Neptune

After the reclassification of Pluto as a dwarf planet in 2006, Neptune became the farthest planet from the sun, orbiting at an average distance of 30.1 astronomical units. It is one of the four gas giants of the solar system, consisting mainly of gas rather than solid matter, and is the densest of the four. Neptune is 17.147 Earths in mass and 57.74 Earths in volume, with a surface gravity only slightly higher than Earth’s (1.14 g).

The core of Neptune is itself about 1.2 times the mass of Earth, and consists of iron, nickel, and silicates at extreme temperatures (about 5,100 degrees Celsius) and very high pressure. This core is surrounded by a mantle that, though referred to as icy, is a very hot, dense, ocean of water and ammonia. Experimental data suggests the possibility of a base layer of an ocean of liquid diamond dotted with solid masses of floating diamond crystals formed by decomposing methane. The convective fluid motion of the electrically conductive liquids in Neptune’s core is believed to be responsible for the planet’s tilted magnetosphere, which is tilted to 47 degrees relative to the rotational axis.

Neptune takes 164.79 years to orbit the sun, which means its seasons, such as they are, last more than 40 years. In 2011, the planet completed its first full orbit since its discovery in 1846. Its axial tilt of 28.32 degrees is comparable to Earth’s (23.5 degrees). Neptune’s orbit makes it a significant gravitational force on the Kuiper belt, the region of small icy planet-like objects that lie beyond it.

Atmosphere and Climate

Neptune has a distinct blue appearance, due in large part to the presence of atmospheric methane, which absorbs red light. Neptune’s atmosphere makes up most of its volume and 5 to 10 percent of its mass. Like the other gas giants, its atmosphere is overwhelmingly hydrogen and helium with trace elements. Neptune and Uranus are sometimes also called ice giants because they contain proportionally more water, ammonia, and methane than Jupiter and Saturn. Unlike Uranus, Neptune’s atmosphere is subject to active, energetic weather patterns. The Great Dark Spot, a dark blue anticyclonic storm similar to Jupiter’s Great Red Spot and first observed by Voyager 2 in 1989, is the result of such weather, and includes the strongest winds in the solar system—as high as 2,100 kilometers per hour. Unlike the Great Red Spot, it is not observable from Earth, and so little is known about how long it has persisted.

The atmosphere has two distinct regions. In the upper stratosphere, the temperature gets hotter at higher altitudes. In the lower troposphere, the temperature decreases at higher altitudes. The troposphere is banded by clouds of ammonia, water, ammonium sulfide, and hydrogen sulfide. At high altitudes, clouds may be found to cast shadows on the cloud bands below them. Hydrocarbons keep Neptune warmer than Uranus. The south pole is significantly warmer than the rest of Neptune, as a result of an axial tilt that keeps the south pole exposed to the sun for forty Earth years at a time, which results in methane thawing from ice into gas and leaking out into space. In 2023, Neptune's high-altitude clouds disappeared, promoting a study of observation using the Hubble Space Telescope. Early results of the study concluded that Neptune's high-altitude cloud activity was bound to Solar cycles.

Planetary Rings and Moons

Neptune’s ring system was first suspected in the 1960s and was confirmed by Voyager 2 in 1989. The rings are smaller and less elaborate than those of Saturn, and are thought to consist mainly of ice colored by particles of carbon or silica. The three main rings—Adams at 63,000 kilometers, Le Verrier at 53,000 kilometers, and Galle at 42,000 kilometers—are named for Neptune’s discoverers. The rings are not perfectly smooth, which is thought to be the result of gravitational disturbance from Neptune’s moons.

Neptune has fourteen known moons, the largest of which is Triton. Triton consists of more than 99 percent of the lunar mass orbiting Neptune. It was discovered days after Neptune itself was first observed, and of the known moons in the solar system, it is the only moon of significant mass that is believed to have been captured by gravity rather than accumulating in place. It is most likely a former Kuiper belt object, and like the Earth’s moon, it has a synchronous rotation, meaning that the same side of Triton faces Neptune at all times.

The remaining Neptunian moons are small and irregularly shaped. Only Triton and Nereid were known until Voyager 2’s exploration of Neptune and discovery of six new moons. The remainder were discovered in the twenty-first century.

History of Observation and Exploration

Neptune is never visible to the naked eye, which accounts for its late discovery, although modern binoculars are sufficiently strong to make it out. Neptune had been observed without being understood on a number of occasions; Galileo, for instance, mistook it for a fixed star when recording its appearance in conjunction with Jupiter in 1612. Neptune has the distinction of having been discovered through mathematics rather than direct observation. Shortly after the discovery of Uranus in 1791, French astronomer Alexis Bouvard deduced from its orbit that some previously undiscovered body must be acting on it through gravitational perturbation. John Couch Adams and Urban Le Verrier built on Bouvard’s work, calculating Neptune's estimated position in order to guide the search for it, and in 1846, Johann Galle successfully observed it from the Berlin Observatory. The discovery of a planet has always been a significant event, but in this case, the confirmation of the mathematical predictions contributed additional drama.

Its distance and dimness made Neptune a mystery to visual observation until the approach of the Voyager 2 probe in 1989. The probe conducted a flyby of Triton and had a near-encounter with Nereid, and was able to confirm the existence and tilt of the planet’s magnetic field. Images from Voyager 2 were broadcast as part of an all-night Public Broadcasting Service event, Neptune All Night. Advances in Earth-based telescopes have made observation of Neptune easier, and the Hubble Space Telescope has contributed significantly to the study of the planet. It was only after such advances that Neptune’s rings became visible from Earth, for instance.

Bibliography

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Chambers, John, and Jacqueline Mitton. From Dust to Life: The Origin and Evolution of Our Solar System. Princeton UP, 2013.

Cole, George H. A., and Michael M. Woolfson. Planetary Science: The Science of Planets around Stars. CRC, 2013.

Dick, Steven J. Discovery and Classification in Astronomy. Cambridge UP, 2013.

Encrenaz, Therese. Planets: Ours and Others. World Scientific, 2013.

Esposito, Larry. Planetary Rings: A Post-Equinox View. Cambridge UP, 2014.

Milone, Eugene F., and William J. F. Wilson. Solar System Astrophysics: Planetary Atmospheres and the Outer Solar System. Springer, 2014.

"Neptune’s Disappearing Clouds Linked to the Solar Cycle." NASA, 17 Aug. 2023, science.nasa.gov/missions/hubble-space-telescope/neptunes-disappearing-clouds-linked-to-the-solar-cycle. Accessed 24 Oct. 2024.

"Neptune Facts." NASA, 2024, science.nasa.gov/neptune/neptune-facts/. Accessed 24 Oct. 2024.