Scientists discover a planetary waltz – 01/16/2024 – Marcelo Viana

About 100 light-years from Earth, in the constellation Cabeleira de Berenice, is the star HD110067, similar to our Sun but with an unusual planetary system. In an article published in the journal Nature in November, scientists from the University of Chicago report that in Six planets orbit around it, designated by the letters B, C, D, E, F and G, all with similar sizes: their radii are 2 to 3 times the radius of the Earth. We don’t know their composition, but we know that they are very close to the star, closer than Mercury is to the Sun, so they are too hot to support life.

What makes this system remarkable is something else: the durations of the six planets’ orbits, which are completely synchronized. For every 3 revolutions of B around the star, C makes exactly 2 revolutions. The same 3:2 proportion applies to the orbits of C and D, and also to the orbits of D and E. Then, it changes: between the orbits of E and F the proportion is exactly 4:3, and between the orbits of F and G too.

Such perfect synchrony —which has lasted for more than a billion years!—may seem like a magical coincidence, but the mechanism that caused it to arise is common in nature, and its mathematics is well understood. Periodic systems – that is, those that repeat the same configurations at regular time intervals – in mutual interaction usually exchange energy with each other until the ratios between their periods are given by fractions of small and whole numbers. We say they resonate.

A familiar example occurs when we push a swing: we quickly realize that it is best for the period of the push to be equal to the swing period of the swing, or at least a small multiple of it, as this maximizes the energy we transmit to the toy. In the planetary system, the “pushes” correspond to the mutual gravitational attraction between the planets.

However, although the case of HD110067 is not unique, it is rare: less than 1% of known planetary systems have such strong synchronies. And our Solar System is not one of them. This is because in most cases the movement of planets is affected by dramatic events, such as collisions or the approach of another star. In our case, it is known that the formation of the giant planet Jupiter messed up (a lot!) the evolution of the system.

Still, there is resonance in the Solar System. The ratio between the orbital periods of Jupiter and Saturn is approximately 2:3, for example, and we also know that the Asteroid Belt has gaps caused by resonances with Jupiter.

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