HomeSkyIntroducing Neptune, the Stormy Ice World Charles November 7, 2016 Sky Poor Neptune. The beautiful sapphire-blue world often gets forgotten about, having only been visited once by a spacecraft as it wonders lonely through the furthest reaches of the solar system. Neptune lies, on average, 2.8 billion miles (4.5 billion km) away from the sun, or about thirty times further away than Earth. Despite its serene deep blue hue, the gas-giant world is also notable for being home to the fiercest storms in the solar system. It’s also the smallest of the gas giant planets, with a radius a little under four times that of Earth. Like the other gas giants, however, Neptune also has an extensive collection of at least 13 moons. Planets like Neptune are often known as ice giants nowadays, owing to their distinctly different composition compared to gas giants like Jupiter and Saturn. Most notably, they’re composed of around 20% hydrogen, whereas the gas giants are close to 90% hydrogen. Other Neptune-sized planets, some of which are almost certainly ice giants as well, have been found around other stars. Diving into Neptune’s Icy Blue Clouds Kelvinsong A diagram showing Neptune’s composition, as well as its rings and the orbit of some of its moons. Neptune is the only planet that cannot be seen by the naked eye, although it was discovered in the early seventeenth century using telescopes. It’s likely that Galileo was the first to observe the planet, although he assumed it to be a star. To see anything resembling an actual disk, you’ll need excellent viewing conditions combined with a telescope with an aperture of at least 8 inches. Owing to its colour, Neptune was named after the Roman god of the Sea, the same god as the Greek Poseidon, hence the trident symbol being attributed to it. Little is known about the blue planet, but it’s not as similar to its sister planet Uranus as many people assume. The enormously successful Voyager 2 probe reached Neptune in 1989 after a 12-year journey and, to date, it’s been the only close-up exploration of the planet. Nonetheless, the probe did make some fascinating discoveries while also taking the only close-up images of its surface and that of Triton, its largest moon. Being a gas giant planet, Neptune has no solid surface to stand upon, although there is likely a rocky core, about the size of the Earth’s moon, at the heart of the planet. However, under the intense pressures of thousands of miles of atmosphere, we’ll only ever be able to guess what lies on any such a solid surface. Neptune’s upper atmosphere consists mostly of hydrogen and about 19 percent helium. The trace amounts of methane in the atmosphere are what make the planet a stunning deep blue. This is because the methane absorbs the redder light from the sun, reflecting blue light out into space instead. The upper reaches of the atmosphere have a temperature of -218°C, making Neptune the coldest planet in the solar system. Descending into Neptune’s atmosphere, one would notice that the gravity is close to that of Earth, being only about fourteen percent stronger. This is because Neptune has a very low density, which consequently gives it a very low mass despite its size. In fact, you could squeeze Earth into Neptune almost 60 times over, although the planet is only 17 times heavier. One of the few other things that Neptune has in common with Earth is that it has similar seasonal variations due to its axial tilt. Nonetheless, being so far from the sun, the seasons last decades, fitting into a year 165 times longer than our own. The Neptunian day, however, only lasts 16 hours, and this faster rotation is also partly responsible for its high wind speeds. The Neptunian atmosphere is home to one of the most ferocious climates in the solar system. The supersonic winds can reach a blustery 1,300 mph (2092 kph), and vast, continent-sized anticyclonic storms ravage the upper atmosphere for decades at a time. When Voyager 2 visited Neptune, it took close-up photos of the Great Dark Spot, a huge anticyclonic storm some 8,000 miles in diameter. Although that particular storm has since disappeared, others always seem to sprout up around the planet. Exploring the Ice Volcanoes of Triton ESO. Calçada A relatively calm view of Neptune from the surface of Triton. Note how dim the sun is at a distance 30 times greater than that of Earth. Like all the gas giants, Neptune has many moons. In total, thirteen are known, although all except one of these are extremely small and are most likely captured asteroids. Because these moons are so small, they do not even exert enough gravitational force to pull themselves into a spherical shape, making them little more than floating rocks. The only noteworthy natural satellite is Triton, which was discovered barely a couple of weeks after Neptune was first sighted. Triton is the seventh-largest moon in the solar system, with a diameter of almost 1,700 miles, a little less than our moon. It is notable mostly because it is the only moon of any significant size which has a retrograde orbit. This means that it orbits in the opposite direction to its host planet’s rotation. According to current scientific models, this implies that the moon was actually a dwarf planet, captured from the nearby Kuiper Belt, which itself consists of several known dwarf planets and numerous asteroids. Triton also has a very low density, and at least 15% of its composition is water ice. The surface is one of the coldest of any moon or planet in the solar system, averaging at -237°C, even colder than Neptune’s upper clouds. What makes Triton most interesting is that it is also one of the few geologically active moons in the solar system – it’s not just another dead rock. There aren’t many impact craters due to the young age of the moon, but the high amount of volcanism has a profound effect on the surface. Triton’s volcanoes are nothing like those of Earth, however. These are cryovolcanoes which, instead of throwing liquid rock up from beneath the surface, produce spectacular geysers of water and ammonia. Some of these geysers can reach heights of five miles. As is the case with other geologically active moons in the solar system, Triton’s volcanoes could be a product of tidal heating caused by Neptune’s gravitational pull. Triton accounts for 99.7 percent of all the orbital mass around Neptune. Some of the smaller moons orbit in Neptune’s faint rings. In fact, Neptune boasts five rings, although they are very faint, and their existence was not even confirmed until the Voyager 2 probe photographed them. They are made of dust, rocks and icy debris which may be the remnants of a destroyed moon. Exploring Neptune and Beyond Created with Celestia An artist’s impression of the so-far undiscovered Planet Nine, orbiting in the furthest reaches of the Kuiper Belt beyond Neptune. Neptune remains by far the least explored planet in our solar system. Voyager 2 got remarkably close to the north pole of the planet, flying by the planet’s north pole at 3,080 miles (4,950 km). Voyager 2 took the only close-up photos of Neptune and Triton in existence while also telling us more about its atmosphere and other characteristics. There are currently no missions to Neptune planned in the near future. Due to the enormous distance from Earth, it took Voyager 2 twelve years to get there, and while a dedicated mission using today’s technology could get there much faster, the costs have so far proven prohibitive. With little to offer at this point, Neptune is not likely to receive any more visits for decades to come. As evidence mounts for the existence of Planet Nine, it seems that Neptune might not be the furthest planet in the solar system. Estimated to be around 10 times more massive than Earth, this so-far unproven planet, lying up to 40 times further away from the sun than even Neptune, is thought to be responsible for the gravitational anomalies effecting several dwarf planets in the Kuiper Belt. Should such a planet exist, it would be far lonelier and even more inhospitable than the stormy ice world. 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