You may have learned in previous sections of this web site that between every two objects there is an attraction due to gravity. The smaller the objects, the weaker the attraction. The larger the objects, the stronger the attraction. Also, the force of gravity depends on the distance between the two objects. If they are in close proximity to each other, the attraction is stronger, and vice versa. Now that we have that straight, consider the attraction between planets. Since planets are massive objects, there will obviously be a huge attraction between them.
For more on Newton's law of universal gravitation see What is Gravity? Page 4.
