Planets Can Be Bells: The Aftermath of Major Mergers

What would happen to a gaseous exoplanet if it was struck by a protoplanet? Recent research suggests that it would ring like a bell, and that just maybe, we’d be able to observe this ringing back on Earth.

Heavy Metal Mystery

Previous studies have hinted that giant planets have more heavy elements in their cores than we’d expect if they had formed from the exact same stew of gas and dust as their host stars. How exactly they end up with these enriched compositions is something of a mystery, though. Astronomers have proposed two potential theories: one somewhat dull, and one that involves dramatic collisions between the massive protoplanets.

In the first case, perhaps growing giant planets clear out the gas along their orbits, then steadily and quietly accrete the dust that spills over into the gaps. In the latter case, theorists have conjured a model in which growing gas giant planets repeatedly collide and merge with large and somewhat rocky protoplanets. Both of these mechanisms would produce the enhanced heavy element contents observed today, so how can we tell these two paths apart? Recent work by a team led by J.J. Zanazzi (University of California, Berkeley) proposes one such test.

Ringing Planets

Through mathematical derivations, the researchers demonstrate that if a gas giant were to collide with a protoplanet, the impact would cause the planet’s surface to physically oscillate inwards and outwards, and for its surface temperature to vary. In other words, the planet would “ring” like a bell struck with a hammer, and as it physically convulses in the aftermath of the merger it would grow brighter and fainter in a repeating, periodic pattern. In principle, if the ringing lasts for long enough and the oscillations in brightness were large enough, observers back on Earth could record the amplitude and period of the changes and back out some constraints on what had happened and when.

Excitingly, the researchers demonstrate that not only would the oscillations last for millions of years, they would also be bright enough for JWST to detect for one specific planet named Beta Pictoris b. Should this world have undergone a merger in the last 18 million years, just a few hours of observations with JWST’s Near-Infrared Camera would reveal 45-minute pulsations in which the planet’s brightness changes by about 1%. Since the planet is too far away from its host star for these oscillations to be caused by any gravitational interactions, detecting any periodic signal would be a slam-dunk for the major merger theory.

Beta Pictoris b is a young planet in a system that’s still forming, and it orbits its host star on an eccentric path that takes it through dense regions of the disk. Hopefully, future observations will test whether this treacherous path produced any massive collisions in the relatively recent past. Until then, though, we can all enjoy charming visions of massive planets colliding with one another and ringing through the night.

Citation

“Seismic Oscillations Excited by Giant Impacts in Directly Imaged Giant Planets,” J. J. Zanazzi et al 2025 ApJ 993 3. doi:10.3847/1538-4357/ae04ec