Each planet in the solar system exerts some force on the Earth, however small that force may be. Most of it comes from the sun and the moon. For example, the moon is the primary reason we have tides. As you may know, twice per month there is a higher than usual tide. This occurs when the sun and the moon are lined up and acting to "tug" at the Earth in the same direction -- causing more pronounced tides. No other planets contribute to the Earth's tides because they are very far away, and gravitational force varies inversely with the square of the distance. Even if all other planets were to line up, the combined magnitude of their gravity would be insignificant compared to that of the sun and moon. It wouldn't even be detectable! So, in short, if there is a planetary conjunction in the sky, go out and watch it in comfort and with appreciation. For more "coverage", please see the popular astronomy magazines or their websites.