Neptune is unlikely to ever contend for the unofficial title of "most popular planet." It is the most distant from the sun of the eight planets in the solar system, and the only one that is never visible with the unaided eye. Even Pluto, despite being demoted from a planet to a dwarf planet by the International Astronomy Union in 2006, seems to garner more attention even now than Neptune, named after the Roman god of the sea (the Greek version of whom, by the way, is called Poseidon).
Neptune is the third-heaviest planet and the fourth largest in terms of volume, being slightly smaller but more dense than its nearest solar-system neighbor, Uranus. These two planets along with Jupiter and Saturn are called "gas giants," but as you'll soon learn, in some ways this name is somewhat misleading.
The Solar System: An Overview
The literal and descriptive center of the solar system is the sun (the Latin for sun is "sol"), which is a fairly non-remarkable star other than its existence being absolutely necessary for the presence of any and all life on Earth. The solar system also includes the eight planets, five dwarf planets, the moons of these planets and a smattering of asteroids (about 781,000, actually), meteoroids and comets.
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In order from innermost to outermost, the eight planets are Mercury, Venus, Mars, Jupiter, Saturn, Uranus and Neptune. Mercury's orbit is "only" about 31 million miles from the sun, while Neptune, orbiting at a distance of 2.8 billion miles, is about 900 times farther away than Neptune. The asteroid belt lies between Mars and Jupiter, while the ice-and-rock comets orbit beyond the reaches of Pluto in a loose aggregation called the Oort Cloud. Every planet besides Mercury has an atmosphere, as do many of the moons. Neptune's atmosphere consists chiefly of hydrogen and helium, the two lightest elements.
Mercury, Venus, Mars, Jupiter and Saturn are not only visible from Earth but with few exceptions appear brighter than the brightest of stars. They are also distinctive, with Mercury being reddish, Mars a deeper red, Venus almost white, and Saturn and Jupiter yellowish. Uranus is faintly visible to most people but requires a trained eye (and a good chart of the sky) to find; Neptune, alas, can only be seen with magnifying instruments.
The Inner Planets vs. the Outer Planets
If nothing else, the whims of nature have imposed a great deal of symmetry on the arrangement of the solar system, with human astronomers aiding in this process by ejecting Pluto from the pantheon of the planets after its 76-year tenure. This makes it easier to remember basic details about the solar system for those without a lot of background in astronomy.
As noted, the asteroid belt divides the inner four planets from the outer four. But the distinctions between the inner quartet and the outer quartet would be striking even without the asteroid belt serving as a reminder that, from the standpoint of the planets, there are really two mini-solar systems.
Mercury, Venus, Earth and Mars are all within 131 million miles of the sun, meaning that even Mars is less than 1/20th of the distance to Neptune. All of these planets have a diameter of less than 8,000 miles (12,800 km). They consist almost entirely of hardened rock and are called "terrestrial planets" for this reason.
Jupiter, Saturn, Uranus and Neptune, in contrast, are all at least 498 million (just under half a billion) miles from the sun. All four have a diameter of at least 30,000 miles, which is about four times that of Earth, the largest of the terrestrial planets. And perhaps most notably, they consist of a mixture or solid, liquid and gaseous material. The gases, being the lightest, are on the outside, and this foursome as a group is knows as the "gas giants."
The Gas Giants
Jupiter, Saturn, Uranus and Neptune – an order that happens to rank them from largest to smallest in addition to being the order in which their orbits appear – have been called "gas giants" since the science-fiction author James Blish came up with the nickname. They are also classified in some schemes as "Jovian planets," which means "Jupiter-like." (In more recent years, scientists have determined that Uranus and Neptune are really not very Jupiter-like beyond the superficial level, but the name has stuck, and despite the ways in which they vary, each is far more similar to the other gas giants than any of them is to a terrestrial planet.)
Although hydrogen and helium, the most abundant elements in the outer parts of the gas giants, normally exist in the gaseous state, the considerable gravity of these massive planets creates sufficient pressure to squeeze most of the hydrogen and helium into their liquid states. Most of the gas giants, therefore, actually consist of liquid. All of them also have solid cores, but only Uranus and Neptune, being colder than Jupiter and Saturn, have a layer of ice surrounding the core to form a frozen mantle. This has led some scientists to call the pair "ice giants."
Neptune, as noted, is about 2.8 billion miles from the sun; despite electromagnetic radiation traveling at 186,000 miles per second, it takes over four hours for sunlight to reach Neptune. Its period of revolution around the sun is 165 Earth-years, meaning that as of the second decade of the 20th century, only one complete Neptunian year had passed since the planet's discovery in 1846. Despite its girth, Neptune completes one full rotation about its axis in 16 hours, making a Neptunian day only two-thirds as long as Earth's despite the far smaller size of latter. With Neptune having a circumference of four times Earth's, this means that Neptune's rotational speed at its equator is a whopping six times that of Earth.
This high rotation speed has climatological consequence. Neptune is considered the windiest planet of the eight, with winds reaching speeds of some 1,200 miles per hour near Neptune's surface, about one and a half times the speed of sound and close to three times as fast as most commercial airliners fly.
Neptune is also not a place to bother looking for life, with the planet featuring an average surface temperature of -353 degrees Fahrenheit (-214 C). Neptune has six faint rings and, as of 2018, 14 known moons.
Neptune has been the subject of a near-encounter with only one Earth-launched spacecraft. In 1989, the U.S. project Voyager 2 made a flyby and captured the first close-up photos of Neptune in history. Voyager 2 also transmitted back information about the planet's rings, moons and rotation. Since then, the Hubble Telescope has taken revealing images of the planet from a much greater distance.
Neptune is tilted about 28 degrees on its axis from the vertical, similar to the Earth's 23-degree tilt. This means that even within the context of an already brutal climate, Neptune experiences something akin to seasons.
Of Neptune's moons, only one, Triton, is of any consequence. This large satellite was captured by Neptune's gravity very early in the solar system's life and is believed to be one of the solar system's very coldest bodies of any sort.