The densest planet in our solar system is Earth. This density results from its composition, which includes a significant amount of heavy metals like iron and nickel in its core, surrounded by a silicate mantle and crust. Earth's average density is about 5.5 grams per cubic centimeter, which is higher than that of any other planet in the solar system. This high density contributes to Earth's strong gravitational pull, which plays a crucial role in retaining its atmosphere and sustaining life.