Answer:
density
Explanation:
mecurys is significantly smaller and less massive than earth
Gravitational force between sun and planet is given by,

Thus, The force of gravitation is inversely proportional to the distance.
Kepler's law states that the square of period of planet is proportional to cube of semi major axis of its orbit.
The time period of planet is more when the gravitational force is less.
Thus, planet Y is closer to earth because sun exert greater gravitational force than planet Z.