The exoplanet GJ 367b orbits its star in an extremely short time. A year on this planet is just eight hours. There is no planet like it in the Solar System: Mercury is the fastest planet here with an orbital period of 88 days. Compared to GJ 367b it is a snail. GJ 367b is a rocky planet that is much denser than Earth and similar in structure to Mercury. It probably has a large iron core. This precise characterisation is based on high-precision measurements of radius and mass – not a matter of course for exoplanets. GJ 367b orbits a dwarf star that is about half the size of the Sun. The radiation on the planet is enormous due to the small distance to the star: On the side of the planet facing the star, the temperature is between 1300 and 1500 Celsius. At such temperatures, iron and rocks melt.