The correct answer is Mercury.
Key Points
- Mercury is the closest planet to the Sun.
- The planet probably received this name because it moves so quickly across space.
- Temperature variations on Mercury are the most extreme in the solar system ranging from 90 K to 700 K.
- Mercury is the second densest major body in the solar system, after Earth.

Important Points
- The solar system is made up of the sun and everything that orbits around it, including planets, moons, asteroids, comets, and meteoroids.
- It extends from the sun, called Sol by the ancient Romans, and goes past the four inner planets, through the Asteroid Belt to the four gas giants and on to the disk-shaped Kuiper Belt and far beyond to the teardrop-shaped heliopause.
- Scientists estimate that the edge of the solar system is about 9 billion miles (15 billion kilometers) from the sun.
- Beyond the heliopause lies the giant, spherical Oort Cloud, which is thought to surround the solar system.
Additional Information
- The sun is at the center of our solar system and is its largest object, accounting for 99.8% of the solar system's mass.
- Our sun is a giant, raging ball of fire powered by nuclear reactions, and it provides the energy that sustains life on Earth.
- The life-giving star is a yellow dwarf star made up of gas: about 91% hydrogen and 8.9% helium, according to NASA.
- Compared with other stars, the size of the sun is relatively small and it's just one of the hundreds of billions of stars in our home galaxy, the Milky Way.