A Sun-like star is home to two huge planets both about the size of Saturn. This pair of planets are locked in orbital resonance, with one planet taking almost exactly twice as long to orbit the star as the other.
The two planets both orbit their star at distances far closer than even that between Mercury and our Sun. The inner planet revolves around the star once ever 19.2 days, and the outer planet takes 38.9 days to do the same. The fact the planets orbit in an almost exact 2:1 ratio suggests they are in orbital resonance, a celestial phenomenon in which bodies exert unusual gravitational influence on each other's orbits. The most obvious sign of this is that the planets' orbits can be expressed as the ratio of very small integers.
These resonance systems can be either stable or unstable, depending upon the masses of the two objects and the external gravitational influences (like that of their star). The astronomers who discovered the planets ran multiple simulations of this system could evolve over time, and they consistently found that this system should remain stable for billions of years. These simulations also suggest both planets have slightly less mass than Saturn and are roughly the same size.
In a podcast with Science, astronomer Matthew Holman explained how the two planets most likely formed:
"We think mostly likely they formed with the star itself but probably they didn't form where we presently see the planets. Most likely they formed further out beyond the point which is called the snow line - the point at which a lot of gases formed ices - and those ices and the rocky material with it would form the cores of giant planets. Once those cores are formed, they begin to accumulate gas, the gas from the surrounding disc. So our understanding is that the way giants planets...form beyond that snow line which is several times further from the star than say the Earth is from the Sun, they form there and then they migrate inward by some dynamical process. And that dynamical process is either scattering of planets or interaction with the remaining disc that is part of this planet formation process.
"That we see these two planets near this 2:1 orbital resonance, that's a signature to theorists that there was some kind of migration that occurred, that having planets be captured into these orbital resonances is a consequence of something we call convergent migration where two planets are moving either toward the star or away from the star but they're doing so in a fashion that the planets are getting closer to each other as time progresses."
The planets were discovered by the Kepler Space Observatory, and so the star has been designated Kepler-9, with the two planets Kepler-9b and Kepler-9c. It's very possible there's also a Kepler-9d as well - the data suggests the two planets can't fully account for each other's gravitational perturbations, which means there must be another planet affecting both of them.
The astronomers believe there's a third planet much, much closer to Kepler-9, which might have an orbital period of just 1.6 days. This planet would most likely be a "Super Earth", a massive rocky planet several times the mass of Earth. However, the results so far are inconclusive as to the precise nature of this third planet, so its existence is still considered unconfirmed.
[Science Express; artist's conception courtesy of NASA/Ames/JPL-Caltech.]