Artistic representation of a hot Neptune orbiting close to its parent star. Kepler-223 has four such planets, all with orbital periods of less than 20 days. Mercury’s orbit, on the other hand, lasts 88 days.
How did our solar system reach its current configuration? One of the main candidates to explain things like the sparseness of the asteroid belt and the small size of Mars is the great tack, with Jupiter originally migrating inward toward the sun until interactions with Saturn pulled them both out again.
The idea that giant planets wander near their star has received some support from many of the exosolar systems we’ve discovered. We’ve seen densely packed systems of large planets when there probably wasn’t enough material in the region to form them all, suggesting they formed elsewhere and then migrated into place.
But this idea raises some questions. What keeps the planets from wandering, keeps them from colliding with each other, and prevents them from falling into their parent star? A phenomenon known as orbital resonance may be the answer, and researchers claim it explains the presence of four exoplanets, all with orbits less than 20 days.
The exosolar system is called Kepler-223 and has been known for a while. But it was a bit difficult to study, since the signals from the transiting planets are hard to distinguish from the fluctuations of its host star (which is 6 billion years old and resembles the sun). Nevertheless, enough Kepler data is available to clearly distinguish their signals 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 times reduces 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 passes make minor corrections if one of the bodies begins to drift out of place. These kinds of resonances have been observed before (for example, between some of Saturn’s moons), but this is the first four-planet to have been observed.
The regular gravitational interactions also create so-called “transit timing variations”. Because of the different gravitational pulls and drags, the planets complete their orbits a little faster or slower than we would expect if they orbited the Earth alone. These orbits appear from our perspective as differences in the time when the planet begins to move in front of its host star.
The timing variations allow the authors to determine the masses of each planet, all of which fall in 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), we can calculate their density.
An interesting feature is that each planet’s density decreases as it moves further away from its host star. The authors estimate that the outer planet is 10 percent hydrogen and helium, and that amount of gas could have been obtained only in the cooler regions of the exosolar system, well beyond its current 20-day orbit. This idea strongly argues that migration plays a key role in the formation of the system.
Further evidence comes from simulations of the system’s formations. The authors orbit the planets at different locations in a gas-filled disk, which creates a drag that slows their orbits. As a result, the planets migrate inward and do not stabilize until they reach the 3:4:6:8 ratio of orbital times.
But the resonance is extremely fragile. If there are only a few small bodies (planetesimals) crossing the orbits of these planets, it could be enough to throw off the resonances. Actually just about each significant orbital interactions can throw off the resonances. “Various mechanisms,” the authors write, “including disk dissipation, planet-planet scattering, tidal dissipation, and planetesimal scattering can break migration-induced resonances.”
So their argument is that many exosolar systems probably experienced planetary migrations and orbital resonances between their planets. While these things play an important role in structuring the systems, they are also fragile, and small perturbations tend to throw off the resonances as the system ages. As a result, resonances do not dominate the population of exosolar systems Kepler imaged, although they are more common than chance.
The main conclusion is that Kepler-223 could have reached its current state only through migration, providing some support for the “grand tack” of gas giants through our own solar system.
Nature2016. DOI: 10.1038/nature17445 (About DOIs).