Venus is a planet.
Venus . It has the longest rotation period ( 243 days ) of any planet in the Solar System and rotates in the opposite direction to most other planets . Venus is a terrestrial planet and is sometimes called Earth 's `` sister planet '' because of their similar size , mass , proximity to the Sun , and bulk composition . Venus was the first planet to have its motions plotted across the sky , as early as the second millennium BC . As the closest planet to Earth , Venus has been a prime target for early interplanetary exploration .