We’re learning a lot more about how planets interact with each other gravitationally. ‘Resonance’ is the operative term here. When planets are locked in a 2:1 orbital resonance, the outer planet orbits the host star once for every two orbits of the inner planet. A 3:2 resonance occurs when the outer planet orbits the star twice for every three orbits of the inner planet.

Resonance (technically ‘mean motion resonance’) prevents close encounters between planets and provides long-term orbital stability. And if the 2:1 resonance is the most common pattern, it’s also true that things can change when planets migrate to different parts of their system. John Johnson (Caltech) describes the result of fast inner migration:

“Planets tend to get stuck in the 2:1. It’s like a really big pothole. But if a planet is moving very fast it can pass over a 2:1. As it moves in closer, the next step is a 5:3, then a 3:2, and then a 4:3.”

Johnson’s work on resonance has born fruit in a new paper in which he and his colleagues discuss the discovery of two solar systems where gas giants in relative proximity to each other have become locked into resonance. Studying the matter helps us understand how solar systems evolve, as planets farther out in the protoplanetary disk migrate inwards, causing gravitational disturbances that can only become stable in orbital resonance.

Studying the star 24 Sextantis, some 244 light years from Earth, using radial velocity methods, the researchers have found two gas giants separated by about 0.75 AU, roughly 113 million kilometers. You can contrast this with the spacing between the largest planets in our system. Jupiter and Saturn are never closer than 531 million kilometers. The planets orbit the star with periods of 455 days and 910 days and are locked in a 2:1 orbital resonance.

A second gas giant pairing occurs around the star HD 200964, some 223 light years from Earth. Here the distance between the two gas giants can close to 0.35 AU (53 million kilometers). Johnson likens the latter pairing to that of Titan and Hyperion, two Saturnian moons, which also show a 4:3 resonance, but notes that the planets orbiting HD 200964 interact far more strongly, each being 20,000 times more massive than the combined mass of Titan and Hyperion. The planets in this system have orbital periods of 630 and 830 days respectively. Johnson adds:

“This is the tightest system that’s ever been discovered, and we’re at a loss to explain why this happened. This is the latest in a long line of strange discoveries about extrasolar planets, and it shows that exoplanets continuously have this ability to surprise us. Each time we think we can explain them, something else comes along.”

Gravitational interactions in this environment are quite powerful. This Caltech news release notes that the gravitational tug between HD 200964’s two planets is 3 million times greater than the gravitational force between Earth and Mars, 700 times larger than that between the Earth and the Moon, and 4 times larger than the pull of the Sun on the Earth.

As to the history of these worlds, the paper on this work notes their current positions and their likely changes over time:

In both the 24 Sex and HD 200964 systems, the planets reside well within the so-called snow line, beyond which volatiles in the protoplanetary disk can condense to provide the raw materials for protoplanetary core growth. For a pre-main-sequence, 1.5 M [solar mass] star the snow line is located beyond 2-3 AU… It is therefore likely that the planets around 24 Sex and HD 200964 formed at larger semimajor axes and subsequently experienced inward orbital migration.

Both of these stars are massive and dying, subgiants that have evolved off the main sequence and have run out of hydrogen for nuclear fusion. The eventual fate of such stars is to become a red giant, but neither of the stars has progressed that far. While red giants are problematic for radial velocity methods because their pulsations mask the spectral shifts that would reveal orbiting planets, subgiants have not expanded to that point and planet hunting remains possible. In fact, using the Keck Subgiants Planet Survey, Johnson and team are learning a great deal about such systems:

“Right now, we’re monitoring 450 of these massive stars, and we are finding swarms of planets. Around these stars, we are seeing three to four times more planets out to a distance of about 3 AU — the distance of our asteroid belt — than we see around main-sequence stars. Stellar mass has a huge influence on frequency of planet occurrence, because the amount of raw material available to build planets scales with the mass of the star.”

The paper is Johnson et al., “Retired A Stars and Their Companions VI. A Pair of Interacting Exoplanet Pairs Around the Subgiants 24 Sextan[t]is and HD 200964,” accepted for publication in The Astronomical Journal (abstract).