The odd one out is Pluto. It is a dwarf planet, while the others are planets. Pluto was classified as a planet from its discovery in 1930 until 2006, when the International Astronomical Union (IAU) reclassified it as a dwarf planet. The IAU’s definition of a planet requires that an object be in orbit around the Sun, have sufficient mass for its self-gravity to overcome rigid body forces so that it assumes a hydrostatic equilibrium (nearly round) shape, and have cleared the neighborhood around its orbit. Pluto does not meet the third criterion, as it shares its orbit with many other objects in the Kuiper belt.
Mars is the fourth planet from the Sun and the second-smallest planet in the Solar System. It is a terrestrial planet with a thin atmosphere, having surface features reminiscent both of the impact craters of the Moon and the valleys, deserts and polar ice caps of Earth.
The Sun is a star, a giant ball of hot plasma held together by gravity. It is the center of our solar system and the source of all the light and heat on Earth.
Saturn is the sixth planet from the Sun and the second-largest planet in the Solar System. It is a gas giant with an average radius of about 9.5 times that of Earth. Saturn is known for its extensive ring system, which is composed of countless small particles ranging in size from dust grains to small moons.
Mercury is the smallest planet in the Solar System and the closest to the Sun. It is a terrestrial planet with a solid surface. Mercury has no atmosphere, and its surface is heavily cratered.