:6:8. The gravitational interactions of bodies in orbital resonance help stabilize their orbits, as the regular passages apply slight corrections if any of the bodies start to drift out of place. These sorts of resonances have been seen before (between some of Saturn's moons, for example), but this is the first four-planet one that has been observed.

The regular gravitational interactions also create what are called "transit timing variations." Because of the various gravitational pulls and tugs, the planets complete orbits somewhat faster or slower than we'd expect if they were orbiting alone. These orbits show up as differences in the time at which the planet starts to transit in front of its host star from our perspective.

The timing variations allow the authors to determine the masses of each planet, all of which fall into the super-Earth/sub-Neptune region—from four to eight times Earth's mass. Combined with the data on their radii (obtained by determining how much of their star's light they block out), we can figure out their density.

An interesting feature is that the density of each planet goes down as it moves out from the host star. The authors estimate that the outermost planet is 10 percent hydrogen and helium and could only have obtained that much gas in the cooler regions of the exosolar system, far beyond its current 20-day orbit. This idea makes a strong case for migration playing a key role in the system's formation.

Further evidence comes from simulations of the system's formations. The authors start the planets orbiting at different locations in a gas-filled disk, which creates a drag that slows their orbits. As a result, the planets migrate inward and only stabilize once they reach the 3:4:6:8 ratio of orbital periods.

But the resonance is extremely fragile. If there are just a few small bodies (planetesimals) that cross the orbits of these planets, it could be enough to throw off the resonances. In fact, just about any significant orbital interactions can throw off the resonances. "Various mechanisms," the authors write, "including disk dissipation, planet–planet scattering, tidal dissipation, and planetesimal scattering could break migration-induced resonances."

So their argument is that many exosolar systems have probably experienced planetary migrations and orbital resonances among their planets. While these things play an important role in the structuring of the systems, they're also fragile, and small perturbations tend to throw off the resonances as the system ages. As a result, resonances don't dominate the population of exosolar systems that Kepler has imaged, although they do show up more frequently than chance.

The main conclusion is that Kepler-223 could only have reached its current state via migration, which provides some support for the "grand tack" of gas giants through our own solar system.

Nature, 2016. DOI: 10.1038/nature17445  (About DOIs).

This post originated on Ars Technica