NEPTUNE vs. MARS Neptune, the eighth planet from the sun, is more than 7 times the size of Mars, and more than 150 times as heavy. Here are some comparisons: Neptune's Radius: 24,766 km, 7.3 times as wide as Mars Neptune's Mass: 1.02 x 1026 kg, 158 times as massive as Mars. Neptune's Density: 1.64, 42% as dense as Mars.