Neptune is the eight planet from our Sun, one of the four gas giants, and one of the four outer planets in our Solar System. Since the “demotion” of Pluto by the IAU to the status of a dwarf planet – and/or Plutoid and Kuiper Belt Object (KBO) – Neptune is now considered to be the farthest planet in our Solar System.
As one of the planets that cannot be seen with the naked eye, Neptune was not discovered until relatively recently. And given its distance, it has only been observed up close on one occasion – in 1989 by the Voyager 2 spaceprobe. Nevertheless, what we’ve come to know about this gas (and ice) giant in that time has taught us much about the outer Solar System and the history of its formation.
Discovery and Naming:
Neptune’s discovery did not take place until the 19th century, though there are indications that it was observed before long that. For instance, Galileo’s drawings from December 28th, 1612, and January 27th, 1613, contained plotted points which are now known to match up with the positions of Neptune on those dates. However, in both cases, Galileo appeared to have mistaken it for a star.
In 1843, English astronomer John Couch Adams began work on the orbit of Uranus using the data he had and produced several different estimates in the following years of the planet’s orbit. In 1845–46, Urbain Le Verrier, independently of Adams, developed his own calculations, which he shared with Johann Gottfried Galle of the Berlin Observatory. Galle confirmed the presence of a planet at the coordinates specified by Le Verrier on September 23rd, 1846.
The announcement of the discovery was met with controversy, as both Le Verrier and Adams claimed responsibility. Eventually, an international consensus emerged that both Le Verrier and Adams jointly deserved credit. However, a re-evaluation by historians in 1998 of the relevant historical documents led to the conclusion that Le Verrier was more directly responsible for the discovery and deserves the greater share of the credit.
Claiming the right of discovery, Le Verrier suggested the planet be named after himself, but this met with stiff resistance outside of France. He also suggested the name Neptune, which was gradually accepted by the international community. This was largely because it was consistent with the nomenclature of the other planets, all of which were named after deities from Greco-Roman mythology.
Neptune’s Size, Mass and Orbit:
With a mean radius of 24,622 ± 19 km, Neptune is the fourth largest planet in the Solar System and four times as large as Earth. But with a mass of 1.0243×1026 kg – which is roughly 17 times that of Earth – it is the third most massive, outranking Uranus. The planet has a very minor eccentricity of 0.0086, and orbits the Sun at a distance of 29.81 AU (4.459 x 109 km) at perihelion and 30.33 AU (4.537 x 109 km) at aphelion.
Neptune takes 16 h 6 min 36 s (0.6713 days) to complete a single sidereal rotation, and 164.8 Earth years to complete a single orbit around the Sun. This means that a single day lasts 67% as long on Neptune, whereas a year is the equivalent of approximately 60,190 Earth days (or 89,666 Neptunian days).
Because Neptune’s axial tilt (28.32°) is similar to that of Earth (~23°) and Mars (~25°), the planet experiences similar seasonal changes. Combined with its long orbital period, this means that the seasons last for forty Earth years. Also owing to its axial tilt being comparable to Earth’s is the fact that the variation in the length of its day over the course of the year is not any more extreme than it on Earth.
Neptune’s orbit also has a profound impact on the region directly beyond it, known as the Kuiper Belt (otherwise known as the “Trans-Neptunian Region”). Much in the same way that Jupiter’s gravity dominates the Asteroid Belt, shaping its structure, so Neptune’s gravity dominates the Kuiper Belt. Over the age of the Solar System, certain regions of the Kuiper belt became destabilised by Neptune’s gravity, creating gaps in the Kuiper belt’s structure.
There also exists orbits within these empty regions where objects can survive for the age of the Solar System. These resonances occur when Neptune’s orbital period is a precise fraction of that of the object – meaning they complete a fraction of an orbit for every orbit made by Neptune. The most heavily populated resonance in the Kuiper belt, with over 200 known objects, is the 2:3 resonance.
Objects in this resonance complete 2 orbits for every 3 of Neptune, and are known as plutinos because the largest of the known Kuiper belt objects, Pluto, is among them. Although Pluto crosses Neptune’s orbit regularly, the 2:3 resonance ensures they can never collide.
Neptune has a number of known trojan objects occupying both the Sun–Neptune L4 and L5Lagrangian Points – regions of gravitational stability leading and trailing Neptune in its orbit. Some Neptune trojans are remarkably stable in their orbits, and are likely to have formed alongside Neptune rather than being captured.
Neptune’s Atmosphere:
At high altitudes, Neptune’s atmosphere is 80% hydrogen and 19% helium, with a trace amount of methane. As with Uranus, this absorption of red light by the atmospheric methane is part of what gives Neptune its blue hue, although Neptune’s is darker and more vivid. Because Neptune’s atmospheric methane content is similar to that of Uranus, some unknown atmospheric constituent is thought to contribute to Neptune’s more intense coloring.
Neptune’s atmosphere is subdivided into two main regions: the lower troposphere (where temperature decreases with altitude), and the stratosphere (where temperature increases with altitude). The boundary between the two, the tropopause, lies at a pressure of 0.1 bars (10 kPa). The stratosphere then gives way to the thermosphere at a pressure lower than 10-5 to 10-4 microbars (1 to 10 Pa), which gradually transitions to the exosphere.
Neptune’s spectra suggest that its lower stratosphere is hazy due to condensation of products caused by the interaction of ultraviolet radiation and methane (i.e. photolysis), which produces compounds such as ethane and ethyne. The stratosphere is also home to trace amounts of carbon monoxide and hydrogen cyanide, which are responsible for Neptune’s stratosphere being warmer than that of Uranus.
For reasons that remain obscure, the planet’s thermosphere experiences unusually high temperatures of about 750 K (476.85 °C/890 °F). The planet is too far from the Sun for this heat to be generated by ultraviolet radiation, which means another heating mechanism is involved – which could be the atmosphere’s interaction with ion’s in the planet’s magnetic field, or gravity waves from the planet’s interior that dissipate in the atmosphere.
Because Neptune is not a solid body, its atmosphere undergoes differential rotation. The wide equatorial zone rotates with a period of about 18 hours, which is slower than the 16.1-hour rotation of the planet’s magnetic field. By contrast, the reverse is true for the polar regions where the rotation period is 12 hours.
This differential rotation is the most pronounced of any planet in the Solar System, and results in strong latitudinal wind shear and violent storms. The three most impressive were all spotted in 1989 by the Voyager 2 space probe, and then named based on their appearances.
Initially, only Triton – Neptune’s largest moon – could be observed. But by the mid-20th century and after, thanks to improvements in ground-based telescopes and the development of robotic space probes, many more moons would be discovered. Neptune now has 14 recognized satellites, and in honor of of their parent planet, all are named for minor water deities in Greek mythology.
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