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When a planet revolves around a star, it will have a fixed time period if the only gravitational force acting on it is by the star around which it orbits. For example, if in our solar system, only one planet were there, the it would have a fixed time period. But since there are more than one planet, the gravitational force between the planets affects its time period. In our solar system, this effect is in the order of seconds and hence negligible. 
PH3C is 2300 lightyears from its star. and it has massive neighbouring planets such as PH3B and PH3D whose gravitational force influence is much higher. Since their relative position changes at different times, the mutual gravitational forces also changes. In simpler words, there are gravitational force acting on PH3C apart from the gravitational force of the star around which it orbits. So the time period varies. 
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but arount the eart there r also planet and all planets have neighbouring planet so why it could not happen to other planet
and if gravitational forces increse then it will immediately and force fully attracted by sun then it merge and if decrese then it moves on another path same lik electron do in nucleus
Sun is much more massive than any planet in our solar system.So the gravitational force by other planets on earth are negligible compared to the gravitational force by sun. SO their effects are negligible.

The fluctuations are very small compared to the gravitational force of the sun.
And electron revolving around nucleus is analogous to the planetary system. It is not the same.
yup thanks