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Photo of Neptune taken by Voyager 2 in 1989
Symbol Neptune symbol.svg
Date of discovery September 23, 1846[1][2]
Name of discoverer Johann Gotfried Galle[1][2]
Name origin Roman god of the sea[1]
Orbital characteristics
Primary Sun
Order from primary 9
Perihelion 4,444,450,000 km (29.709 AU)[2]
Aphelion 4,545,670,000 km (30.386 AU)[2]
Semi-major axis 4,495,060,000 km (30.048 AU)[2]
Titius-Bode prediction 38.8 AU
Circumference 188.925 AU
Orbital eccentricity 0.0113[2]
Sidereal year 164.79 a[2]
Synodic year 367.49 da[2]
Avg. orbital speed 5.432 km/s[2]
Inclination 1.769°[2] to the ecliptic
Rotational characteristics
Sidereal day 16.11 h[2][3]
Axial tilt 28.32°[2]
Physical characteristics
Mass 1.0243 * 1026 kg[2]
Density 1,638 kg/m³[2]
Mean radius 24,622 km[2]
Equatorial radius 24,764 km[2]
Polar radius 24,341 km[2]
Surface gravity 11.00 m/s²[2]
Escape speed 23.50 km/s[2]
Surface area 7,619,000,000 km²
Mean temperature 53 K
Number of moons 13
Composition 80% hydrogen, 19% helium, 1.5% methane, 192 ppm hydrogen deuteride, 1.5 ppm ethane, aerosol traces of ammonia ice, water ice, ammonia hydrosulfide, and methane ice.[2]
Color deep blue
Albedo 0.41[2]
Magnetic flux density 0.142 G (1.42 * 10-5 T)[2]
Magnetic dipole moment at present 1.5 * 1024 N-m/T[4]
Magnetic dipole moment at creation 2.42 * 1025 N-m/T[5]
Decay time 2200 a[6]
Half life 1525 a[6]

Neptune is the eighth and farthest planet from the sun. It was discovered in 1846 by German astronomer Johann Gotfried Galle based on orbital calculations by French mathematician Urbain Le Verrier.

The U.S. spacecraft Voyager 2 flew by Neptune in 1989. The planet is named after the Roman god of the sea. It is 17 times the mass of the Earth and is composed of 80 percent hydrogen and 19 percent helium.


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.[7]

In 1781, William Herschel discovered Neptune's twin Uranus. But astronomers noticed that that planet's orbit deviated from the strict Newtonian model. John Couch Adams and Le Verrier independently predicted another planet beyond Uranus to account for these irregular motions. Though Adams never published his predictions, Le Verrier did. Galle and his student 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.[8]

A protracted dispute between the British and French arose as to the naming rights and the proposed name of the planet.[7] 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.[7]

Orbital characteristics

Neptune maintains an average distance from the sun of 4.49506 billion kilometers (2.7931 billion miles, or 30.048 AU).[2] 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.[2]

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.[2][3]

Physical characteristics

Neptune is 24,764 km (15,388 miles) in radius. Its 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.[7][9] The source of the energy that fuels these winds remains unsettled.[7]

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.[7][8] 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.[7][9]

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.[7]


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.[9]

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.[7][9]

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.[8] 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.


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.

Difficulties for uniformitarian theories

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.[10]

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.[11] 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.[12]

Desch,[13] 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.


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[4] 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.


The only spacecraft to visit Neptune has been 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.[14]

The Hubble Space Telescope and several very powerful Earth-based telescopes continue to observe Neptune from Earth.[15]


  1. 1.0 1.1 1.2 "Gazetteer of Planetary Nomenclature: Planetary Body Names and Discoverers." US Geological Survey, Jennifer Blue, ed. March 31, 2008. Accessed April 17, 2008.
  2. 2.00 2.01 2.02 2.03 2.04 2.05 2.06 2.07 2.08 2.09 2.10 2.11 2.12 2.13 2.14 2.15 2.16 2.17 2.18 2.19 2.20 2.21 2.22 2.23 2.24 Neptune Fact Sheet, NASA, November 29, 2007. Accessed May 6, 2008.
  3. 3.0 3.1 "Neptune." Encyclopædia Britannica. 2008. Encyclopædia Britannica Online. Accessed May 6, 2008.
  4. 4.0 4.1 Humphreys, D. R. "Beyond Neptune: Voyager II Supports Creation." Institute for Creation Research. Accessed April 30, 2008
  5. Humphreys, D. R. "The Creation of Planetary Magnetic Fields." Creation Research Society Quarterly 21(3), December 1984. Accessed April 29, 2008.
  6. 6.0 6.1 Calculated from observed present magnetic dipole moment and expected magnetic dipole moment at creation.
  7. 7.0 7.1 7.2 7.3 7.4 7.5 7.6 7.7 7.8 Arnett, Bill. "Neptune." The Nine 8 Planets, September 2, 2004. Accessed May 6, 2008.
  8. 8.0 8.1 8.2 Smith, Bradford A. "Neptune." World Book Online Reference Center, NASA, 2004. Accessed May 6, 2008.
  9. 9.0 9.1 9.2 9.3 Hamilton, Calvin J. "Entry for Neptune." Views of the Solar System, 2001. Accessed May 7, 2008.
  10. R.N., Birth of Uranus and Neptune, Astronomy '28'(4):30, 2000
  11. Boss, Alan P. "Formation of gas and ice-giant planets." Earth and Planetary Science Letters 202(3-4):513-523, September 30, 2002. doi:10.1016/S0012-821X(02)00808-7 Accessed May 7, 2008.
  12. Thommes, Edward W., Duncan, Martin J., and Levison, Harold F. "The formation of Uranus and Neptune among Jupiter and Saturn." Astron. J. 123:2862, 2002. arXiv:astro-ph/0111290v1 Accessed May 7, 2008.
  13. Desch, S. J. "Mass Distribution and Planet Formation in the Solar Nebula." Arizona State University, September 21, 2007. Accessed April 29, 2008.
  14. Sanders, Jane. "Solar System Secrets: Nuclear-Powered Mission to Neptune Could Answer Questions About Planetary Formation." Press Release, Georgia Institute of Technology, December 9, 2004. Accessed May 8, 2008.
  15. "Hubble Makes Movie of Neptune's Dynamic Atmosphere." Space Telescope Science Institute, release no. STScI-2005-22, September 1, 2005. Accessed May 8, 2008.

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