Neptune

Young Astronomers Blog, Volume 29, Number 10.

If we were to travel out in the Solar System, we would eventually reach the last of the official planets. I know, many would argue there is still one more planet out there, but that’s another story.

Neptune is the only planet to be discovered through mathematical analysis. The orbit of Uranus was found to be shifted by an unknown object farther out in the Solar System. The disturbance in Uranus’ orbit was strong enough that this object was possibly a new planet. Both Urban Le Verrier and John Couch Adams performed extensive calculations to determine the position of the alleged planet. In the end, Le Verrier was the quicker of the two. In September 1846, he sent his predicted position to Johann Gottfried Galle at the Berlin Observatory. Galle found and confirmed the new planet within a few days.

Neptune, along with the other planets, formed around 4 ½ billion years ago. Neptune, according to the “Nice” model of planetary formation, was initially found inside the orbit of Uranus and closer to the Sun than it is today. Over time, Jupiter and Saturn fell into a 2:1 resonance, where Jupiter completed two orbits to Saturn’s one orbit. The gravitational impact of this pushed Neptune pass Uranus and farther out in the Solar System to where it is found today.

Today, Neptune is some 2.8 billion miles from the Sun. It along with Uranus are similar in size, composition, and appearance. Neptune is around four times the diameter of the Earth. It has an atmosphere composed mostly of Hydrogen and Helium, making it one of the gas giants. It also has some methane, which gives it its blue color. Although, astronomers are not quite sure why Neptune is a deeper blue than Uranus. Like Uranus, Neptune has icy material (water, methane, and ammonia) in its interior. So, both are classified as ice giants.

Neptune rotates quickly like the other gas giants with a day equal to only 16 hours. Neptune has a tilt in its axis of around 28^o giving it seasons like the Earth, Mars, and Saturn. Because of its distance from the Sun, it has a long orbital period of 165 years. In fact, it has only completed a little over one complete orbit since its discovery in 1846.

Although, Neptune’s orbit is more circular than the Earth’s, Pluto’s elongated orbit brings it inside the orbit of Neptune for a period of 20 years every 248 years. Sorry, if you’re waiting for this to happen, it last occurred from 1979 and 1999, and won’t reoccur until 2227.

Neptune is often classified as the windiest of the planets. Winds are estimated to range from 700 to over 1,000 mph depending on the source. These winds are thought to result from heat radiating from Neptune’s interior. It is also the coldest planet with temperatures at -330^o F. Although, the dwarf planet Pluto is colder at -375^o F.

Neptune, like all the giant planets, has a series of rings (Galle, Leverrier, Lassell, Arago and Adams). The five rings are rather thin and the last ring, Adams, is broken into four distinct arcs named Liberté (Liberty), Egalité (Equality), Fraternité (Fraternity) and Courage.

Only one spacecraft, Voyage 2, has visited Neptune, and that was back in 1989. During Voyager’s flyby it took images of the planet, revealing what has become known as the “Great Dark Spot” and a white area called “Scooter”. Later observations by the Hubble Telescope in 1994 showed that these spots, along with a smaller dark one, had disappeared. Over time, four other dark spots have appeared, including a new “great dark spot”, which was observed forming from 2015 to 2018.

These spots are thought to be storms around the size of the Earth, although astronomers have also theorized that they are breaks in Neptune’s atmosphere. Associated with the spots are higher altitude white methane clouds that precede the formation of the spot. Unlike Jupiter’s Great Red Spot that remains in one place and has lasted hundreds of years, Neptune’s dark spots drift in its atmosphere and last only a few years.

Sixteen moons are found orbiting Neptune. Two new moons were discovered in 2024. Triton is the largest and is one of the seven largest moons in the Solar System. Unlike the other six, Triton orbits Neptune in the opposite direction (retrograde) from Neptune’s rotation. Its surface is thought to consist of frozen nitrogen with geysers that stream icy material onto its surface. Astronomers think Triton might be a dwarf planet that was captured by Neptune’s gravity sometime in the ancient past.