Neptune

When you turn around, you can glimpse the Sun at the opposite end of the campus, 450 metres away. Neptune, which is 5 mm across here, has a bright blue-green colour and is only slightly smaller than Uranus. It was once thought that Neptune was a much darker blue, but recent colour analyses of the old images show that the two planets are in fact almost identical in colour. The planet consists of a rocky core about the size of Earth, surrounded by a thick, fluid layer of water, ammonia, and methane ice.

Neptune’s clouds contain a mixture of hydrogen, helium, and some methane gas. Since astronomers believe that the two planets have a fairly similar internal structure, Neptune behaves in a somewhat surprising and unexplained way: it is much warmer inside than Uranus. This means that the two planets have approximately the same surface temperature, even though Neptune is more than one billion kilometres farther from the Sun and receives much less sunlight. The process responsible for this additional heat production in Neptune’s interior remains unclear.

The result is, at times, a more active atmosphere than Uranus has, with more visible features. These were photographed by the Voyager 2 spacecraft when it flew past Neptune in 1989. The images showed some banded structure, several faint, white, elongated clouds, and a large dark spot. The spot was about the size of Earth. It has since disappeared and been replaced by one or more similar dark spots that appear at regular intervals.

Whatever its colour or dark spots, Neptune can certainly boast about its wind speeds. With winds exceeding 2,100 km/h, the planet holds the Solar System’s absolute wind-speed record. It is also worth noting that Neptune takes a full 165 Earth years to complete just one orbit around the Sun, meaning that it has completed only a single Neptunian year since its discovery in 1846. The planet is surrounded by 16 known moons and a few very faint, modest rings. The largest moon, Triton, is a fascinating world: it orbits Neptune “the wrong way,” is one of the few bodies in the Solar System that is volcanically active—with geysers that erupt nitrogen—and may contain a liquid ocean beneath its frozen surface.

Now that you have reached the outermost known planet in the Solar System, it may be interesting to place our second-closest star, Proxima Centauri, on the same scale. The Sun is our closest star. Click here to learn more and experience an even greater perspective.