Specs
| What | Data |
|---|---|
| Name | Neptune |
| Class | Ice Giant (see note below) |
| Planet number | 8 |
| Diameter | 49,244 km |
| Mass | 1.024 × 10²⁶ kg (17× Earth) |
| Distance from the Sun | 4.5 billion km / 30.1 AU |
| Orbital period | 164.8 years |
| Rotation period | 16h 6m |
| Known moons | 16 |
| Largest moon | Triton |
The farthest planet
Neptune is the eighth and most distant planet in the solar system. It was the first planet found by mathematical prediction rather than observation: irregularities in the orbit of Uranus pointed astronomers to its position before it was ever seen through a telescope in 1846.
It is a world of extremes — the fastest winds in the solar system race across its deep-blue atmosphere at up to 2,100 km/h.
Ice giant — or magma-ocean giant?
For decades Neptune and Uranus have been classified as ice giants: a hydrogen/helium atmosphere over a vast mantle of “ices” (water, ammonia, methane) and a rocky core. A 2026 study from the University of California challenges that picture.
The researchers note the three-layer ice-giant model is not the only way to explain the planets’ properties. Objects in the Kuiper Belt — thought to preserve the material from where Neptune and Uranus formed — are mostly rock, not ice. Their best-fitting model instead suggests a well-mixed magma ocean with hydrogen dissolved into it at the bottom and a hydrogen-dominated envelope on top. At high pressure, hydrogen gas can dissolve into magma to form a single well-mixed fluid, which could explain the planets’ density without an ice-rich interior.
If correct, Neptune and Uranus might be better described as “magma-ocean giants.” The idea could also help explain the interiors of the many sub-Neptune exoplanets across the Milky Way.
Source: Scientists Think Neptune and Uranus May Not Be the Ice Giants We Imagined (Slashdot, via Gizmodo).