Mercury
Neptune
Diameterkm
4,879
49,528
Mass (Earth = 1)x
0.06
17.15
Surface Gravitym/s²
3.70
11.15
Mean Temperature°C
+167
-214
Distance from SunAU
0.39
30.07
Orbital Perioddays
87.97
59,800
Day Lengthhours
4222.60
16.11
Known Moons
0
16
Axial Tilt°
0.03
28.32
Orbital Velocitykm/s
47.36
5.43
Quick Facts
Mercury
- -Mercury's day-night cycle lasts 176 Earth days -- longer than its year of 88 Earth days.
- -A person weighing 68 kg on Earth would weigh only 25.7 kg on Mercury.
- -Mercury has ice in permanently shadowed craters near its poles despite surface temperatures reaching 430°C.
- -Mercury is shrinking -- its circumference has decreased by about 14 km as its core slowly cools and contracts.
Neptune
- -Neptune has the strongest winds in the solar system, reaching speeds of up to 2,100 km/h.
- -Neptune was the first planet discovered through mathematical prediction rather than observation.
- -One year on Neptune lasts about 165 Earth years -- it has completed just one orbit since its discovery in 1846.
- -Neptune's moon Triton orbits backward (retrograde), suggesting it was captured from the Kuiper Belt.