Neptune: The Planet Exterior to Uranus

By Riddle, Bob | Science Scope, November 2007 | Go to article overview

Neptune: The Planet Exterior to Uranus


Riddle, Bob, Science Scope


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A planet and a former dwarf planet

This month, the three most outer planets--Jupiter, Uranus, and Neptune--and one of the larger dwarf planets, Pluto, will be in the evening skies after sunset. While only Jupiter and Uranus will be visible through small telescopes or binoculars, this part of the night sky can be viewed with the understanding that one is looking in the direction of some of the distant members of the solar system. Pluto will be located just above the planet Jupiter and to the west of the constellation Sagittarius. Neptune will lie within the northern boundaries of the constellation Capricornus, and Uranus will be amongst the stars of Pisces (see Figure 1).

Discovery of Neptune

Following the discovery of Uranus (See October's "Scope on the Skies"), astronomers spent a few years calculating and plotting more precisely its orbital path around the Sun. They quickly noticed that the orbit appeared to be affected, or perturbed, by a more distant and massive object--causing Uranus to appear to speed up and slow down. In 1841, British astronomer and mathematician John Couch Adams correctly calculated the orbit and position of an eighth planet that would account for the perturbations in Uranus's orbit. Due to perhaps a miscommunication between Adams and the British Royal astronomer (Sir George Airy), Adams' calculations were never used. Several years later, in 1843, French astronomer and mathematician Urbain Le Verrier, working independently of Adams, made similar calculations of an eighth planet perturbing the planet Uranus. Then, on September 23, 1846, working with German astronomer Johan Galle, Adams and Le Verrier located the planet Neptune in the boundaries of the constellation Aquarius, and within 1[degrees] of its calculated position. For a short time after the discovery, Britain and France argued over who should get credit for the planet's discovery; this was eventually settled by crediting both Le Verrier and Adams with the discovery of Neptune. However, there is still some controversy about this, with the suggestion that the men should not receive equal credit, or that others share the credit (such as Johan Galle and possibly Galileo, who recorded observing Neptune but did not realize it was another planet).

Neptune was the first planet discovered through the use of mathematical calculations. As the eighth planet, Neptune is the last of the giant gaseous outer planets and like its near twin, Uranus, is often referred to as an ice giant planet to distinguish it from the much larger Jupiter and Saturn. Neptune is 17 times the mass of the Earth, and is approximately 4.5 billion km (2.8 billion miles) from the Sun. The temperature at the top of its clouds is -218[degrees]C and winds were clocked at speeds of 2,100 km/h by the Voyager 2 spacecraft. It has 13 known moons, six of which were discovered by the Voyager 2 spacecraft. Neptune has an axial tilt of 29.6[degrees], has a rotational period of 16.11 hours, and takes nearly 165 Earth years to complete one revolution around the Sun. Neptune has an atmosphere composed primarily of hydrogen, with some helium, and enough methane to absorb the red wavelengths of light from the Sun giving Neptune its distinctive bluish color.

Neptune has a ring system composed of six rings extending outward from the planet to a distance of about 63,000 km (39,000 miles). The rings are composed of darker ices and rock than Saturn's rings and therefore appear darker and are very difficult to observe from Earth. The composition of the ring system is unknown; however, within the rings is a lot of dust from collisions between the rocky and icy ring particles, which obscures parts of the rings, giving the appearance from Earth-based observations that the rings are incomplete and seemingly made of arcs of ring material. The flyby of the Voyager 2 spacecraft in August 1989 showed that the rings are indeed complete. …

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