Do Young Planets Make and Break Orbital Chains?

V1298 Tau is the youngest known planetary system containing multiple exoplanets. What can simulations tell us about how this system likely formed?

animation of the resonances of jupiter's moons

This animation (not to scale) shows the resonances exhibited by three of Jupiter’s moons. Alignments between the moons are highlighted by color changes. [Wikipedia]

Chasing Young Planetary Chains

One of the many enduring mysteries in the study of planetary systems is how they form. Some theories suggest that the movement of young planets within a protoplanetary disk causes the planets to form resonant chains — a setup in which the planets’ orbital periods are integer multiples of each other. Jupiter’s moons Ganymede, Europa, and Io are an example of this arrangement; the orbital periods of Europa and Ganymede are two and four times as long, respectively, as Io’s orbital period.

However, observations show that less than 1% of mature planetary systems are arranged in resonant chains. To understand whether planets make and then break resonant chains — or whether these chains form at all — we need to study young (<100 million years old) systems with three or more planets, only two of which are currently known.

Seeking Stability

At just 23 million years old, V1298 Tau hosts the youngest multi-planet system discovered so far. Given the orbital periods of the four known planets in the system — roughly 8, 12, 24, and 50 days — some researchers have suggested that the planets in this system are arranged in a resonant chain. To test this theory, a team of astronomers led by Roberto Tejada Arevalo (Princeton University) incorporated new estimates of the planets’ masses into a dynamical model to test the stability of the current arrangement and probe the true orbital properties of the system.

plot of posterior distributions for resonant conditions

Distributions of stable (black) and unstable (red) resonant configurations for different samples of orbital elongation or eccentricity, e, and the ratio of total planetary mass to stellar mass, μ. Click to enlarge. [Adapted from Tejada Arevalo et al. 2022]

In their analysis, Tejada Arevalo and collaborators used observations of planetary transits made by Kepler and the Transiting Exoplanet Survey Satellite (TESS) to constrain the possible orbital parameters of the system. The transits captured by Kepler and TESS give us only snapshots of the system’s behavior, and orbital parameters can oscillate over time, so the authors determined the set of orbital parameters that could lead to the transits we’ve observed. Using this set of parameters, the team then tested the stability of each orbital setup. Ultimately, the authors find that only 1% of stable orbital configurations that could generate the observations are consistent with a resonant chain, making it unlikely that V1298 Tau’s planets are arranged in such a configuration.

Finding a Solution that Resonates

What do these results imply about the possible creation — and subsequent breaking — of resonant chains in young planetary systems? The V1298 Tau system’s lack of a resonant chain arrangement implies that either the planets were never in that configuration or the breaking of the chain occurred early in the system’s formation. The dissipation of a protoplanetary disk, which tends to occur after just a few million years, may provide a natural way for resonant chains to become unstable. The team’s analysis suggests that the system’s nearly resonant configuration is consistent with a chain-breaking instability early in the system’s history.

plot of the target star locations for the Cluster Difference Imaging Photometric Survey

TESS observing footprint (gray) and target stars (blue) for the Cluster Difference Imaging Photometric Survey — a search for young exoplanets that might uncover resonant chains. Click to enlarge. [Bouma et al. 2019]

Ultimately, characterizing just one planetary system isn’t enough to draw conclusions about the formation of planetary systems as a whole. To do that, we’re going to need more detections of young planets — and, luckily, several searches are underway.


“Stability Constrained Characterization of the 23 Myr Old V1298 Tau System: Do Young Planets Form in Mean Motion Resonance Chains?,” Roberto Tejada Arevalo et al 2022 ApJL 932 L12. doi:10.3847/2041-8213/ac70e0