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movement of Mercury

Coincidences on Mercury

The planet Mercury (foreground) and a record of its orbit around the sun

NASA / JHU / Carnegie InstituteThe planet Mercury (foreground) and a record of its orbit around the sunNASA / JHU / Carnegie Institute

An international group of astronomers, including the Frenchman Julien Frouard, who has just finished his post-doctoral studies at the Universidade Estadual Paulista (Unesp) in Rio Claro, believe they have found the explanation for one of the most mysterious coincidences in the solar system: the celestial movement of Mercury. Only one-third the size of Earth, Mercury is the smallest planet and lies closest to the sun. The researchers wanted to understand the relation between Mercury’s spin period – or how long it takes the planet to complete one revolution around its axis – and its orbital period – which is the time it takes the planet to go around the sun. Its spin period of 58 Earth days is precisely two-thirds of its 88-day orbital period. In other words, every three Mercury days the planet goes through two Mercury years. Since the planet lies so close to the sun, it was expected that the intense gravitational force of this star would slow Mercury’s spin down until the length of its day equaled that of its year. The question has been why this process stopped just when it reached the proportion of three to two. In October 2013, at a meeting of the American Astronomical Society, Frouard and his colleagues presented a new model for the history of Mercury, which was born some five billion years ago. The model takes into account details like changes in the rocky body of the planet, which were caused by the sun’s gravitational pull on it. The scientists concluded that the influence of the sun, combined with Mercury’s significantly elongated orbit, caused the planet to get trapped at its current spin-orbit ratio early on in its history – that is, only tens of millions of years after it formed.

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