Neptune: Discovery, Characteristics, and Planetary Overview

Introduction

Neptune is the eighth and furthest planet from the sun. This blue gas giant has been named following the tradition of other planets, with a name taken from mythology: Neptune is the Roman god of the sea. The planet has an interesting history and characteristics that clearly differentiate it from other planetary bodies. The aim of this paper is to examine the planet, looking at its discovery as well as its characteristics and how it compares to Earth.

Early Observations: Telescope and Galileo

The first discovery of Neptune occurred only after the development of the telescope. Although five planets of the solar system may be observed easily with the naked eye, and Uranus has the potential to be seen unaided when observed from a perfectly dark place, Neptune does not have sufficient brightness to be observed without a telescope. With a brightness magnitude between +7.7 and +8, the planet will never be visible to the naked eye (Williams, 2014; Grosser, 1962). Not only does the planet have low magnitude brightness; the planet's own moons are able to outshine it (Williams, 2014). Therefore, initial observation of Neptune only occurred after the telescope came into use.

The first observations of Neptune appear to have taken place in 1613, when Galileo Galilei recorded the observation of an object in a drawing on December 28 (Grosser, 1962). Galileo made a similar observation on January 27, 1613, recording an object that appeared to be very close to Jupiter (Grosser, 1962). Traditionally, it is believed that Galileo thought the planet, which would later be named Neptune, was a fixed star, as it appeared to be stationary in the sky rather than moving in orbit as would be expected of a planet (Jones, 2014). However, at the point of discovery, Neptune had just entered the retrograde stage of its cycle on the day it was initially observed, explaining its apparent lack of movement (Jones, 2014).

In more recent research undertaken by Australian physicist David Jamieson, it appears that Galileo may have been aware that his observation was significant. The notation for January 28, 1613 is made in different ink, indicating that it was differentiated, and further observations including a sketch on January 6 appear to indicate he was observing this heavenly body in a systematic manner (Jones, 2014). However, without any specific notes and no evidence of further observational attempts, the evidence remains inconclusive regarding his interpretation of the observation (Jones, 2014).

Galileo was not the only individual studying the heavens who noted the presence of an object that would later become known as Neptune. In all cases, the telescopes were of limited strength, so the relatively dim planet would have appeared as a small and difficult object to observe. However, it is also recorded that the planet was observed in 1795 by Jérôme Lalande from the Paris Observatory and then in 1830 by John Herschel, although neither recognized the body as a planet; both classified it as a star.

Mathematical Discovery and Official Recognition

The official discovery of Neptune in 1846 differs markedly from other planets discovered up to that point. Instead of being observed and then studied, the presence of a planet was identified using mathematical calculations based on indirect evidence rather than through direct telescopic observation.

The mathematical discovery that Neptune should exist began with the publication of astronomical tables by Alexis Bouvard. These tables, published in 1821, recorded the orbit of Uranus and provided a basis for comparing theoretical orbit with observed positions. The observations indicated significant deviations from the expected path, which led Bouvard to hypothesize that other gravitational influences were impacting the planet's orbit (Jones, 2014).

In 1843, John Couch Adams examined the data regarding Uranus's orbit. Adams collected additional data from the Astronomer Royal, Sir George Airy, and used the tables along with observational data taken by the Cambridge Observatory to hypothesize the presence of a new planet, continuing his work through 1846 (Adams, 1846). Independently in France, astronomer Urbain Le Verrier was also working on a similar project, observing differences between the movements of Uranus and the theoretical path according to the tables, but his work was undertaken without support from his peers (Jones, 2014).

Atmospheric Conditions and Weather

Upon reviewing the findings of Adams and Le Verrier and finding significant alignment between their independent works, Sir George Airy approached James Challis, asking him to search for the planet. Despite searching between August and September 1846, Challis did not find it (Jones, 2014; Grosser, 1962). At the same time, Le Verrier had written to Johann Galle at the Berlin Observatory, asking him to observe the sky in the region he predicted to determine if the displacement characteristics matched those of a planet. On September 23, 1846, the same day Galle received the letter, the planet was observed approximately 1 degree from Le Verrier's prediction and approximately 12 degrees from Adams's prediction (Airy, 1846). Subsequent to this discovery, Challis realized he had observed the planet twice on August 8 and August 12, but his less rigorous approach meant he failed to recognize it. Controversy existed regarding the attribution of the discovery, which was eventually given to both Adams and Le Verrier.

Neptune is a gas giant, and similar to other gas giants, its composition has two regions without a firm surface (Kerrod, 2000). The "surface" is defined at the point where the pressure on the planet equals the pressure on Earth at sea level (Kerrod, 2000). The atmospheric composition by volume consists of 80 percent molecular hydrogen with a 3.2 percent uncertainty, 19 percent helium with a 3.2 percent uncertainty, and 1.5 percent methane with a 0.5 percent uncertainty (Williams, 2014). There are also trace elements of hydrogen deuteride and methane, with aerosols of ammonia ice, water ice, ammonia hydrosulfide, and potential methane ice (Williams, 2014). The temperature at the surface is approximately 55 Kelvin, or −201 degrees Celsius, and as altitude increases, the temperature decreases (Williams, 2014). The higher surface temperature reflects how more heat is created by the planet's rotation than is gained from the sun's rays; however, upon reaching the upper stratosphere, the temperature starts to increase again (Fletcher et al., 2010).

Neptune has clouds that vary with altitude. At the highest levels are frozen methane cirrus clouds, which have been observed casting shadows on clouds at an altitude 35 miles lower (Fletcher et al., 2010). Beneath these methane clouds are believed to be clouds made up of ammonium sulfide, hydrogen sulfide, and water (Fletcher et al., 2010).

Physical Characteristics and Orbital Parameters

The planet experiences powerful storms, with winds of up to 1,305 miles per hour, giving Neptune some of the most extreme weather in the solar system—winds that are nine times faster than those recorded on Earth and three times stronger than those on Jupiter. Voyager 2 observed a massive storm approximately the size of Earth, and a storm was observed that lasted for hundreds of years as a spot on the surface (Lindal, 1992).

Neptune has a mass that is 17.15 times that of Earth, at 102,410,000,000,000,000 billion kilograms. The equatorial diameter is 49,528 kilometers, and the polar diameter is 48,682 kilometers. The circumference at the equator is 155,600 kilometers, with the planet having an ellipticity of 0.01708 (Williams, 2014). The mean density of the planet is 1.638 grams per cubic centimeter. The planet has a ring system with five known rings; the three major rings are named after the official discoverers: the Adams Ring, the Le Verrier Ring, and the Galle Ring. Neptune also has 14 known moons, although only one is spherical (Williams, 2014). A day on Neptune—one full planetary rotation—is 16 hours.

Neptune's mean orbital distance is 30.10 AU, or 4,498,396,441 kilometers, with the perihelion (shortest distance to the sun) at 29.81 AU and the aphelion distance (furthest from the sun) at 30.31 AU (Williams, 2014). The orbital period is 164.79 Earth years (Williams, 2014). The mean orbital velocity is 5.43 kilometers per second, with a maximum of 5.5 kilometers per second and a minimum of 5.37 kilometers per second, and an orbital inclination of 1.769 degrees (Williams, 2014). There is an orbital eccentricity of 0.0085858 (Williams, 2014).