By Jon Stefansson Updated Mar 24, 2022
Neptune is the eighth planet from the Sun and one of only two that cannot be seen with the naked eye. It is almost four times larger than Earth and roughly 17 times more massive. The planet completes an orbit in 165 Earth years, and a single Neptune day lasts about 16 hours.
The striking blue “surface” visible in spacecraft imagery is actually the top of a permanent cloud layer. Below this, Neptune’s atmosphere consists mainly of hydrogen, helium, and methane, which sit atop an icy mantle.
Neptune’s mantle is a layer of water, ammonia, methane, and silica ices—possibly the closest thing the planet has to a true surface. Scientists debate whether water is abundant enough to form a subsurface ocean or whether the mantle is a deep, compressed gas layer extending to the core.
Temperatures in the mantle are estimated at about –223 °C, but they rise as one moves toward the core, which still retains heat from the planet’s formation. Consequently, Neptune emits almost three times the thermal energy it receives from the Sun.
Wind speeds at the mantle level can reach up to 700 mph—far stronger than any storm on Earth—driven by the temperature gradient between the upper atmosphere and the core. These winds produce the violent swirling patterns observed in Neptune’s cloud bands.
In 1846, Neptune was first observed thanks to predictions made by John C. Adams and Urbain J. J. Leverrier, who calculated the planet’s position from perturbations in Uranus’s orbit. Their work marked the first planet to be discovered by mathematical calculation rather than direct observation.
