neptune planet facts 

Neptune is named for the Roman god of the sea, who is identifi ed with the Greek deity Poseidon, a son of the Titan  Cronus (the Roman god Saturn) and a brother of Zeus (the  Roman god Jupiter). It is the second planet to have been  found by means of a telescope. Its discovery in 1846 was a  remarkable combination of the application of solid Newtonian  physics and a belief in a numerological scheme that later  proved to be scientifi cally unfounded. Neptune’s orbit is  almost perfectly circular; as a result, its distance from the Sun  varies comparatively little over its nearly 164-year period of  revolution. Although the dwarf planet Pluto’s mean distance  from the Sun is greater than Neptune’s, its orbit is so eccen- tric (elongated) that for about 20 years of each revolution  Pluto is actually nearer the Sun than is Neptune.  

 Neptune is almost four times the size of Earth but slightly smaller than Uranus, which makes it the smallest in diameter  of the four giant, or Jovian, planets. It is more massive than  Uranus, however, having a density roughly 25 percent higher.  Like the other giant planets, Neptune consists primarily of  hydrogen, helium, water, and other volatile compounds, along  with rocky material, and it has no solid surface. It receives less than half as much sunlight as Uranus, but heat escaping from its interior makes Neptune slightly warmer than Uranus.  

The heat liberated may also be responsible for the storminess in Neptune’s atmo- sphere, which exhibits the fastest winds  seen on any planet in the solar system.  

 Neptune has 13 moons (natural satellites), only two of which had been discovered before the Voyager 2 space- craft flew past the planet in 1989, and a  system of rings, which had been uncon- firmed until Voyager’s visit. As is the  case for Uranus, most of what astrono- mers know about Neptune, including its  rotation period and the existence and  characteristics of its magnetic field  and magnetosphere, was learned from a single spacecraft encounter. In recent years new knowledge of the Neptunian  system has come as a result of advances  in Earth-based observational technology.

Neptune’s orbital period of 163.72 years means that by mid-2010 it will have cir- cled the Sun only once since its discovery  in September 1846. Consequently, astron- omers expect to be making refinements  in calculating its orbital size and shape  well into the 21st century. Voyager 2’s  encounter with Neptune resulted in a  small upward revision of the planet’s esti- mated mean distance from the Sun, which  is now thought to be 4,498,250,000 km  (2,795,083,000 miles). Its orbital eccen- tricity of 0.0086 is the second lowest of  the planets; only Venus’s orbit is more  circular. Neptune’s rotation axis is tipped  toward its orbital plane by 29.6°, some- what larger than Earth’s 23.4°. As on Earth,  the axial tilt gives rise to seasons on  Neptune, and, because of the circularity  of Neptune’s orbit, the seasons (and the  seasons of its moons) are of nearly equal  length, each nearly 41 years in duration.

The atmosphere

Like the other giant planets, Neptune’s

outer atmosphere is composed predomi-

nantly of hydrogen and helium. Near the

one-bar pressure level in the atmosphere,

these two gases contribute nearly 98 per-

cent of the atmospheric molecules. Most

of the remaining molecules consist of

methane gas. Hydrogen and helium are

nearly invisible, but methane strongly

absorbs red light. Sunlight reflected off

Neptune’s clouds therefore exits the

atmosphere with most of its red colours

removed and so has a bluish cast.

Although Uranus’s blue-green colour is

also the result of atmospheric methane,

Neptune’s colour is a more vivid, brighter

blue, presumably an effect of the pres-

ence of an unidentified atmospheric gas.

The temperature of Neptune’s atmo-

sphere varies with altitude. A minimum

temperature of about 50 kelvins (K; −370

°F, −223 °C) occurs at a pressure near 0.1

bar. The temperature increases with

decreasing pressure—i.e., with increasing

altitude—to about 750 K (890 °F, 480 °C)

at a pressure of a hundred-billionth of a

bar, which corresponds to an altitude of

2,000 km (1,240 miles) as measured from

the one-bar level, and it remains uniform

above that altitude. Temperatures also

increase with increasing depth below the

0.1-bar level to about 7,000 K (12,000 °F,

6,700 °C) near the centre of the planet,

where the pressure may reach five

megabars. The total amount of energy

radiated by Neptune is equivalent to that

of a nonreflecting sphere of the same size

with a uniform temperature of 59.3 K

(−353 °F, −214 °C). This temperature is

called the effective temperature.

Neptune is more than 50 percent

farther from the Sun than is Uranus and

so receives less than half the sunlight of

the latter. Yet the effective temperatures

of these two giant planets are nearly

equal. Uranus and Neptune each reflect—

and hence also must absorb—about the

same proportion of the sunlight that

reaches them. As a result of processes not

fully understood, Neptune emits more

than twice the energy that it receives

from the Sun. The added energy is gener-

ated in Neptune’s interior. Uranus, by

contrast, has little energy escaping from

its interior.

Moons

Prior to Voyager 2’s encounter, Neptune’s

only known moons were Triton, discov-

ered visually through a telescope in

October 1846 by English astronomer

William Lassell shortly after the discov-

ery of Neptune, and Nereid, discovered in

telescopic photographs more than a cen-

tury later in 1949 by American astronomer

Gerard Kuiper. In 1989, Voyager’s obser-

vations added six previously unknown

moons to Neptune’s system. All are less

than half of Triton’s distance from

Neptune and are regular moons—i.e., they

travel in prograde, nearly circular orbits

that lie near Neptune’s equatorial plane.

These moons are probably synchronous

rotators; that is, their rotational and

orbital periods are the same.

In 2002–03, five additional tiny

moons, estimated to be about 15–30 km

(9–18 miles) in radius, were discovered in

Earth-based observations. These are

irregular, having highly eccentric orbits

that are inclined at large angles to the

planet’s equator; three also orbit in the

retrograde direction. Their mean dis-

tances from Neptune lie roughly between

15 million and 48 million km (9 million

and 30 million miles), well outside the

orbit of Nereid.

The Ring System

Evidence that Neptune has one or more

rings arose in the mid-1980s when stellar

occultation studies from Earth occasion-

ally showed a brief dip in the star’s

brightness just before or after the planet

passed in front of it. Because dips were

seen only in some studies and never sym-

metrically on both sides of the planet,

scientists concluded that any rings pres-

ent do not completely encircle Neptune

but instead have the form of partial rings,

or ring arcs.

Images from Voyager 2, however,

revealed a system of six rings, each of

which in fact fully surrounds Neptune.

The putative arcs turned out to be bright

regions in the outermost ring, named

Adams, where the density of ring parti-

cles is particularly high. Although rings

also encircle each of the other three giant

planets, none displays the striking

clumpiness of Adams. The arcs are found

within a 45° segment of the ring. From

leading to trailing, the most prominent

are named Courage, Liberté, Egalité 1,

Egalité 2, and Fraternité. They range in

length from about 1,000 km (600 miles)

to more than 10,000 km (6,000 miles).

Although the moon Galatea, which orbits

just planetward of the inner edge of

Adams, may gravitationally interact with

the ring to trap ring particles temporar-

ily in such arclike regions, collisions

between ring particles should eventually

spread the constituent material relatively

uniformly around the ring. Consequently,

it is suspected that the event that

supplied the material for Adams’s enig-

matic arcs—perhaps the breakup of a

small moon—occurred within the past

few thousand years.

The other five known rings of

Neptune—Galle, Le Verrier, Lassell,

Arago, and Galatea, in order of increas-

ing distance from the planet—lack the

nonuniformity in density exhibited by

Adams. Le Verrier, which is about 110 km

(70 miles) in radial width, closely resem-

bles the nonarc regions of Adams. Similar

to the relationship between the moon

Galatea and the ring Adams, the moon

Despina orbits Neptune just planetward

of the ring Le Verrier. Each moon may

gravitationally repel particles near the

inner edge of its respective ring, acting as

a shepherd moon to keep ring material

from spreading inward.

Galle, the innermost ring, is much

broader and fainter than either Adams

or Le Verrier, possibly owing to the

absence of a nearby moon that could

provide a strong shepherding effect.

Lassell consists of a faint plateau of ring m

that extends outward from Le Verrier about 

halfway to Adams. Arago  

is the name used to distinguish a nar-

row, relatively bright region at the outer

edge of Lassell. Galatea is the name gen-

erally used to refer to a faint ring of 

material spread all along the orbit of the  

moon Galatea.

Neptune’s discovery

Neptune is the only giant planet that is

not visible without the aid of a telescope.

Having an apparent magnitude of 7.8, it is

approximately one-fifth as bright as the faintest stars visible to the unaided eye.

Hence, it is fairly certain that there were

no observations of Neptune prior to the

use of telescopes. Galileo is credited as

the first person to view the heavens with

a telescope in 1609. His sketches from a

few years later, the first of which was

made on Dec. 28, 1612, suggest that he

saw Neptune when it passed near Jupiter

but did not recognize it as a planet.

Prior to the discovery of Uranus by

the English astronomer William Herschel

in 1781, the consensus among scientists

and philosophers alike was that the plan-

ets in the solar system were limited to

six—Earth plus those five planets that had

been observed in the sky since ancient

times. Knowledge of a seventh planet

almost immediately led astronomers and

others to suspect the existence of still

more planetary bodies. Additional impe-

tus came from the mathematical curiosity

known as Bode’s law.

Some astronomers were so impressed

by the seeming success of Bode’s law in

explaining the distances of Ceres and

Uranus that they proposed the name

Ophion for the large planet that the law

told them must lie beyond Uranus, at a

distance of 38.8 AU.

In addition to this scientifically

unfounded prediction, observations of

Uranus provided actual evidence for the

existence of another planet. Uranus was

not following the path predicted by

Newton’s laws of motion and the gravita-

tional forces exerted by the Sun and the

known planets. Furthermore, more than 20 recorded prediscovery sightings of

Uranus dating back as far as 1690 dis-

agreed with the calculated positions of

Uranus for the respective time at which

each observation was made. It appeared

possible that the gravitational attraction

of an undiscovered planet was perturbing

the orbit of Uranus.

In 1843 the British mathematician

John Couch Adams began a serious

study to see if he could predict the loca-

tion of a more distant planet that would

account for the strange motions of

Uranus. Adams communicated his results

to the astronomer royal, George B. Airy,

at Greenwich Observatory, but they

apparently were considered not precise

enough to begin a reasonably concise

search for the new planet. In 1845 Urbain-

Jean-Joseph Le Verrier of France, unaware

of Adams’s efforts in Britain, began a sim-

ilar study of his own.

By mid-1846 the English astronomer

John Herschel, son of William Herschel,

had expressed his opinion that the math-

ematical studies under way could well

lead to the discovery of a new planet. Airy,

convinced by Herschel’s arguments,

proposed a search based on Adams’s cal-

culations to James Challis at Cambridge

Observatory. Challis began a systematic

examination of a large area of sky sur-

rounding Adams’s predicted location.

The search was slow and tedious because

Challis had no detailed maps of the dim

stars in the area where the new planet

was predicted. He would draw charts of

the stars he observed and then compare them with the same region several nights

later to see if any had moved.

Le Verrier also had difficulty convinc-

ing astronomers in his country that a

telescopic search of the skies in the area

he predicted for the new planet was not a

waste of time. On Sept. 23, 1846, he com-

municated his results to the German

astronomer Johann Gottfried Galle at

the Berlin Observatory. Galle and his

assistant Heinrich Louis d’Arrest had

access to detailed star maps of the sky

painstakingly constructed to aid in the

search for new asteroids. Galle and

d’Arrest identified Neptune as an

uncharted star that same night and veri-

fied the next night that it had moved

relative to the background stars.

Later observations

from Earth

Earth-based observations of Neptune

before Voyager 2’s flyby suffered greatly

as a consequence of the planet’s enor-

mous distance from both Earth and the

Sun. Its average orbital radius of 30.1 AU

means that the sunlight reaching its

moons and its upper atmosphere is

barely 0.1 percent as bright as that at

Earth. Pre-Voyager telescopic viewing of

Neptune through the full thickness of

Earth’s atmosphere could not resolve fea-

tures smaller than about one-tenth of

Neptune’s diameter, even under the best

observing conditions. Most such obser-

vations concentrated on determining

Neptune’s size, mass, density, and orbital

parameters and searching for moons. In

the early 21st century specialized inter-

ferometric techniques have routinely

improved spatial resolution of distant

objects by factors of 10–100 over earlier

surface-based observations.

Spacecraft exploration

Voyager 2 is the only spacecraft to have

encountered the Neptunian system. This

spacecraft and its twin, Voyager 1—both

launched in 1977—originally were slated

to visit only Jupiter and Saturn, but the

timing of Voyager 2’s launch gave its tra-

jectory the leeway needed for the

spacecraft to be redirected, with a gravity

assist from Saturn, on extended missions

to Uranus and then to Neptune.

Voyager 2 flew past Neptune and its

moons on Aug. 24–25, 1989, observing the

system almost continuously between

June and October of that year. It mea-

sured the planet’s radius and interior

rotation rate and detected its magnetic

field, determining that the latter is both

highly inclined and offset from the

planet’s rotation axis. It confirmed that Neptune has rings and discovered six new

moons. Neptune previously had been

thought too cold to support active weather

systems, but Voyager’s images of the

planet revealed the highest atmospheric

winds seen in the solar system and several

large-scale storms, one the size of Earth.

Because Neptune was Voyager 2’s

last planetary destination, mission scien-

tists risked sending the spacecraft closer

to it than to any other planet during the

mission. Voyager passed about 5,000 km

(3,100 miles) above Neptune’s north pole.

A few hours later it passed within 40,000

km (24,800 miles) of Triton, which allowed

it to gather high-resolution images of the

moon’s highly varied surface as well as

precise measurements of its radius and

surface temperature. As of 2009, no future 

missions to  Neptune are planned.