Mercury

"The Sun may be a mass of incandescent gas, but in the plasmatic reaches of its solar winds spins another seemingly glowing (but relatively minute) orb. The last of the five naked-eye planets discovered in ancient times, Mercury has long been an elusive, enigmatic world. As seen from the Earth, it never emerges far from the Sun, and astronomers in the telescopic era found it challenging to work out basic data such as its rotation period, the inclination of its axis, and whether or not it possessed an atmosphere."--