Young planetary systems resemble an orchestra that has just started rehearsing: all the musicians strive to play in the same rhythm. Astronomers, observing them with the TESS telescope, noticed that planets around young stars often move in sync: their orbits are arranged so that for every two orbits of one planet, there is exactly one orbit of another. But by the age of a hundred million years, this order falls apart, and the orbits start to resemble the discord after a long rehearsal break. Why does this happen?
Scientists have proposed a two-stage explanation. First, something gives the planets a slight "nudge," tilting their orbits a bit, like someone quietly bumping a music stand, causing the sheet music to shift. This is enough to trigger the second stage: the mutual gravitational pull of the planets starts to swing their orbits more and more, and within a hundred million years, order completely collapses. Such a "nudge" could come from impacts of small bodies, each about the size of Mercury, falling onto the planets. The total mass of these bodies is only a few percent of the entire system's mass, but that is sufficient.
These impacts also explain another oddity: in some young systems, the planets' orbits fall just short of exact resonance, like a musician hitting a note a semitone flat. Astronomers have also noticed that over time, systems lose planets: some may be ejected or fall into the star. This is similar to an orchestra losing musicians if the rehearsal drags on.
Thus, a picture emerges: planetary systems do not freeze immediately but go through a turbulent youth. The chaos we see in adult stars is not a coincidence but a consequence of ancient collisions. And perhaps our own solar system once experienced such a shake-up.
as written by the authors arXiv:2604.05035
TESS is enabling the discovery of transiting planets around young stars. These observations suggest that most close-in planets were born in chains of mean-motion resonances that break on a characteristic timescale of order 100 Myr. This observation is surprising because the same dissipative forces that capture planets into resonance render their orbits long-term stable. We explore a two-stage disruption scenario for resonant chains of super-Earths. First, the chains have their (free) eccentricities excited by some mechanism. We show that any such mechanism that seeds eccentricities of a few percent sets in motion a second stage of dynamical instability on a $$\sim$$100 Myr timescale. A possible stage-one mechanism is the accretion of a handful of Mercury-sized bodies totaling a few percent of the planetary system mass, which excites the requisite eccentricities and triggers a stage two that reproduces the observed decline in the incidence of resonance. Impacts from such bodies can also explain why some young systems have period ratios narrow of commensurability. We sketch how these impactors may have grown out of debris left over from an earlier epoch of planet formation. We also identify two new trends in the observational data: a decline in multiplicity on the same timescale as the decline in the incidence of resonance, and an increase in the occupation of resonances with multiplicity.
An impact from a Mercury-sized body on a planet releases enough energy to boil an ocean.
In Isaac Asimov's novel "The Currents of Space," planets are knocked out of their orbits by an external influence — similar to the scenario in this article.