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You are, of course, welcome to consult and make use of my pages at will. However, if you have found them useful in any appreciable way, please consider sending a small disbursement (even something as small as 1 Euro/Pound/Dollar/etc. would be received with appreciation).

B. Daugherty, 31 MH, Portsmouth PO5 3JG, Britain.

The Outer Planets of the Solar System

The outer planets are gas giants, apart from Pluto. Nowadays, it is known that all gas giants have rings, which appear to be held in place by 'shepherd satellites'.


Jupiter rotates very quickly - its rotation period is about 9h 50m - the shortest 'day' of all planets (rotation near the poles is 5 minutes less than at the Equator).

Jupiter has dark belts and light zones. These belts and zones, like similar features on other gas giants, are caused by atmospheric convection and the rapid rotation of the atmosphere. The zones are regions of upflowing gas which forms clouds at the top of their 'columns'. The bands are where the gas is 'falling' back down again.

Great Red Spot on Jupiter

The Great Red Spot (14,000 km wide and 25,000 km long) has been studied for over 300 years, since the work of Giovanni Cassini (it was possibly seen by Robert Hooke in 1664). It rotates counter-clockwise and moves around the planet. Despite its name, it is only strikingly red at intermittent periods. It is cooler than its surroundings but is about 25 km above it. The basic nature of these storms are known from similar phenomena on Earth. On Earth, we have cyclones, which are areas of low pressure into which neighboring gases spiral, and anticyclones, a region of high pressure, out of which gases spiral. The Great Red Spot is an anticyclone. It lies in the South Tropical Zone.

The color is less red nowadays then formerly, seemingly described as a 'vivid red' in the 19th century. It can noticebaly change hue within a short period. At the latitude of the Red Spot, the rotation period is 9h 56m.

Clouds are largely ammonia

The strong metallic field is asumed to be caused by Metallic Hydrogen - a form adopted by hydrogen under the extreme conditions in Jupiter's interior.

It is currently estimated to have a molten silicate core of about 10-15 ME.

It has large gravitational effects on comets.


Jupiter has four prominent satellites - Io, Europa, Ganymede, Callisto, which are known collectively as the Galilean Satellites (Galileo discovered them on 7th January 1610). They lie in the Equatorial Plane and are easy to detect with a telescope. Europa is the brightest. Ganymede is the largest moon in the Solar System, and is larger than Mercury.

The four satellites show decreasing levels of activity with distance from Jupiter, a feature of other satellite systems of the outer planets.


Mean Radius

Orbital Period
(Earth days)




volcanic - but with Sulfur




smoothest body in SS




largest moon in SS




These satellites can obviously disappear from view by moving behind Jupiter (occultation), but they can also disappear by being eclipsed before moving around the back of the planet. In fact the three nearest satellites are eclipsed once every orbit, but Callisto can go without being eclipsed for months or even years.


Voyager 1 detected active volcanoes on Io, and now more than 80 have been detected.


Europa is covered by dark cracks. There are very few craters. It has the appearance that broken sea ice has on Earth.


Ganymede is not as smooth as Europa, it is more heavily cratered than Io and Europa. Parallel ridges suggest some sort of activity in past, these markings looking similar to those made by glaciers on Earth.


Callisto is covered in impact craters, the largest of which is of diameter 600km. No evidence of volcanic activity.


At opposition it can reach -0.3.

Similar in size to Jupiter, and has a similar rotation period of 10h 39m. Belts and zones are visible on the surface, although the general details are less distinct than Jupiter. Other features of Jupiter also appear on Saturn - the planet is cooler though so there is less visible turbulence in its cloud formations.

It has the lowest average density of all the planets, and could float in water.

A ring system is definitely visible (except in those years when the ring system is edge-on to us.

It was in 1657 that Christaan Huygens was able to see that these 'projections' which had been mystifying observers, were actually rings. These rings extend out to about 65,000 km but are only a few kilometers thick - it is possible to view stars thru them. Three rings are visible from Earth - the A (the outermost),B, C rings. The Cassini Division is the gap between the two brightest rings, A and B. There is a narrower gap in ring A called the Encke Gap, which separates the A ring into an outer dark and an inner brighter ring. The C ring is also referred to as the Crepe Ring, beacause it is very thin and translucent.

The White Spot was famously discovered by the British comedian, Will Hay. This spot was not seen by the Voyagers in 1980 and 1981, although a similar feature was observed in 1990. This particular spot was viewed to spread out into a series of linked vortices.

Moons of Saturn

Because of Saturn's tilt, its satellites can be observed above or below the planet (in contrast to Jupiter's which are observed to be more or less 'in line').


By far the biggest moon is Titan, first discovered by Huygens, and the second largest moon in the Solar System (it is also larger than Mercury). It was visited by the Huygens space probe in 2005. It is the only moon in the Solar System with a substantial atmosphere, this atmosphere being, like Earth, predominantly Nitrogen. This atmosphere prevents us directly viewing the surface.

It circles Saturn in about 16 days, and has a magnitude of 8.


Uranus was discovered in 1781 by the German astronomer William Herschel who was resident in Bath at the time.

Its axis of rotation is at an angle of an astonishing 98° as well as the sense of rotation being retrograde. Along its orbital path, the planet will maintain an aspect just like the gyroscope that it actually is (see diagram below).

The planet looks blue in images because of methane absorbing red light preferentially.

Unlike the other giant planets, it appears to have no internal heat source - Neptune, despite being farther out, has a similar surface temperature. The axis of its magnetic field is noticeably inclined to the axis of rotation and does not even pass thru the center of the planet.

In March 1977, studies of the planet occulting a star lead to the discovery of its ring system.

Orbit of Uranus
Moons of Uranus

The two largest moons are Titania and Oberon which are about half the size of our Moon.


The innermost moon of Uranus orbits the planet at a distance of 129,400 km, every 1.4 days. The moon is dark and icy. Its diameter is 480 km and its density is 1.3 times that of water. Miranda is unique because its surface looks as if it was a jigsaw puzzle that had been assembled in a hurry - it looks as though it has experienced dramatic geological activity. The surface also has some impact craters. Miranda's surface may be mostly water ice, with the low density body being made of silicate rock and methane-related organic compounds.

Other descriptions

  • Its surface is unlike anything in the solar system with features that are jumbled together in a haphazard fashion.

  • looks like a small world that was torn into many pieces and then put back together again the wrong way.

  • Miranda is criss-crossed by huge fault canyons as deep as 20 kilometers, terraced layers and a mixture of old and young surfaces, with patchwork regions of broken terrain indicating intense geological activity in the moon's past. The younger regions might have been produced by incomplete differentiation of the moon, a process in which upwelling of lighter material surfaced in limited areas.

  • Some scientists believe that Miranda may have been shattered as many as five times during its evolution. After each shattering the moon would have reassembled from the remains of its former self with portions of the core exposed and portions of the surface buried.

  • Or else, it is thought that this activity is likely powered by tidal forces from Uranus, but another theory suggests that Miranda was at some point struck by a massive object that partially shattered the moon.


Neptune was discovered at Berlin Observatory in 1846, acting on the predictions of Urban Leverrier.

Its composition is very close very close to that of Uranus, and surprisingly it seems to have similar temperature despite being further from the Sun.

Its atmosphere appears to rotate in the opposite direction to the planet's rotation.

The Great Dark Spot, similar to Jupiter's Great Red Spot, is involved with most violent winds known in Solar System, at 2000 km/h.

A cloud called the Scooter was seen circling the Great Dark Spot by Voyager 2 in 1989. Av Hubble image of 1994 showed that the scooter had moved from the Southern Hemisphere to the Northern Hemisphere.

Its magnetic field is inclined at an angle of 47° to the axis of rotation.


Triton has a synchronous orbit, but is unique among the large satellites in that its orbit is retrograde.

There are very few craters visible; the surface is relatively young. This suggests that Triton has been "repaved". Almost the entire southern hemisphere is covered with an "ice cap" of frozen nitrogen and methane.

Triton has a little atmosphere.

Other descriptions:

  • Scarps and cracks on its ice surface. At this temperature, ice is more rigid than ice on Earth. Very flat areas suggest that these areas were once liquid water.

  • Triton is scarred by enormous cracks.

  • There are extensive ridges and valleys in complex patterns all over Triton's surface. These are probably the result of freezing/thawing cycles.

  • The most interesting (and totally unexpected) features of this world are the ice volcanoes (or geysers). The eruptive material is probably liquid nitrogen, dust, or methane compounds from beneath the surface. One of Voyager's images shows an actual plume rising 8 km above the surface and extending 140 km "downwind". Dark streaks are left on the surface.

Triton was discovered from Liverpool, very shortly after the planet itself was discovered.


Pluto has quite a high eccentricity, such that although nominally further away than Neptune, for part of its orbit it does move closer to the Sun than Neptune (for example, between 1979 and 1999).

Furthermore, its orbit is noticeably inclined to the plane of the Solar System, much more than the other planets.

Its satellite Charon stays fixed over the same point - its orbital period is the same as Pluto's rotational period. Pluto spins on its side, similar to Neptune, so Charon orbits perpendicular to the plane of the Solar System. Charon's diameter is only half the size of Pluto, so probably right to consider it as a 'double planet' system.

In this sense, the rotation period of both is equal to the period of revolution of the pair about each other - this means that the same faces of the two bodies always point at each other. The period is only six days, so they are close together.

Pluto, Charon and Triton appear to be similar. If this so, then they are likely to be members of a much larger class of objects, rather than a small class with Pluto and Charon meeting by chance and Triton encountering Neptune by chance. Since 1992, over 100 Trans-Neptunian objects have been discovered. It is proposed that this is indicative of the existence of a large number of Kuiper Belt objects of which Pluto, Charon and Triton could form a part.

Pluto was discovered in 1930 by Clyde Tombaugh, an amateur who was doing some work as an observer at the Lowell Observatory in Flagstaff, Arizona. Its satellite, Charon, was discovered in 1978 by James Christy at the US Naval Observatory.

Moons of Saturn

Crab Nebula

Big Red Spot on Jupiter