Astronomy Picture of the Day . Venus UnVeiled . ... A comet nucleus, formed from the primordial stuff of the solar system, resembles a very dirty iceberg. ... After taking its spectacular pictures of the outer solar system planets, Voyager 1 looked back at six planets from the inner solar system. ... Pluto: The Frozen Planet . ... Mercury is the closest planet to the Sun, the second hottest planet (Venus gets hotter), and the second smallest planet (Pluto is smaller). The Sun Erupts . ...