Mercury Mercury is the smallest rocky planet (terrestrial planet) in the solar system, orbiting closest to the Sun. Basic Characteristics: Orbital Period: Approximately 88 days (a very short year). Size and Mass: The smallest among the planets in the solar system. Surface Temperature: Because it is close to the Sun, daytime temperatures rise to about 430°C, while nighttime temperatures drop to minus 180°C, resulting in extreme temperature fluctuations. Atmosphere: Due to its low gravity and the effects of solar winds and high temperatures, it has almost no atmosphere.