Mercury

The innermost planet of the Solar system, Mercury is the only planet in Sol that has no permanent population. Due to the planet's material composition, its magnetic field causes severe interference with instruments and its proximity to the sun makes terraformation extremely difficult. During the age of expansion, when mankind was spreading through Sol assisted by new technologies, Mercury was explored as a potential terraforming candidate, but the colonization effort fell apart quickly for the aforementioned reasons and it has not been attempted again. The lone, small planet has drifted through space ever since, an empty rock devoid of life.