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.