Neptune: An Ice Giant With Diamond Rain

Astronomers used to divide the planets into two broad categories. The first, called the terrestrial planets, included Mercury, Earth, Venus and Mars. These four planets mostly consist of metals or silicate rocks and they've got solid outer surfaces.

Before the 1990s, Uranus and Neptune were lumped together with the other giant planets Jupiter and Saturn into the second group: the gas giants. "Giant" is an appropriate label.

Jupiter is easily the biggest planet in our solar system, but Neptune is no runt. It has a radius of 15,299.4 miles (24,622 kilometers), making it four times wider than Earth and the fourth largest planet in the solar system.

By the millennium's end, however, scientists realized half of the "gas giants" were fundamentally different from the other half. True, all four planets lack solid exteriors. But Jupiter and Saturn are predominantly made of hydrogen and helium gas. The same cannot be said of Uranus or Neptune. These two outer planets are actually made up of more heavier elements.

Neptune's outer atmosphere (as well as Uranus') is made of helium, hydrogen and methane, but beneath it Neptune has a thick mantle. This layer is loaded with slushy methane ice — along with the solid, crystallized forms of ammonia and water. Neptune's interior includes an inner core that might be rocky and Earth-sized.

Today, Uranus and Neptune are no longer considered gas giants. Instead, they've been relegated to a third category of planets astronomers call ice giants.

You wouldn't mistake one ice giant for the other. Uranus looks blue-green to our eyes, whereas Neptune has more of an azure complexion. Both planets contain atmospheric methane clouds, which simultaneously absorb red light waves and reflect blue ones.

This is what gives the two ice giants their bluish color schemes — but it doesn't explain why Neptune is visibly darker in hue. (Perhaps there's a mystery ingredient hanging out in the Neptune's atmosphere.)

Here's another key distinction. Uranus doesn't release much excess heat into space, yet Neptune — like Jupiter and Saturn — emits more energy than it receives from the sun. Even so, Neptune is considered the coldest planet in the solar system.

In some parts of Neptune's outer atmosphere, temperatures are liable to hit minus 218 degrees Celsius (minus 360 degrees Fahrenheit). Maybe that helps account for Neptune's ultra-fast wind speeds: The atmospheric coldness is thought to reduce friction, allowing wind speeds to zip around more freely.

Speaking of weather patterns, when the Voyager 2 spacecraft visited Neptune in 1989, it photographed an oval-shaped storm system 8,000 miles (13,000 kilometers) across. Nicknamed "the Great Dark Spot," it vanished by the time the Hubble Space Telescope took a new round of pictures in 1994.

Hubble discovered another massive vortex in Neptune's northern hemisphere in 2018 wider than the Atlantic Ocean. A year later scientists again recorded it drifting south toward Neptune's southern hemisphere where these storms typically vanish.

But in August 2020, Hubble confirmed the storm system had changed direction again and was moving north, surprising researchers. Altogether, half a dozen Neptunian storm systems of this kind have been documented over 30 years.

Lots of interesting things are happening above Neptune as well. The ice giant has 14 known moons, including one that wasn't discovered until 2013. This newcomer was named Hippocamp, in honor of an aquatic beast from Greek mythology.

Neptune borrows its own name from the Roman god of the sea. For that reason, astronomers like to name the planet's moons after lesser sea gods and nymphs in Greek mythology, including Poseidon.

One of Neptune's moons is named for Poseidon's son Triton. Triton also is Neptune's largest moon. At 1,680 miles (2,700 kilometers) in diameter, it's bigger than the dwarf planet Pluto.

Triton also is the only large moon in the entire solar system to orbit its planet in the opposite direction to its planet's rotation (a retrograde orbit). Temperatures on Triton are extremely cold, and hover around minus 391 degrees Fahrenheit (minus 235 degrees Celsius). These frozen temperatures make it unusual that Triton has active geysers, a rarity in our solar system.

Neptune's gravity is actually dragging Triton closer to the planet, which means in millions of years, Triton will be so close, Neptune's gravity will rip it apart, possibly leaving a ring.

We know Neptune has five planetary rings and four prominent ring arcs. Neptune's rings are the most recent to be discovered of all rings around major planets in the solar system.

Dark and faint, Neptune's rings — Galle, Leverrier, Lassell, Arago and Adams — are presumed to be young. According to Universe Today, they might be the remains of a moon that was somehow destroyed.

The four ring arcs — Liberté, Egalité, Fraternité and Courage — are in the outermost ring. Scientists think the gravitational effects of Galatea, a moon just inward from the ring, stabilizes these ring arcs.

Neptune's orbit is very unusual because of its relationship with dwarf planet Pluto. First, Neptune takes about 16 hours to rotate once (a Neptunian day). But it takes Neptune about 165 Earth years to make a complete orbit around the sun (a Neptunian year).

The planet Neptune also is tilted 28 degrees on its plane in its orbit around the sun. This means that Neptune has seasons like Earth; however, since a year is so long on Neptune, each of the four seasons lasts for over 40 Earth years.

Another interesting aspect about Neptune's orbit: Dwarf planet Pluto's elliptical orbit brings Pluto inside Neptune's orbit every 248 Earth years. Pluto remains inside Neptune's orbit for 20 years but won't ever crash into Neptune. Why? Because for every three times Neptune orbits the sun, Pluto orbits it twice.

The last time Pluto's elliptical orbit was inside Neptune's was from 1979 to 1999. It won't occur again until 2227.

Unlike other planets in our solar system, Neptune is the only planet never visible to the naked eye. Yet when it was first spotted via telescope in the year 1846, the discovery didn't come as a surprise.

Keen observers had noticed certain irregularities in Uranus' orbit. As the seventh planet moved around the sun, Uranus was deviating from its anticipated pathway.

So in the early 18th century, mathematicians reasoned that a distant planet must be gravitationally tugging on Uranus. And they were right: It was Neptune.

The existence of this azure world — with its savage winds and disappearing storms — is still the only planet predicted by math before its discovery. College kids, let that fact inspire you when finals week rolls around.