Mercury and Venus are the two planets that can be seen as morning and evening star in the sky.

Mercury being the closest to the Sun is hidden in the glare of the sun and is rarely visible. However, it can be observed just before the sunrise or after the sunset near the horizon.

Venus appears in the eastern sky before sunrise and appears in the western sky after the sunset. 

Due to their visibilty before sunrise and after sunset, they are called evening and morning star. Also, Mercury and Venus lie inside the orbit of the earth.