Neptune

Neptune is the eighth and farthest true planet from the sun. Long after its discovery, astronomers had no notion of how remarkable Neptune would prove to be, until Voyager 2 flew by Neptune in 1989. Neptune continues to perplex astronomers, and yield more evidence for a young solar system, with virtually every observation made.

Discovery
In 1613, Galileo Galilei first observed what he took to be a star close to Jupiter. On the next two nights this "star" had moved with respect to another true star. Then the object fell from his field of view, and bad weather prevented Galileo from observing it again. He would never know the magnitude of his oversight.

In 1781, William Herschel discovered Neptune's twin Uranus. But astronomers noticed that that planet's orbit deviated from the strict Newtonian model. discoverer::John Couch Adams and discoverer::Urbain Le Verrier independently predicted another planet beyond Uranus to account for these irregular motions. Though Adams never published his predictions, Le Verrier did. Johann Gottfried Galle and his student discoverer::Heinrich L. d'Arrest used those predictions to help him search the sky, and he found the eighth planet within one degree of where Le Verrier's numbers predicted it to be.

A protracted dispute between the British and French arose as to the naming rights and the proposed name of the planet. Galle at one point proposed naming the planet after Le Verrier. Eventually, however, the community of astronomers chose the name Neptune, for the Roman god of the sea and brother of Jupiter. They also chose as a symbol for Neptune the trident, the three-pronged sceptre that the mythical Neptune carried.

In fact, the Adams and Le Verrier predictions would not have held much longer, and the best reason why Galle found Neptune when he did is that he acted very soon after Le Verrier published his predictions, when they would still hold.

Orbital characteristics
Neptune maintains an average distance from the sun of 4.49506 billion kilometers (2.7931 billion miles, or 30.048 AU). It therefore is the first and only spheroidal body in orbit around the Sun whose orbital placement infringes the Titius-Bode Law. The dwarf planet Pluto actually lies close to the place that the Titius-Bode law predicts for the ninth object in orbit around the Sun.

Neptune's orbit is more nearly circular than that of any other satellite of the Sun except Venus. Its sidereal year is 164.79 Julian years. Its orbit is slightly inclined to the ecliptic, by 1.769 degrees.

Rotational characteristics
Neptune's sidereal day is 16.11 hours long. Voyager 2 determined this by measuring the period of Neptune's magnetic field, whose axis is inclined 49 degrees from the planet's axis of rotation.

Physical characteristics
Neptune is 24,764 km (15,388 miles) in radius. It’s atmosphere is about 80% hydrogen, 19% helium, and 1.5% methane. Despite being the farthest planet from the sun, Neptune radiates more than twice as much heat as it receives from the Sun. The winds of Neptune are the strongest in the solar system, at 2000 km/hr. The source of the energy that fuels these winds remains unsettled.

The planet’s most prominent feature was its Great Dark Spot, a storm similar to Jupiter’s Great Red Spot. In 1989, when Voyager 2 revealed it, it moved westward at 300 km/s. Voyager 2 also revealed an irregular, eastward-moving cloud, called the "scooter," evidently moving with the planet's rotation. Most astronomers believe that this is a plume rising from a deeper deck of clouds.

When the Hubble Space Telescope photographed Neptune in 1994, the Great Dark Spot had disappeared. Another great storm of comparable size formed a few months later.

Magnetosphere
Neptune's magnetic field has a magnetic dipole moment variously estimated as 1.5 * 1024 or 2.1 * 1024 N-m/T. The axis of the dipole is also inclined about 47 degrees from the axis of rotation, and displaced from the center by about 13,500 km.

Ring system
Like all the gas giants, Neptune has a ring system. Observations from Earth never resolved complete rings, but Voyager 2 did. Most of the rings are named after the astronomers involved in the discovery of Neptune; from the outermost to the innermost they are named after Adams, Le Verrier, and Galle.

The Adams ring is subdivided into three major prominent arcs, named Liberty, Equality, and Fraternity. This clumping of the rings into arcs of uneven brightness, which once caused scientists to suspect that the rings were incomplete, remains unexplained. Curiously, the Fraternity ring arc appears braided, though JPL staff insist that the braiding is an artifact of the photograph. A JPG version of the photograph appears at left, so that the viewer may decide. This would not be the only braided ring in the solar system; Saturn also has braided rings.

Satellites
Neptune has thirteen satellites in all. The first to be discovered was Triton, a most unusual moon that is the only dwarf-planet-sized natural satellite with a retrograde motion about its primary.

Formation of the planet
The nebular hypothesis would not even have allowed Neptune and its twin, Uranus, to form at its tremendous distance from the Sun in the time generally allotted. As the journal Astronomy stated the problem:Pssst … astronomers who model the formation of the solar system have kept a dirty little secret: Uranus and Neptune don’t exist. Or at least computer simulations have never explained how planets as big as the two gas giants could form so far from the sun. Bodies orbited so slowly in the outer parts of the sun’s protoplanetary disk that the slow process of gravitational accretion would need more time than the age of the solar system to form bodies with 14.5 and 17.1 times the mass of Earth.

Of course, the time allotted for the formation of the solar system is generally taken to be the alloted age of the Earth. The problem is that Uranus and Neptune would require more than twice the supposed age of the earth to accrete their present masses. In theory, the solar system could be older than the earth itself, but not much older. The solar system has other bodies in it that are much "younger." Nor does the above account for the body of evidence that the solar system is in fact quite young.

Boss, in 2002, suggested reviving the old disk-instability hypothesis of planet formation, in which Uranus and Neptune formed through gravitational instability in a gaseous protoplanetary disk, after which a passing star blasted away much of the gaseous envelopes of the two worlds, leaving them much as they are today. Thommes et al. suggested another theory: that Uranus and Neptune formed at the same general distance from the Sun as Jupiter and Saturn and then migrated to their present orbits.

Desch, on the other hand, supports a radical alteration of the nebula hypothesis that calls for a decretion disk, not an accretion disk. But even his model requires that Uranus and Neptune exchange places early in the process, primarily because Neptune, though about 10 AU further out from the Sun than is Uranus, is yet more massive.

Weather
Scientists first thought that, so far out in the solar system, Neptune would be a cold, dead planet. Neptune's internal heat source, tremendous winds, and storms that form, dissipate, and then form again, belie that characterization.

Magnetic field
The strength of Neptune's magnetic field contradicts the popular "dynamo" theory of celestial magnetic fields. That model requires a liquid core, but Neptune's core is solid, not liquid. However, after Voyager 2 demonstrated that Uranus (which also has a solid core) had a magnetic field, astronomers seem to have revised their models to suggest that any planet with an internal heat source could have a magnetic field. Russell Humphreys predicted the strength of Neptune's magnetic field to well within observational tolerances, five years before Voyager 2 visited Neptune. In his second paper Humphreys acknowledged that the then-current model for planetary magnetic fields seems to have predicted Neptune's field equally well. Nevertheless, Humphreys published his prediction two years before astronomers had any data on Uranus to suggest to them that their models might be flawed and require revision.

Exploration
The only spacecraft to visit Neptune has been Visiting mission::Voyager 2. No further deep-space missions to Neptune are currently planned. However, NASA is funding a feasibility study of a plan to send a nuclear-powered deep-space probe to Neptune to study Neptune and its satellites, especially Triton, in depth.

The Hubble Space Telescope and several very powerful Earth-based telescopes continue to observe Neptune from Earth.

Related Links

 * NASA "Neptune"
 * Neptune by Wikipedia
 * Answers in Genesis "Neptune: monument to creation"

Neptune