Mercury

Mercury is the smallest and innermost planet of the Solar System. Despite being closest to the Sun, it is not the hottest planet — that distinction goes to Venus, which has a thick CO₂ atmosphere. Mercury’s essentially non-existent atmosphere (a tenuous exosphere of sodium, potassium, hydrogen and helium) lets heat escape rapidly; temperatures drop to ~100 K on the night side.

The planet is locked in a 3:2 spin-orbit resonance, meaning it rotates three times on its axis for every two orbits around the Sun. A single solar day on Mercury (sunrise to sunrise) lasts about 176 Earth days.

Mercury’s density (5.43 g/cm³) is second only to Earth’s and implies an enormous iron core making up some 85% of its radius. The Caloris Basin, 1,550 km across, is one of the largest impact features in the Solar System; antipodal to it lie the “weird terrain” hills thought to have formed when seismic waves from the impact converged on the opposite hemisphere. NASA’s MESSENGER mission (orbital phase 2011-2015) mapped the planet in detail and confirmed water ice in permanently shadowed polar craters. The ESA/JAXA BepiColombo spacecraft, launched in 2018, is scheduled to enter orbit in November 2026.