The Outer Planets

Jupiter is the fifth planet from the Sun and by far the largest. Jupiter is more than twice as massive as all the other planets combined (318 times Earth). 
Orbit: 778,330,000 km (5.20 AU) from Sun.
Diameter: 142,984 km (equatorial)
Mass: 1.900e27 kg
Jupiter is the fourth brightest object in the sky (after the Sun, the Moon and Venus; at some times Mars is also brighter). It has been known since prehistoric times. Galileo's discovery, in 1610, of Jupiter's four large moons Io, Europa, Ganymede and Callisto (now known as the Galilean moons) was the first discovery of a centre of motion not apparently cantered on the Earth. It was a major point in favour of Copernicus's heliocentric theory of the motions of the planets; Galileo's outspoken support of the Copernican theory got him in trouble with the Inquisition. 
Jupiter was first visited by Pioneer 10 in 1973 and later by Pioneer 11, Voyager 1, Voyager 2 and Ulysses. The spacecraft Galileo is currently in orbit around Jupiter and will be sending back data for at least the next two years. 
The gas planets do not have solid surfaces, their gaseous material simply gets denser with depth (the radii and diameters quoted for the planets are for levels corresponding to a pressure of 1 atmosphere). What we see when looking at these planets is the tops of clouds high in their atmospheres (slightly above the 1 atmosphere level). 
Jupiter is about 90% hydrogen and 10% helium (by numbers of atoms, 75/25% by mass) with traces of methane, water, ammonia and "rock". This is very close to the composition of the primordial Solar Nebula from which the entire solar system was formed. Saturn has a similar composition, but Uranus and Neptune have much less hydrogen and helium. 
Our knowledge of the interior of Jupiter (and the other gas planets) is highly indirect and likely to remain so for some time. (The data from Galileo's atmospheric probe goes down only about 150 km below the cloud tops.) 
Jupiter probably has a core of rocky material amounting to something like 10 to 15 Earth-masses. 
Above the core lies the main bulk of the planet in the form of liquid metallic hydrogen. This exotic form of the most common of elements is possible only at pressures exceeding 4 million bars, as is the case in the interior of Jupiter (and Saturn). Liquid metallic hydrogen consists of ionized protons and electrons (like the interior of the Sun but at a far lower temperature). At the temperature and pressure of Jupiter's interior hydrogen is a liquid, not a gas. It is an electrical conductor and the source of Jupiter's magnetic field. This layer probably also contains some helium and traces of various "ices". 
The outermost layer is composed primarily of ordinary molecular hydrogen and helium which is liquid in the interior and gaseous further out. The atmosphere we see is just the very top of this deep layer. Water, carbon dioxide, methane and other simple molecules are also present in tiny amounts. 
Recent experiments have shown that hydrogen does not change phase suddenly. Therefore the interiors of the jovian planets probably have indistinct boundaries between their various interior layers. 
Three distinct layers of clouds are believed to exist consisting of ammonia ice, ammonium hydrosulfide and a mixture of ice and water. However, the preliminary results from the Galileo probe show only faint indications of clouds (one instrument seems to have detected the topmost layer while another may have seen the second). But the probe's entry point (left) was unusual -- Earth-based telescopic observations and more recent observations by the Galileo orbiter suggest that the probe entry site may well have been one of the warmest and least cloudy areas on Jupiter at that time. 
Data from the Galileo atmospheric probe also indicate that there is much less water than expected. The expectation was that Jupiter's atmosphere would contain about twice the amount of oxygen (combined with the abundant hydrogen to make water) as the Sun. But it now appears that the actual concentration much less than the Sun's. Also surprising was the high temperature and density of the uppermost parts of the atmosphere. 
Jupiter and the other gas planets have high velocity winds which are confined in wide bands of latitude. The winds blow in opposite directions in adjacent bands. Slight chemical and temperature differences between these bands are responsible for the collared bands that dominate the planet's appearance. The light collared bands are called zones; the dark ones belts. The bands have been known for some time on Jupiter, but the complex vortices in the boundary regions between the bands were first seen by Voyager. The data from the Galileo probe indicate that the winds are even faster than expected (more than 400 mph) and extend down into as far as the probe was able to observe; they may extend down thousands of kilometres into the interior. Jupiter's atmosphere was also found to be quite turbulent. This indicates that Jupiter's winds are driven in large part by its internal heat rather than from solar input as on Earth. 
The vivid colours seen in Jupiter's clouds are probably the result of subtle chemical reactions of the trace elements in Jupiter's atmosphere, perhaps involving sulfur whose compounds take on a wide variety of colours, but the details are unknown. 
The colours correlate with the cloud's altitude: blue lowest, followed by browns and whites, with reds highest. Sometimes we see the lower layers through holes in the upper ones. 
The Great Red Spot (GRS) has been seen by Earthly observers for more than 300 years (its discovery is usually attributed to Cassini, or Robert Hooke in the 17th century). The GRS is an oval about 12,000 by 25,000 km, big enough to hold two Earths. Other smaller but similar spots have been known for decades. Infrared observations and the direction of its rotation indicate that the GRS is a high-pressure region whose cloud tops are significantly higher and colder than the surrounding regions. Similar structures have been seen on Saturn and Neptune. It is not known how such structures can persist for so long. 
Jupiter radiates more energy into space than it receives from the Sun. The interior of Jupiter is hot: the core is probably about 20,000 K. The heat is generated by the Kelvin-Helmholtz mechanism, the slow gravitational compression of the planet. (Jupiter does NOT produce energy by nuclear fusion as in the Sun; it is much too small and hence its interior is too cool to ignite nuclear reactions.) This interior heat probably causes convection deep within Jupiter's liquid layers and is probably responsible for the complex motions we see in the cloud tops. Saturn and Neptune are similar to Jupiter in this respect, but oddly, Uranus is not. 
Jupiter is just about as large in diameter as a gas planet can be. If more material were to be added, it would be compressed by gravity such that the overall radius would increase only slightly. A star can be larger only because of its internal (nuclear) heat source. (But Jupiter would have to be at least 80 times more massive to become a star.) 
Jupiter has a huge magnetic field, much stronger than Earth's. Its magnetosphere extends more than 650 million km (past the orbit of Saturn!). (Note that Jupiter's magnetosphere is far from spherical -- it extends "only" a few million kilometres in the direction toward the Sun.) Jupiter's moons therefore lie within its magnetosphere, a fact which may partially explain some of the activity on Io. Unfortunately for future space travellers and of real concern to the designers of the Voyager and Galileo spacecraft, the environment near Jupiter contains high levels of energetic particles trapped by Jupiter's magnetic field. This "radiation" is similar to, but much more intense than, that found within Earth's Van Allen belts. It would be immediately fatal to an unprotected human being. 
The Galileo atmospheric probe discovered a new intense radiation belt between Jupiter's ring and the uppermost atmospheric layers. This new belt is approximately 10 times as strong as Earth's Van Allen radiation belts. Surprisingly, this new belt was also found to contain high energy helium ions of unknown origin. 
Jupiter has rings like Saturn's, but much fainter and smaller (right). They were totally unexpected and were only discovered when two of the Voyager 1 scientists insisted that after travelling 1 billion km it was at least worth a quick look to see if any rings might be present. Everyone else thought that the chance of finding anything was nil, but there they were. It was a major coup. They have since been imaged in the infra-red from ground-based telescopes and by Galileo. 
Unlike Saturn's, Jupiter's rings are dark (albedo about .05). They're probably composed of very small grains of rocky material. Unlike Saturn's rings, they seem to contain no ice. 
Particles in Jupiter's rings probably don't stay there for long (due to atmospheric and magnetic drag). The Galileo spacecraft found clear evidence that the rings are continuously resupplied by dust formed by micrometeor impacts on the four inner moons, which are very energetic because of Jupiter's large gravitational field. The inner halo ring is broadened by interactions with Jupiter's magnetic field. 
In July 1994, Comet Shoemaker-Levy 9 collided with Jupiter with spectacular results (left). The effects were clearly visible even with amateur telescopes. The debris from the collision was visible for nearly a year afterward with HST. 
When it is in the nighttime sky, Jupiter is often the brightest "star" in the sky (it is second only to Venus, which is seldom visible in a dark sky). The four Galilean moons are easily visible with binoculars; a few bands and the Great Red Spot can be seen with a small astronomical telescope. There are several Web sites that show the current position of Jupiter (and the other planets) in the sky. More detailed and customized charts can be created with a planetarium program such as Starry Night. 

Jupiter's Satellites
Jupiter has 39 known satellites: the four large Galilean moons, 23 smaller named ones, plus 12 more small ones discovered recently but not yet named. 
Jupiter is very gradually slowing down due to the tidal drag produced by the Galilean satellites. Also, the same tidal forces are changing the orbits of the moons, very slowly forcing them farther from Jupiter. 
Io, Europa and Ganymede are locked together in a 1:2:4 orbital resonance and their orbits evolve together. Callisto is almost part of this as well. In a few hundred million years, Callisto will be locked in too, orbiting at exactly twice the period of Ganymede (eight times the period of Io). 
Jupiter's satellites are named for other figures in the life of Zeus (mostly his lovers). 
Many more small moons have been discovered recently but have not as yet been officially confirmed or named. 

Distance Radius Mass
Satellite (000 km) (km) (kg) 
--------- -------- ------ ------- 
Metis       128 20 9.56e16 
Adrastea  129 10 1.91e16 
Amalthea 181 98 7.17e18 
Thebe      222 50 7.77e17 
Io 422 1815 8.94e22 
Europa 671 1569 4.80e22 
Ganymede 1070 2631 1.48e23 
Callisto 1883 2400 1.08e23 
Leda 11094 8 5.68e15 
Himalia 11480 93 9.56e18 
Lysithea 11720 18 7.77e16 
Elara 11737 38 7.77e17 
Ananke 21200 15 3.82e16 
Carme 22600 20 9.56e16 
Pasiphae 23500 25 1.91e17 
Sinope 23700 18 7.77e16 
Values for the smaller moons are approximate. 

Jupiter's Rings
Distance Radius Mass
Ring (km) (km) (kg)
---- -------- ----- ------
Halo 100000 22800 ?
Main 122800 6400 1e13
Gossamer 129200 214200 ?
(distance is from Jupiter's centre to the ring's inner edge) 


Saturn is the sixth planet from the Sun and the second largest: 
Orbit: 1,429,400,000 km (9.54 AU) from Sun.
Diametre: 120,536 km (equatorial)
Mass: 5.68e26 kg
Saturn is visibly flattened (oblate) when viewed through a small telescope; its equatorial and polar diameters vary by almost 10% (120,536 km vs. 108,728 km). This is the result of its rapid rotation and fluid state. The other gas planets are also oblate, but not so much so. 
Saturn is the least dense of the planets; its specific gravity (0.7) is less than that of water. 
Like Jupiter, Saturn is about 75% hydrogen and 25% helium with traces of water, methane, ammonia and "rock", similar to the composition of the primordial Solar Nebula from which the solar system was formed. 
Saturn's interior is similar to Jupiter's consisting of a rocky core, a liquid metallic hydrogen layer and a molecular hydrogen layer. Traces of various ices are also present. 
Saturn's interior is hot (12000 K at the core) and Saturn radiates more energy into space than it receives from the Sun. Most of the extra energy is generated by the Kelvin-Helmholtz mechanism as in Jupiter. But this may not be sufficient to explain Saturn's luminosity; some additional mechanism may be at work, perhaps the "raining out" of helium deep in Saturn's interior. 
The bands so prominent on Jupiter are much fainter on Saturn. They are also much wider near the equator. Details in the cloud tops are invisible from Earth so it was not until the Voyager encounters that any detail of Saturn's atmospheric circulation could be studied. Saturn also exhibits long-lived ovals (red spot at centre of image at right) and other features common on Jupiter. In 1990, HST observed an enormous white cloud near Saturn's equator which was not present during the Voyager encounters; in 1994 another, smaller storm was observed (left). 
Two prominent rings (A and B) and one faint ring (C) can be seen from the Earth. The gap between the A and B rings is known as the Cassini division. The much fainter gap in the outer part of the A ring is known as the Encke Division (but this is somewhat of a misnomer since it was very likely never seen by Encke). The Voyager pictures show four additional faint rings. Saturn's rings, unlike the rings of the other planets, are very bright (albedo 0.2 - 0.6). 
Though they look continuous from the Earth, the rings are actually composed of innumerable small particles each in an independent orbit. They range in size from a centimetre or so to several meters. A few kilometre-sized objects are also likely. 
Saturn's rings are extraordinarily thin: though they're 250,000 km or more in diameter they're less than one kilometre thick. Despite their impressive appearance, there's really very little material in the rings -- if the rings were compressed into a single body it would be no more than 100 km across. 
The ring particles seem to be composed primarily of water ice, but they may also include rocky particles with icy coatings. 
Voyager confirmed the existence of puzzling radial inhomogeneities in the rings called "spokes" which were first reported by amateur astronomers (left). Their nature remains a mystery, but may have something to do with Saturn's magnetic field. 
Saturn's outermost ring, the F-ring, is a complex structure made up of several smaller rings along which "knots" are visible. Scientists speculate that the knots may be clumps of ring material, or mini moons. The strange braided appearance visible in the Voyager 1 images (right) is not seen in the Voyager 2 images perhaps because Voyager 2 imaged regions where the component rings are roughly parallel. 
There are complex tidal resonances between some of Saturn's moons and the ring system: some of the moons, the so-called "shepherding satellites" (i.e. Atlas, Prometheus and Pandora) are clearly important in keeping the rings in place; Mimas seems to be responsible for the paucity of material in the Cassini division, which seems to be similar to the Kirkwood gaps in the asteroid belt; Pan is located inside the Encke Division. The whole system is very complex and as yet poorly understood. 
The origin of the rings of Saturn (and the other jovian planets) is unknown. Though they may have had rings since their formation, the ring systems are not stable and must be regenerated by ongoing processes, probably the breakup of larger satellites. 
Like the other jovian planets, Saturn has a significant magnetic field. 
When it is in the nighttime sky, Saturn is easily visible to the unaided eye. Though it is not nearly as bright as Jupiter, it is easy to identify as a planet because it doesn't "twinkle" like the stars do. The rings and the larger satellites are visible with a small astronomical telescope. There are several Web sites that show the current position of Saturn (and the other planets) in the sky. More detailed and customized charts can be created with a planetarium program such as Starry Night. 

Saturn's Satellites
Saturn has 18 named satellites plus 12 recently discovered and as yet unnamed ones: 
Of those moons for which rotation rates are known, all but Phoebe and Hyperion rotate synchronously. 
The three pairs Mimas-Tethys, Enceladus-Dione and Titan-Hyperion interact gravitationally in such a way as to maintain stable relationships between their orbits: the period of Mimas' orbit is exactly half that of Tethys, they are thus said to be in a 1:2 resonance; Enceladus-Dione are also 1:2; Titan-Hyperion are in a 3:4 resonance. 
In addition to the 18 named satellites, a dozen more have been reported and given provisional designations but not yet named. 

Distance Radius Mass
Satellite (000 km) (km) (kg) 
--------- -------- ------ ------- 
Pan 134 10 ? 
Atlas 138 14 ? 
Prometheus 139 46 2.70e17 
Pandora 142 46 2.20e17 
Epimetheus 151 57 5.60e17 
Janus 151 89 2.01e18 
Mimas 186 196 3.80e19 
Enceladus 238 260 8.40e19 
Tethys 295 530 7.55e20 
Telesto 295 15 ? 
Calypso 295 13 ? 
Dione 377 560 1.05e21 
Helene 377 16 ? 
Rhea 527 765 2.49e21 
Titan 1222 2575 1.35e23 
Hyperion 1481 143 1.77e19 
Iapetus 3561 730 1.88e21 
Phoebe 12952 110 4.00e18 

Saturn's Rings
Radius Radius approx. approx.
Name inner outer width position mass (kg)
---- ------ ------ ----- -------- --------
D-Ring 67,000 74,500 7,500 (ring)
Guerin Division 
C-Ring 74,500 92,000 17,500 (ring) 1.1e18
Maxwell Division 87,500 88,000 500 (divide)
B-Ring 92,000 117,500 25,500 (ring) 2.8e19
Cassini Division 115,800 120,600 4,800 (divide)
Huygens Gap 117,680 (n/a) 285-440 (subdiv)
A-Ring 122,200 136,800 14,600 (ring) 6.2e18
Encke Minima 126,430 129,940 3,500 29%-53%
Encke Division 133,580 325 78%
F-Ring 140,210 30-500 (ring)
G-Ring 165,800 173,800 8,000 (ring) 1e7?
E-Ring 180,000 480,000 300,000 (ring)
* distance is kilometres from Saturn's centre.
* the "Encke Minima" is a slang term used by amateur astronomers, not an official IAU designation.
This categorization is actually somewhat misleading as the density of particles varies in a complex way not indicated by a division into neat regions: there are variations within the rings; the gaps are not entirely empty; the rings are not perfectly circular. 


Uranus is the seventh planet from the Sun and the third largest (by diameter). Uranus is larger in diameter but smaller in mass than Neptune. 
Orbit: 2,870,990,000 km (19.218 AU) from Sun.
Diameter: 51,118 km (equatorial)
Mass: 8.683e25 kg
Careful pronunciation may be necessary to avoid embarrassment; say "YOOR a nus" , not "your anus" or "urine us". 
Most of the planets spin on an axis nearly perpendicular to the plane of the ecliptic but Uranus' axis is almost parallel to the ecliptic. At the time of Voyager 2's passage, Uranus' south pole was pointed almost directly at the Sun. This results in the odd fact that Uranus' polar regions receive more energy input from the Sun than do its equatorial regions. Uranus is nevertheless hotter at its equator than at its poles. The mechanism underlying this is unknown. 
Actually, there's an ongoing battle over which of Uranus' poles is its north pole! Either its axial inclination is a bit over 90 degrees and its rotation is direct, or it's a bit less than 90 degrees and the rotation is retrograde. The problem is that you need to draw a dividing line *somewhere*, because in a case like Venus there is little dispute that the rotation is indeed retrograde (not a direct rotation with an inclination of nearly 180). 
Uranus is composed primarily of rock and various ices, with only about 15% hydrogen and a little helium (in contrast to Jupiter and Saturn which are mostly hydrogen). Uranus (and Neptune) are in many ways similar to the cores of Jupiter and Saturn minus the massive liquid metallic hydrogen envelope. It appears that Uranus does not have a rocky core like Jupiter and Saturn but rather that its material is more or less uniformly distributed. 
Uranus' atmosphere is about 83% hydrogen, 15% helium and 2% methane. 
Like the other gas planets, Uranus has bands of clouds that blow around rapidly. But they are extremely faint, visible only with radical image enhancement of the Voyager 2 pictures (right). Recent observations with HST (left) show larger and more pronounced streaks. Further HST observations show even more activity. Uranus is no longer the bland boring planet that Voyager saw! It now seems clear that the differences are due to seasonal effects since the Sun is now at a lower Uranian latitude which may cause more pronounced day/night weather effects. By 2007 the Sun will be directly over Uranus's equator. 
Uranus' blue colour is the result of absorption of red light by methane in the upper atmosphere. There may be collared bands like Jupiter's but they are hidden from view by the overlaying methane layer. 
Like the other gas planets, Uranus has rings. Like Jupiter's, they are very dark but like Saturn's they are composed of fairly large particles ranging up to 10 meters in diameter in addition to fine dust. There are 11 known rings, all very faint; the brightest is known as the Epsilon ring. The Uranian rings were the first after Saturn's to be discovered. This was of considerable importance since we now know that rings are a common feature of planets, not a peculiarity of Saturn alone. 
Voyager 2 discovered 10 small moons in addition to the 5 large ones already known. It is likely that there are several more tiny satellites within the rings. 
Uranus' magnetic field is odd in that it is not cantered on the centre of the planet and is tilted almost 60 degrees with respect to the axis of rotation. It is probably generated by motion at relatively shallow depths within Uranus. 
Uranus is sometimes just barely visible with the unaided eye on a very clear night; it is fairly easy to spot with binoculars (if you know exactly where to look). A small astronomical telescope will show a small disk. There are several Web sites that show the current position of Uranus (and the other planets) in the sky, but much more detailed charts will be required to actually find it. Such charts can be created with a planetarium program such as Starry Night. 

Uranus' Satellites
Uranus has 20 named moons (plus 1 recently discovered one which as yet has not been given an official name). 
Unlike the other bodies in the solar system which have names from classical mythology, Uranus' moons take their names from the writings of Shakespeare and Pope. 
They form three distinct classes: the 11 small very dark inner ones discovered by Voyager 2, the 5 large ones (right), and the newly discovered much more distant ones. 
Most have nearly circular orbits in the plane of Uranus' equator (and hence at a large angle to the plane of the ecliptic); the outer 4 are much more elliptical. 

Distance Radius Mass
Satellite (000 km) (km) (kg) 
--------- -------- ------ ------- 
Cordelia 50 13 ? 
Ophelia 54 16 ? 
Bianca 59 22 ? 
Cressida 62 33 ? 
Desdemona 63 29 ? 
Juliet 64 42 ? 
Portia 66 55 ? 
Rosalind 70 27 ? 
Belinda 75 34 ? 
Puck 86 77 ? 
Miranda 130 236 6.30e19 
Ariel 191 579 1.27e21 
Umbriel 266 585 1.27e21 
Titania 436 789 3.49e21 
Oberon 583 761 3.03e21 
Caliban 7169 40 ? 
Stephano 7948 15 ? 
Sycorax 12213 80 ? 
Prospero 16568 20 ? 
Setebos 17681 20 ? 

Uranus' Rings
Distance Width
Ring (km) (km)
------- -------- -----
1986U2R 38000 2,500
6 41840 1-3
5 42230 2-3
4 42580 2-3
Alpha 44720 7-12
Beta 45670 7-12
Eta 47190 0-2
Gamma 47630 1-4
Delta 48290 3-9
1986U1R 50020 1-2
Epsilon 51140 20-100
(distance is from Uranus' centre to the ring's inner edge)


Neptune is the eighth planet from the Sun and the fourth largest (by diameter). Neptune is smaller in diameter but larger in mass than Uranus. 
Orbit: 4,504,000,000 km (30.06 AU) from Sun.
Diameter: 49,532 km (equatorial)
Mass: 1.0247e26 kg
Because Pluto's orbit is so eccentric, it sometimes crosses the orbit of Neptune making Neptune the most distant planet from the Sun for a few years. 
Neptune's composition is probably similar to Uranus': various "ices" and rock with about 15% hydrogen and a little helium. Like Uranus, but unlike Jupiter and Saturn, it may not have a distinct internal layering but rather to be more or less uniform in composition. But there is most likely a small core (about the mass of the Earth) of rocky material. Its atmosphere is mostly hydrogen and helium with a small amount of methane. 
Neptune's blue colour is largely the result of absorption of red light by methane in the atmosphere but there is some additional as-yet-unidentified chromophore which gives the clouds their rich blue tint. 
Like a typical gas planet, Neptune has rapid winds confined to bands of latitude and large storms or vortices. Neptune's winds are the fastest in the solar system, reaching 2000 km/hour. 
Like Jupiter and Saturn, Neptune has an internal heat source -- it radiates more than twice as much energy as it receives from the Sun. 
At the time of the Voyager encounter, Neptune's most prominent feature was the Great Dark Spot (left) in the southern hemisphere. It was about half the size as Jupiter's Great Red Spot (about the same diameter as Earth). Neptune's winds blew the Great Dark Spot westward at 300 meters/second (700 mph). Voyager 2 also saw a smaller dark spot in the southern hemisphere and a small irregular white cloud that zips around Neptune every 16 hours or so now known as "The Scooter" (right). It may be a plume rising from lower in the atmosphere but its true nature remains a mystery. 
However, HST observations of Neptune (left) in 1994 show that the Great Dark Spot has disappeared! It has either simply dissipated or is currently being masked by other aspects of the atmosphere. A few months later HST discovered a new dark spot in Neptune's northern hemisphere. This indicates that Neptune's atmosphere changes rapidly, perhaps due to slight changes in the temperature differences between the tops and bottoms of the clouds. 
Neptune also has rings. Earth-based observations showed only faint arcs instead of complete rings, but Voyager 2's images showed them to be complete rings with bright clumps. One of the rings appears to have a curious twisted structure (right). 
Like Uranus and Jupiter, Neptune's rings are very dark but their composition is unknown. 
Neptune's rings have been given names: the outermost is Adams (which contains three prominent arcs now named Liberty, Equality and Fraternity), next is an unnamed ring co-orbital with Galatea, then Leverrier (whose outer extensions are called Lassell and Arago), and finally the faint but broad Galle. 
Neptune's magnetic field is, like Uranus', oddly oriented and probably generated by motions of conductive material (probably water) in its middle layers. 
Neptune can be seen with binoculars (if you know exactly where to look) but a large telescope is needed to see anything other than a tiny disk. There are several Web sites that show the current position of Neptune (and the other planets) in the sky, but much more detailed charts will be required to actually find it. Such charts can be created with a planetarium program such as Starry Night. 

Neptune's Satellites
Neptune has 8 known moons; 7 small ones and Triton. 
Distance Radius Mass
Satellite (000 km) (km) (kg) Discoverer Date
--------- -------- ------ ------- ---------- -----
Naiad 48 29 ? Voyager 2 1989
Thalassa 50 40 ? Voyager 2 1989
Despina 53 74 ? Voyager 2 1989
Galatea 62 79 ? Voyager 2 1989
Larissa 74 96 ? Voyager 2 1989
Proteus 118 209 ? Voyager 2 1989
Triton 355 1350 2.14e22 Lassell 1846
Nereid 5509 170 ? Kuiper 1949

Neptune's Rings
Distance Width
Ring (km) (km) aka
------- -------- ----- -------
Diffuse 41900 15 1989N3R, Galle
Inner 53200 15 1989N2R, LeVerrier
Plateau 53200 5800 1989N4R, Lassell, Arago
Main 62930 < 50 1989N1R, Adams
(distance is from Neptune's centre to the ring's inner edge)


On August 24th 2006 the International Astronomical Union wrapping up its meeting in Prague declared under their new definitions of what is a planet, Pluto no longer qualified. It is now considered a Dwarf planet.

Pluto is the farthest planet from the Sun (usually) and by far the smallest. Pluto is smaller than seven of the solar system's moons (the Moon, Io, Europa, Ganymede, Callisto, Titan and Triton). 
Orbit: 5,913,520,000 km (39.5 AU) from the Sun (average).
Diameter: 2274 km
Mass: 1.27e22 kg
Pluto is the only planet that has not been visited by a spacecraft. Even the Hubble Space Telescope can resolve only the largest features on its surface (left and above). 
Fortunately Pluto has a satellite Charon. By good fortune Charon was discovered (in 1978) just before its orbital plane moved edge-on toward the inner solar system. It was therefore possible to observe many transits of Pluto over Charon and vice versa. By carefully calculating which portions of which body would be covered at what times, and watching brightness curves, astronomers were able to construct a rough map of light and dark areas on both bodies. 
Pluto's radius is not well known. JPL's value of 1137 is given with an error of +/-8, almost one percent. 
Though the sum of the masses of Pluto and Charon is known pretty well (it can be determined from careful measurements of the period and radius of Charon's orbit and basic physics) the individual masses of Pluto and Charon are difficult to determine because that requires determining their mutual motions around the centre of mass of the system which requires much finer measurements -- they're so small and far away that even HST has difficulty. The ratio of their masses is probably somewhere between 0.084 and 0.157; more observations are underway but we won't get really accurate data until a spacecraft is sent. 
Pluto is the second most contrasty body in the Solar System (after Iapetus). Exploring the origin of that contrast is one of the high-priority goals for the proposed Pluto Express mission. 
Pluto's orbit is highly eccentric. At times it is closer to the Sun than Neptune (as it was from January 1979 thru February 11 1999). Pluto rotates in the opposite direction from most of the other planets. 
Pluto is locked in a 3:2 resonance with Neptune; i.e. Pluto's orbital period is exactly 1.5 times longer than Neptune's. Its orbital inclination is also much higher than the other planets'. Thus though it appears that Pluto's orbit crosses Neptune's, it really doesn't and they will never collide. (Here is a more detailed explanation.) 
Like Uranus, the plane of Pluto's equator is at almost right angles to the plane of its orbit. 
The surface temperature on Pluto varies between about -235 and -210 C (38 to 63 K). The "warmer" regions roughly correspond to the regions that appear darker in optical wavelengths. 
Pluto's composition is unknown, but its density (about 2 gm/cm3) indicates that it is probably a mixture of 70% rock and 30% water ice much like Triton. The bright areas of the surface seem to be covered with ices of nitrogen with smaller amounts of (solid) methane, ethane and carbon monoxide. The composition of the darker areas of Pluto's surface is unknown but may be due to primordial organic material or photochemical reactions driven by cosmic rays. 
Little is known about Pluto's atmosphere, but it probably consists primarily of nitrogen with some carbon monoxide and methane. It is extremely tenuous, the surface pressure being only a few microbars. Pluto's atmosphere may exist as a gas only when Pluto is near its perihelion; for the majority of Pluto's long year, the atmospheric gases are frozen into ice. Near perihelion, it is likely that some of the atmosphere escapes to space perhaps even interacting with Charon. The Pluto Express mission planners want to arrive at Pluto while the atmosphere is unfrozen. 
The unusual nature of the orbits of Pluto and of Triton and the similarity of bulk properties between Pluto and Triton suggest some historical connection between them. It was once thought that Pluto may have once been a satellite of Neptune's, but this now seems unlikely. A more popular idea is that Triton, like Pluto, once moved in an independent orbit around the Sun and was later captured by Neptune. Perhaps Triton, Pluto and Charon are the only remaining members of a large class of similar objects the rest of which were ejected into the Oort cloud. Like the Earth's Moon, Charon may be the result of a collision between Pluto and another body. 
Pluto can be seen with an amateur telescope but it is not easy. There are several Web sites that show the current position of Pluto (and the other planets) in the sky, but much more detailed charts and careful observations over several months will be required to actually find it. Suitable charts can be created with many planetarium programs such as Starry Night. 

Charon ( "KAIR en" ) is Pluto's only known satellite: 
orbit: 19,640 km from Pluto
diameter: 1172 km
mass: 1.90e21 kg
Charon is unusual in that it is the largest moon with respect to its primary planet in the Solar System (a distinction once held by Earth's Moon). Some prefer to think of Pluto/Charon as a double planet rather than a planet and a moon. 
Charon's radius is not well known. JPL's value of 586 has an error margin of +/-13, more than two percent. Its mass and density are also poorly known. 
Pluto and Charon are also unique in that not only does Charon rotate synchronously but Pluto does, too: they both keep the same face toward one another. (This makes the phases of Charon as seen from Pluto very interesting.) 
Charon's composition is unknown, but its low density (about 2 gm/cm3) indicates that it may be similar to Saturn's icy moons (i.e. Rhea). Its surface seems to be covered with water ice. Interestingly, this is quite different from Pluto. 
Unlike Pluto, Charon does not have large albedo features, though it may have smaller ones that have not been resolved. 
It has been proposed that Charon was formed by a giant impact similar to the one that formed Earth's Moon. 
It is doubtful that Charon has a significant atmosphere.


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