Here's why:

* Terrestrial planets are rocky and have high densities due to their composition of metals and silicates. Examples include Mercury, Venus, Earth, and Mars.

* Pluto is classified as a dwarf planet and is primarily composed of rock and ice. Its density is much lower than the terrestrial planets.

Density Comparison:

* Earth: 5.52 g/cm³

* Venus: 5.24 g/cm³

* Mars: 3.93 g/cm³

* Mercury: 5.43 g/cm³

* Pluto: 2.03 g/cm³

As you can see, Pluto's density is significantly lower than any of the terrestrial planets.