When Giants Collide: How Planetary Impacts Can Make Planets Ring Like Bells
The paper by Zanazzi et al. explores why many giant exoplanets—massive planets orbiting stars beyond our Solar System—contain so much heavy material. These planets often have more than 100 Earth masses worth of heavy elements, far exceeding expectations from standard formation theories. One explanation is that these elements came from smaller solid planets that crashed into them during their growth. These dramatic events, known as major mergers, could both add metal to the planet’s interior and trigger seismic oscillations. These vibrations may be visible using powerful telescopes like JWST and help explain properties of planets like Beta Pictoris b.
Planetary Oscillations: Vibrations Inside a Giant World
To understand what kind of seismic waves a giant planet can produce, the authors explore two types of oscillations: fundamental modes (f-modes) and pressure modes (p-modes). These waves are like “ringing” patterns inside the planet, driven by pressure and gravity. Using simulation tools (MESA and GYRE), the authors model a 13-Jupiter-mass planet and calculate the structure of its oscillations. F-modes have no radial nodes and lower frequencies, while p-modes are more complex and higher in frequency. The planet’s outer layers, where radiation escapes, are especially important in shaping the observable effects of these vibrations.
How Big Are the Oscillations After an Impact?
The authors next ask: how strong are oscillations after a giant collision? They simulate what happens when a Neptune-sized object hits a gas giant. The impact delivers a large amount of momentum, exciting the planet’s oscillation modes, especially those with strong surface movement. However, vibrations that are too strong can form shocks and lose energy quickly. The authors calculate both the expected amplitude from the impact and the upper limit before shocks develop. They find that impacts from Neptune-sized bodies can excite visible oscillations in young planets without exceeding this destructive threshold.
How Long Do These Vibrations Last?
Once excited, the oscillations don’t vanish quickly. The authors estimate how long different seismic modes last, considering two damping mechanisms: radiative loss and turbulent mixing. Both processes turn out to be weak for low-frequency modes, meaning these vibrations can persist for 10 million years or more. These timescales are similar to a planet’s cooling time—suggesting that "ringing" from a past impact might still be ongoing in planets like Beta Pictoris b. The analysis highlights that even long after a merger, signs of seismic activity could be detectable.
Seeing the Vibrations: Observability with JWST
Could we actually observe these oscillations? According to the authors, yes—especially in planets like Beta Pictoris b. They calculate how the planet’s brightness would change due to seismic modes, focusing on the most long-lived: the (ℓ = 0, n = 1) p-mode. This mode could cause brightness changes greater than 1% for up to 18 million years after a major impact. Simulated JWST observations show this variability would be visible above the noise, even considering interference from the host star or surrounding disk. These signals could serve as clear evidence of a planet-scale impact.
Implications: Seismology as a Tool for Exoplanets
Finally, the authors discuss how seismic observations could reveal the internal structure of giant planets. Much like how earthquakes inform us about Earth’s interior, these vibrations could help scientists study planetary density, layering, and spin. Measuring mode frequencies constrains average density, while other patterns could reveal deeper features. Although the study focuses on impact-driven oscillations, other forces—like tidal interactions from a nearby star—might also excite detectable seismic waves. In the future, seismology could become a key method for probing the hidden interiors of distant alien worlds.
Source: Zanazzi
