Mercury

Mercury is the smallest planet in our solar system and the closest to the Sun. It orbits the Sun every 88 Earth days at an average distance of about 58 million kilometers. Despite being closest to the Sun, Mercury is not the hottest planet -- that title goes to Venus. Mercury's lack of a substantial atmosphere means it cannot retain heat, leading to extreme temperature swings from -180°C at night to 430°C during the day, the greatest temperature variation of any planet. Mercury's surface is heavily cratered and resembles the Moon in appearance. The Caloris Basin, one of the largest impact craters in the solar system at roughly 1,550 km in diameter, dominates one hemisphere. The planet has a surprisingly large iron core that makes up about 85% of its radius, far more proportionally than any other planet. This oversized core generates a weak magnetic field about 1% the strength of Earth's. Due to its proximity to the Sun, Mercury is difficult to observe from Earth and was one of the least explored planets until NASA's MESSENGER mission orbited it from 2011 to 2015. The BepiColombo mission, a joint project of ESA and JAXA launched in 2018, is currently en route and expected to enter orbit around Mercury in 2025 to continue studying this enigmatic world.