* Both are extreme in their own way: Mercury is the smallest and closest planet to the sun, while Neptune is the farthest and has the strongest winds in the solar system.
* Both have a unique relationship with the Sun: Mercury is heavily influenced by the Sun's gravity, resulting in a very elliptical orbit. Neptune is so far out that the Sun appears as a bright star, and its year is incredibly long (165 Earth years).
* Both have a tenuous atmosphere: Mercury's atmosphere is extremely thin, called an exosphere, and is constantly being replenished by solar wind. Neptune's atmosphere is made up mostly of hydrogen and helium, but it's also incredibly thin compared to planets like Earth.
While these similarities exist, they are overshadowed by the vast differences between the two planets. Mercury is a rocky, cratered world with no moons, while Neptune is a giant gas planet with 14 known moons and a ring system.
