Which planet in the Solar System is the closest to the Sun?

Mercury is the innermost planet, orbiting closest to the Sun at about 0.39 AU (58 million km). Being so near, it moves fastest in its orbit—one Mercury year is just 88 Earth days. Its thin exosphere can’t hold heat, so surface temperatures swing wildly from scorching days (about 430°C) to frigid nights (about −180°C). Fun context: Despite being closest to the Sun, Mercury isn’t the hottest planet—that title goes to Venus, whose thick atmosphere traps heat. Mercury is often hard to spot because it hugs the horizon near sunrise or sunset. Memory tip for planet order: “My Very Educated Mother Just Served Us Nachos”—Mercury comes first.