The densest planet in the solar system is Earth. Its average density is about 5.52 grams per cubic centimeter, which is greater than that of any other planet. This density is primarily due to Earth's metallic core, composed mainly of iron and nickel, along with the silicate mantle and crust. Earth's density allows it to maintain a strong gravitational pull, which retains its atmosphere and supports life. Additionally, the planet's internal structure contributes to its varied geological features, making it a unique and dynamic environment compared to the other planets in our solar system.