Neptune

Neptune is the eighth planet, and - now that Pluto is no longer considered a planet - the farthest from the Sun. It is the second planet (after Uranus) to have been found in modern times. Neptune is a gas giant, composed mainly of hydrogen and helium, and has one large moon, Triton.

Discovery and Observation

Neptune was located through mathematical prediction, rather than by regular observation of the sky. When the motion of Uranus failed to match the exact path astronomers predicted for it, an English astronomer named John Couch Adams and a French mathematician named Urbain Joseph Le Verrier proposed that the difference was caused by the gravity of an as-yet-unknown planet. After being ignored by French astronomers, Le Verrier sent his predictions to Johann Gottfried Galle at the Berlin Observatory, who found Neptune on his first night of searching in 1846. Seventeen days later, Neptune's largest moon, Triton, was also discovered.

While observing Jupiter in 1612 and 1613 with his small telescope, Galileo actually saw Neptune as well, and recorded its position in his notes. However, on both occasions, he mistook Neptune for a fixed star - thus Adams and Le Verrier, and not Galileo, are credited with the discovery of Neptune.

Neptune's average distance from the Sun is 2.8 billion miles (4.5 billion km) - more than 30 times the Earth's - and the period of its orbit around the Sun is nearly 165 Earth years. Neptune can be seen in an amateur-size telescope. However, because of Neptune's great distance, its apparent magnitude is about 8.0 at brightest, and its apparent size is only about 2.5 arc seconds at largest. It will show a bluish disk, but no details.

The only spacecraft to have visited Neptune was Voyager 2, in 1989.

Structure, Composition, and Atmosphere

Like Uranus, Neptune is a gas giant planet with no solid surface. Neptune is slightly smaller than Uranus, at 30,800 miles (49,500 km) in diameter, or 3.9 times the diameter of the Earth. Its rotation period is 16 hours 6 minutes, and its mass - just over 17 times the Earth's - is comprised mainly of hydrogen and helium. Neptune's internal structure is thought to be similar to Uranus's, with a hydrogen- and helium-rich atmosphere overlying a mantle of liquid water, methane, and ammonia; this in turn surrounds an Earth-sized core of rock and metal.

Neptune's electrically-conducting, fluid interior generates a very strong magnetic field, about 27 times more powerful than the Earth's. It is highly tilted, at 47 degrees from Neptune's rotation axis, and its center is offset by at least 0.55 Neptune radii (about 13,500 kilometers or 8,500 miles) from the planet's physical center. Unlike Uranus (but like Jupiter and Saturn), Neptune emits more than 2.5 times as much energy as it receives from the Sun. This keeps Neptune's surface at about the same temperature as Uranus's (-350°F or -214°C), even though Neptune is further from the Sun.
Neptune is a dynamic planet with several large, dark spots reminiscent of Jupiter's hurricane-like storms. The largest spot, known as the Great Dark Spot, is about the size of the Earth, and is similar to the Great Red Spot on Jupiter. Neptune's winds are the strongest of any planet in the solar system. They are three times stronger than Jupiter's, and nine times stronger than Earth's. Near the Great Dark Spot, winds blow up to 2,000 km/h (1,200 miles per hour).

Neptune is the eighth planet, and - now that Pluto is no longer considered a planet - the farthest from the Sun. It is the second planet (after Uranus) to have been found in modern times. Neptune is a gas giant, composed mainly of hydrogen and helium, and has one large moon, Triton.

The Hubble Space Telescope has seen evidence of seasons on Neptune. Neptune's rotational axis is tilted by about 29 degrees to its orbit (compared to 23.5 degrees for the Earth), and because Neptune takes so long to complete one orbit, each hemisphere is exposed to sunlight for many decades. Astronomers believe that the southern hemisphere is brightening - a harbinger of seasonal changes.

The Rings and Moons of Neptune

Like the other gas giant planets, Neptune is encircled by a system of rings. The first evidence of rings was provided by stellar occultation observations in the mid-1980s. Their presence was confirmed by the Voyager 2 spacecraft flyby in 1989. Like those of Uranus, Neptune's rings are faint, and confined into narrow bands by the gravitational action of nearby "shepherd" moons.

The two brightest rings are named Le Verrier and Adams; the outermost, Adams ring contains three prominent arcs, or "clumps". This is a unique feature among planetary ring systems, and is very difficult to understand, because the laws of motion predict that the arcs would spread out into a uniform ring over very short timescales. We now believe that the gravitational effects of Galatea, a moon just inward from the Adams ring, are what confines its arcs.

Neptune has thirteen known moons. The largest and brightest of these, Triton, was the discovered in 1846. Triton orbits in the opposite direction (retrograde) from the direction of Neptune's rotation; it is the only large moon in the solar system which does this. Images from the Voyager 2 spacecraft showed Triton to be geologically active, with a young and complex surface lacking many impact craters.

Neptune's second moon, Nereid, was not discovered until 1949; it is much smaller than Triton, and has a very distant, eccentric orbit around Neptune.

Six more moons were discovered by the Voyager 2 spacecraft in 1989. The new Neptunian moons discovered by Voyager 2 orbit close to the planet, and some orbit within its ring system. The last five were found by Earth-based telescopes using new techniques in 2002 and 2003. The five most recently discovered moons are all small (40 to 60 km in diameter), dark (with albedos of about 0.04), and have eccentric and scattered orbits, like Nereid.