Almost the same size and colour as Uranus, Neptune orbits about 1.6 billion km (1.0 billion miles) further from the Sun than Uranus. German astronomer Johann Galle first spotted the planet in 1846 after mathematicians had calculated where it should be. Neptune has more extreme weather than Uranus, which is thought to be caused by heat from deep within its core. Winds blow up to 2,000 kph (1,250 mph), faster than on any other planet.
Table 17. ESSENTIAL DATA
| Diameter at equator | 49,532 km (30,785 miles) |
| Average distance from Sun | 4,498 million km (2,795 million miles) |
| Orbital period | 164.8 years |
| Rotation period | 16.11 hours |
| Mass (Earth=1) | 17.2 |
| Gravity (Earth=1) | 1.13 |
| Cloud-top temperature | -200°C (-328°F) |
| Number of moons | 13 |
Neptune’s deep blue colour is caused by methane in the atmosphere. Methane also condenses high in the atmosphere to form wisps of cloud, rather like cirrus clouds in Earth’s atmosphere.
Like Uranus, Neptune has a faint set of rings made of tiny dust particles circling its equator. There are four main rings – two broad but faint, and two bright but narrow. The two bright rings, Adams and Leverrier, are named after John Couch Adams and Urbain Leverrier, the mathematicians who calculated where the planet could be found.
Neptune’s largest moon, Triton, has a diameter of 2,710 km (1,685 miles) and has the coldest surface of any body in the Solar System, -235°C (-391°F). Made up of rock and ice, Triton is covered with frozen nitrogen and methane. A pinkish snow-like substance lies over the south polar region and dark streaks show where strange geysers spew out dust. Astronomers think that Triton and the planet Pluto are very similar.
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