Mercury is the closest planet to the sun, but it does not have the title of the hottest planet (see Venus’ description). It is the smallest planet in the Solar System — smaller even than Saturn’s moon Titan and Jupiter’s moon Ganymede.
Mercury also has the most radical temperature changes going from 427°C down to -170°C on the surface away from the Sun. This is due to the lack of atmosphere which would help regulate the temperature by holding on to the heat.
Due to the lack of atmosphere, or exosphere as it’s called in Mercury’s and the other moons cases, it has no protection against asteroids and meteors that stray too close. This is the reason why Mercury looks very similar to the surface of our own Moon, because the atmosphere doesn’t burn up any objects like they do on Earth and other planets.