Which astronomical or cosmological event would explain periodical low-grav-effect on earth-like planet?

Imagine an earth-like planet on which gravity is reduced by 20%-30% every 15 months. The low-grav-effect should start slowly, reaching its peak/bottom of round about 0.7 g after 6 weeks and should slowly diminish again for another 6 weeks, so that it lasts for 3 months in total. The effect exists since the emergence of the planet.The planet itself is located in our galaxy, but much nearer to the center.

Which astronomical or cosmological event would explain such an effect? (It preferably should be visible to observers from the planet´s surface.)

This is serving for fictional worldbuilding. But I want the most scientific approach possible and only stretch or ignore empirical data, where absolutely necessary to fulfill my conceptions, in order to make the whole story function. For example: The 20%-30% grav-relief is only guessing. What I want is a considerable effect on intelligent lifeforms on the planet, so that they could think about the effect like a fifth season, with all of it´s implications.

posted over 6 years ago

Aertemis‭

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