1. The solar system and beyond

3. T h e M e r c u r y Mercury is the innermost and smallest planet in the Solar System , [a] orbiting the Sun once every 87.969 Earth days . The orbit of Mercury has the highest eccentricity of all the Solar System planets, and it has the smallest axial tilt . It completes three rotations about its axis for every two orbits . The perihelion of Mercury's orbit precesses around the Sun at an excess of 43 arcseconds per century; a phenomenon that was explained in the 20th century by Albert Einstein 's General Theory of Relativity . .