Mercury

Mercury is the smallest, innermost, and fastest-orbiting planet in the Solar System. Measuring only about 4,880 km in diameter, it is just slightly larger than Earth’s Moon. Its orbit is highly elliptical, causing it to swing dramatically closer and farther from the Sun during each revolution. A Mercury year is only 88 Earth days, but its own rotation is so slow that one Mercury day lasts 176 Earth days, meaning its daylight cycle is longer than its entire year.

Mercury’s most defining characteristic is its lack of atmosphere. Without a thick blanket of gas to trap heat, the planet experiences the most intense temperature changes of any world: daytime temperatures exceeding 430°C (800°F), while nighttime temperatures drop to –180°C (–290°F). This severe heat swing makes Mercury one of the harshest environments in the Solar System.

Its surface is ancient and heavily cratered, resembling the Moon. One of the most striking features is the Caloris Basin, an enormous impact crater over 1,550 km across—nearly 40% of Mercury’s diameter. The impact was so powerful that it created “weird terrain” on the opposite side of the planet caused by shockwaves traveling through the entire world.

Despite its tiny size, Mercury has a massive iron core, making up 85% of the planet’s radius. This oversized core allows Mercury to generate a weak magnetic field—unusual for such a small planet. Scientists believe the planet may have once had a much thicker crust that was stripped away by a colossal impact early in its formation.