Mercury is the first rock in line from the Sun, and it shows — with almost no atmosphere to insulate it, this tiny world swings from blistering daytime heat to bone-cold nights within the same 176-Earth-day day.
Despite its small size, Mercury holds a giant secret: a massive iron core that makes up roughly 85% of its radius, more proportionally than any other planet. Scientists think an ancient collision may have stripped away much of its outer rock, leaving that oversized metal heart behind.