In contrast to one’s ideas, the planets do not turn around the Sun on fixed orbits like an orrery.
Every body in our Solar System affects the motion of all the others, somewhat as if they were all interconnected via springs: a small perturbation somewhere in the system influences all the planets, which tends to re-establish their original positions. Planetary orbits have a slow rotatory motion around the Sun and oscillate around a mean value.
Since 1989, thanks to the work of Jacques Laskar at the Paris Observatory, astronomers have known that these oscillations are irregular, or chaotic, and that one cannot compute them beyond 60 million years in the past in order to understand the ancient state of the Solar System. The uncertainties arising from the chaotic nature of the solar system are in fact multiplied by 10 every 10 My.
However, this limit has just been crossed thanks to the work of an international multidisciplinary team of scientists to which includes Jacques Laskar [1], a CNRS astronomer at the Institut de Mécanique Céleste et de Calcul des Ephémérides (Institute for Celestial Mechanics and Ephemeris Computation - Observatoire de Paris - PSL / CNRS / Sorbonne Université / Université de Lille). In effect, the French astronomer and his colleagues have shown how geological data can enable one to stretch the predictability horizon up to 200 My.
But just how can geology teach us astronomy? Thanks to the famous Milankovitch parameters: a part of the ancient climatic changes is due to changes in the amount of solar light received on the Earth as a consequence of orbital changes.
It turns out that these climatic changes have left a mark on the Earth, in geological strata. A study of these strata thus enables us to determine the parameters of the terrestrial orbit and thereby deduce the state of the other planets.
Scientists have thus been able to find quantitative information about the past state of the Solar System, thanks to two important cores in the geological layers of the North American continent. Their work shows, for example, that 200 My ago, the eccentricity of the Earth oscillated with a period of 1,7 My, in contrast to today’s period of 2,4 My. This also furnishes a geological proof of the chaotic nature of the Solar System: if the latter had been regular, on would have found the same period as that which is observed today.
The research team now hopes to be able to refine these results via a study of other geological layers at high latitudes, which should give further information about the changes in the orientation of planetary orbits.
Bibliography
The Geological Orrery : Mapping Chaos in the Solar System. Paul E. Olsen, Jacques Laskar, Dennis Kent, Sean Kinney, David Reynolds, Jingeng Sha et Jessica Whiteside. Proceedings of the National Academy of Sciences, semaine du 4 mars 2019.
[1] Jacques Laskar receives the Milutin Milankovitch 2019 Medal for his work on the influence of orbit modifications on the climate and the development of reliable astronomical solutions, important for the paleoclimatology community
Last update on 21 December 2021
