A Tour of The Solar System
As we've seen in other parts of this web site, interstellar gas and dust condense to form a star (or stars) and it is probable that planets (a "solar system") often forms at the same time. This concept has been more or less proven with the discovery of planets orbiting around other stars in our immediate neighbourhood in the Milky Way galaxy of which we are a part. These planetary bodies were initially detected through the gravitational effect they have on their parent star, making the star appear to "wobble", but rapid advances in telescope technology have recently made it possible to actually see some of them - especially the larger ones. However, we have a pretty good example of a solar system right here in our own back yard, so it might be fun to go on a tour around all the planets, stopping at each one so that we can learn a little about them.
We might as well start at the place from which all life and action derives - "our" star - the sun. Nowadays we know that the Earth and all the planets rotate around the sun, but not all that long ago people believed that the Earth was the centre of all things, and that everything rotated around us. As we have said on previous pages, our sun is basically a nuclear furnace, burning hydrogen fuel to make heat and light and driving everything that happens, not just on Earth but also throughout the solar system. Without the energy coming from the sun nothing would work and everything would be dead and lifeless. It’s quite a sobering thought to realize that we are all kept alive by a huge ball of hydrogen and helium gas, 1,390,000 kilometers (869,000 miles) in diameter, burning as it does because of the nuclear fusion reactions which are taking place in its core. It is only in the central core that nuclear fusion takes place, generating temperatures up to 15 million degrees centigrade. In these reactions, hydrogen is converted to helium and massive amounts of energy are released. The core is surrounded by other, much cooler layers and the energy generated in the core is transported through these cooler layers to the surface. The surface has the relatively "cool" temperature of only 6,000 degrees C. Quite chilly really. As many will know the sun goes through periods of higher and lower activity. During highly active periods massive eruptions occur, throwing material thousands of miles into the air, and the particles which are always streaming away from the sun at incredible speeds, increase significantly in intensity. At these times, disruption of Earth based communications can take place, satellites can be disrupted, and any astronauts who happen to be up in space at the time can be placed in significant danger. Space flights are often calculated to take place at times when there is a reduced risk of solar flares. So, how old is the sun? When exactly did OUR star form from a cloud of interstellar gas? Well, the best calculations put the date at about 4,000 million years ago, since when it has been burning its hydrogen gas and producing helium plus a lot of energy. A question we need to ask ourselves is how long will it continue to burn in this way and what are the future prospects for it? Using the knowledge we have accumulated by observing other stars similar to our sun, but in different stages of development, we know that it will continue to burn as it is today for about another 5,000 million years, but then a series of events will occur which cause it to become hotter and brighter, eventually extinguishing all life on Earth. But long before that time we will either have left Earth and colonized other planets or we will no longer be around. In any case I don't think we who are around today have too much to worry about!
THE PLANETS IN THEIR ORBITS
Many people at star parties have asked us to explain how the solar system works, and to explain why Venus appears in the morning and evening skies. So before we begin our journey outwards from the sun, let's take a few minutes to look at how the planets orbit the sun and what exactly does this mean to how we see them in our sky? The first thing to say is that all the planets orbit the sun approximately in what is called the "same plane". What this means is that if you look at the solar system from the side, all the planets would seem to be almost in a line. They would of course be at varying distances from the sun, depending on exactly where they were in their approximately circular orbits. This is best described by a diagram.
In the diagram the planets are orbiting around our "smiley" sun - (and please also note that the diagram is not to scale, but for the purposes of this demonstration it will do). If you are an observer standing on the surface of planet Earth (the blue one near the middle), then it is dark whenever you are on the side of Earth which is facing away from the sun. If you look into the night sky, and if any of the outer planets happen to be at the right point in their orbits - Mars, Jupiter, Saturn, Uranus and Neptune - then you would be able to see the fully illuminated face of the planet. Another fact of life is that all the planets travel at different velocities in their orbit, and so Earth is constantly changing position with reference to any one of them. The effect of this is that the outer planets appear to move against the background of stars as seen from Earth, gradually sliding westwards with time. This applies to all the outer planets and so there are good times and bad times to try to see them in the night sky. This constant movement means that eventually they catch up with the sun and slip behind the sun. When they are the other side of the sun from Earth it is of course impossible to see them, and this situation continues until they have moved a little further and start to rise in the morning sky before sunrise. They then continue their gradual movement against the background of stars, finally reaching a point when they are once again visible in the evening skies. So what about the inner planets, Mercury and Venus. Well, if you are that same observer standing on planet Earth, but you want to look at Venus or Mercury then the rules are a bit different. Both of these planets, as you can see from the diagram, are inside our orbit (they are nearer to the sun that we are). This means that when it is dark on Earth (the side of the planet on which we are standing is facing away from the sun) you will never see these two inner planets if you look directly out into space away from the sun. Both of the them are closer in towards the sun, and so will be "behind" us. In order to see them we must look roughly in the direction of the sun. However, the orbit of Venus in particular does take it quite a long way away from the sun, and when it is at its furthest distance, it can appear from Earth to be quite high in our sky. But that's as far as it gets. As soon as it has reached the furthest extent with respect to planet Earth, the curve of its orbit makes it appear to once again fall back towards the sun. Another way of looking at it is that Venus is either behind the sun (invisible and farthest away from us), between us and the sun (invisible and closest to us), or at points in between. It should be noted that if Venus is on one side of the sun then it appears in our morning sky, and if it is on the other side then it is in the evening sky.
Mercury acts in a very similar way, but is so close to the sun that for most of the time it is lost in the solar glare.
In addition to making the inner planets appear to oscillate up and down in the morning or evening sky, being inside our orbit has another effect - they both show phases, just like the moon. We cannot see phases on the outer planets because all of their surface is always fully illuminated, but depending on where we all are in our respective orbits, both Venus and Mercury present to us a series of illuminations ranging from almost fully illuminated (almost behind the sun on the far side of their orbit), to only partially illuminated (almost between us and the sun so we are effectively looking at the unlighted back side of the planet). We hope that this explanation helps you to understand why we can see the planets on some nights but not on others, why there are big differences between what we see of the outer planets and the inner ones and why the inner planets show phases.
And now it's time to visit those planets and see just what they look like, so let's begin our journey outwards, away from the sun. If we keep our eyes open we will first encounter Mercury, a small, rocky planet, heavily pock-marked with craters, and looking more than a little like our moon. Here is a photograph of Mercury, taken some time ago by a spacecraft. You can see just how scarred the surface is, and these marks are the result of the same cosmic bombardment which took place early in the formation of the solar system, and which gave Earth's moon the appearance which it has. The main reason that Earth's surface does not look like this is not because meteors missed us, but rather because we have an atmosphere and oceans. Both wind from our atmosphere and the erosion effects of the oceans and rain have removed most traces of impact craters from Earth. But some can still be found, and a lot of you will know of the famous Meteor Crater in Arizona which has survived for thousands of years because of the dry conditions in the desert region in which it is located.
Mercury has little or no atmosphere, and certainly no liquid oceans, and so the craters are still there. By the way, did you know that it is just as difficult to launch a spacecraft inwards towards the sun to survey the inner planets as it is to send one away from the sun to explore the outer planets? This is because you first have to raise the speed of the spacecraft to 24,000 mph to escape the pull of Earth’s gravity, but then you have to slow it down again so that it will “fall” inwards towards the sun, and this uses a lot of fuel. You've probably heard of space scientists using a thing called a “slingshot” maneuver to boost the speed of a spacecraft, and this technique has been used very successfully to send spacecraft to most of the outer planets. Well, if you are going to Mercury there are less available planets to use for this, and so we have to use much more fuel. Mercury is a small planet, only 4,900 kilometers (3,060 miles) in diameter. It travels around the sun in 88 of our days in what we call an "elliptical" (egg-shaped) orbit, because the distance varies a bit, but on average it's about 58 million km (36 million miles) from the sun. Whether at it's nearest or farthest distance from the sun, you can bet that Mercury is hot, and no life can exist there. So how and where can you see Mercury in the sky? Well, because it is so very close to the sun it is very difficult to see. In fact, many people go through their whole lives without ever seeing it. It is one of those things which you've got to be looking for to be able to see. As it orbits around the sun inside our orbit, to us it appears to just pop up above the horizon in either the early morning before sunrise, or early evening just after sunset. It appears as a reddish star, very close to the horizon, and only remains visible for a few days, before plunging back to the horizon, only to reappear a short time later on the "other side". Looked at through a telescope it is a disappointing object because of the thick atmosphere of the Earth which we have to look through when observing an object so close to the horizon. Mercury does show phases just like the moon, the phase depending on exactly where it is in its orbit. If you want to see this elusive planet the best thing to do is check when it is supposed to be at what is called "maximum elongation" from the sun (as far east or west as it gets) and then go visit a local astronomy club star party (there is always one somewhere close) and have an astronomer show it to you. But do make sure the sky is clear of clouds all the way to the horizon before you set out.
Leaving Mercury and heading outwards away from the sun our imaginary spacecraft will need to take us a distance almost twice that which we have just covered, until at 108 million km (67 million miles) we reach the planet Venus. Venus has always puzzled astronomers because it is so similar to the Earth in many respects and for a long time it was felt that on Venus we had the best chance of finding life on a planet other than our own. However, answers to that question were not possible for a long time because Venus' surface is always completely covered in a thick blanket of clouds, which have never cleared. The planet itself is almost the same size as Earth - 12,100 km (7,560 miles) and Venus moves in an almost circular orbit, it's "year" being 225 of our days. In this photograph of Venus you can clearly see the thick clouds which completely obscure the surface. The clouds are 35 miles deep, starting at 25 miles from the surface and ending at 60 miles. They are composed of carbon dioxide, with small amounts of oxygen, nitrogen, sulphur dioxide and water vapour.
The surface of Venus is extremely hot, with a temperature high enough to melt lead (464 degrees C). This is partly because it is closer to the sun, but mostly because the thick clouds trap most of the incident sunlight in a type of runaway “greenhouse effect”. So Venus is not a very hospitable planet and the surface pressure caused by the very dense atmosphere is about 90 times what it would be at the Earth’s surface. In effect we would be squashed, even if we could breathe there! Venus has no moons and, like Mercury, orbits inside the Earth’s orbit. But it is much easier to see in the sky because it is so much farther away from the sun than is Mercury. Venus is the very bright object which you can often see in the morning or evening skies, either following the sun down after sunset, or rising before the sun in the morning. In fact it can be so bright that it has been known to cast a shadow, and of course it is all the sunlight which is reflected back into space from the clouds which make it such a bright object. In a telescope Venus can be a very disappointing object, because the clouds reflect back almost all of the light falling on them, making it look just like a bright, white disk. However, because it can approach relatively close to the Earth (as close as 26 million miles) it can present quite a large image in even small telescopes. Venus shows phases, like Mercury, and as it gets closer to us in it's orbit the size grows and the phase decreases. When it is farthest from the sun from our perspective it is a "half Venus" but as it dives down towards the sun over a period of weeks the phase reduces to a large, thin crescent. That is when observing Venus can be the most fun, because the reduced brilliance of the image and the nice "crescent moon" appearance make it visually very attractive. Also, the planet appears to move very quickly in relation to Earth at this point in its orbit, and so the day to day changes can be quick and fascinating to follow.
Continuing our journey out from the sun we arrive at what appears to be an oasis of blue and green in the emptiness of space. We have of course arrived at planet Earth. Here is a photograph of our planet, taken from space.
On this photo you can clearly see Africa and the Red Sea, with the blue oceans standing out against the blackness of space. At an average distance of 150 million km (93 million miles) our home world is positioned just far enough from the sun that liquid water can exist on the surface, but not so far out that it would freeze permanently. Earth is two thirds covered by water and as you know, the oceans are literally teeming with life. The oceans have a significant effect on our weather, creating the clouds from which the rain falls which waters the land and encourages the plant life to grow. The oceans also keep the land less cold during winter months than they otherwise would be, and less hot during summer than they otherwise would be. Without the oceans our weather would be much more severe. A final role played by the oceans is as an absorber and emitter of carbon dioxide, and although this mechanism features strongly in the global warming debate (greenhouse effect) it will not be mentioned further here). You will remember that the absence of oceans is one factor which has left Mercury and our moon scarred forever by meteor impacts. In our continuing search for life throughout the universe, scientists believe that the presence of liquid water is the one thing which a planet must possess if life is to be able to flourish. Earth takes 365.25 days to travel once around the sun, has a diameter of 12,750 km (7,970 miles) and has one moon. Actually, the moon is rather interesting from a solar system perspective. It is one of the largest moons of any of the planets (although one or two are actually larger) and it is positioned at a distance from the Earth which makes it possible, during an eclipse of the sun, for the moon to just exactly cover the solar disc. Just a tiny bit further away and it would appear smaller and therefore unable to do this, and just a tiny bit closer and we would be unable to see the solar corona during an eclipse because the body of the moon would hide it. This must just be an incredible coincidence, because what other explanation could there be?
Jumping aboard our rocket ship we set off once more into the darkness of space, leaving our world behind. This time we are heading for another world where the existence of life has been predicted in the past. We are talking of course about Mars, the red planet. The big difference with Mars is that, unlike Venus our latest discoveries seem to be increasing the chances of finding some form of life there, not decreasing them. Latest photographs and observations from some of the motorised rovers operating on the Martian surface have shown what seems to be very strong indications of flowing water in the recent past, and large quantities of sub-surface water have been found using ground penetrating radar. Remember what we said earlier about the importance of liquid water to the existence of life? Exploration continues using ever more sophisticated spacecraft, and it can only be a matter of time......
The above photographs show Mars from space and also the surface as seen by one of the spacecraft on the surface. Mars is only a small planet (about 6,800 km in diameter), which means that it has only been able to hold on to a thin atmosphere, about the same as that on Earth at an altitude of about 80 miles, so it is not breathable. This means in turn that the surface of the planet is not protected from the dangerous effects of the sun’s cosmic radiation. The famous red colour of the surface is mostly due to the oxygen which was once present in the atmosphere, combining with iron on the surface to produce iron oxide - rust! Mars orbits in a little under two Earth years at an average distance of 228 million km (142 million miles) from the sun. This means, if you consider that it also has very little atmosphere, that it gets pretty cold. Surface temperatures in mid summer near the equator can get as high as perhaps 20 degrees C (68 F), but most of the time it gets extremely cold. So cold in fact that the southern polar cap, which appears every winter is carbon dioxide freezing out of what is left of the atmosphere. We learn more and more about Mars every day. Mars has two tiny moons, Phobos and Deimos. Phobos in particular is so close to Mars that if you were standing on the surface you could just about see it moving and it would rise and set more than once a day. There was some speculation years ago that at least one of these moons was so small that it could be an artificial satellite, launched centuries ago by the now extinct Martians, but we now know that this is not the case. I find that a little sad, because one of the wild theories I have had for a long time is that Mars did once teem with life, but some natural or Martian man made disaster overtook them and destroyed their world. They therefore jumped aboard space ships and headed for Earth - which is how Earth came to be populated. It's far fetched I know, but in my mind there is always just a chance that we are all Martians! Mars takes 780 of our days to orbit the sun and it's day is almost exactly the same as ours, at 24.5 hours. If you want to see Mars in the sky it can either be spectacular or pretty boring, and that's all because it is so very small. It orbits the sun slower than we do, being farther out, and so is constantly lagging behind us. Its distance from us also varies a lot, from only about 56 million km (35 million miles) at closest approach, to nearly 300 million km (188 million miles) when we are on opposite sides of the sun. In a telescope this means that it varies from a tiny red disc - even in the largest amateur telescopes, to quite a nice sized disc on which you can see easily see surface features. In the sky it moves among the constellations, appearing as a bright red star and varying in brightness depending on how far away it is.
Now, something strange happens to us in our journey. We have to cross a huge gap before we reach any other planets. In this gap we pass several small lumps of rock, and at this point in the solar system we are crossing what is known as the "Asteroid Belt". This area has a relatively high concentration of asteroids, and it is likely that they are pieces of a planet which never got itself together. It is also possible that it once was a planet but it was blown apart by something, but current theory is that these are the bits that were left over from the formation of the solar system. We have reached a region far enough from the heat and radiation of the sun, where the temperature has fallen sufficiently for the gases from the original solar nebula to condense, as we will see when we travel just a little bit further. In fact, we have to travel an additional 550 million km (344 million miles) before we finally reach mighty Jupiter, which is also known as the "King of the Planets". Jupiter is the largest planet in the solar system, being more than twice as massive as all the other planets combined! The planet is 143,000 km (89,300 miles) in diameter, more than ten times that of the Earth and it takes 11.86 years to jog around the sun once. It has 318 times more mass than our Earth, and is made almost entirely of hydrogen (90%) and helium (10%) gases. There are also small amounts of ammonia, methane and water, but the make up of Jupiter is very close to what scientists believe was the original "mixture" of elements from which the solar system was formed. Even more interesting is the fact that Jupiter is just about as large in diameter as a gas planet can be. If any 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 than this only because its internal (nuclear) heat source creates an internal pressure, preventing gravitational collapse, but Jupiter would have to be at least 80 times more massive to become a star. It does however have a very strong magnetic field and temperatures within the core are known to be very high, mostly due to compressional forces. Jupiter is indeed a weird and wonderful place.
In this photograph of Jupiter you will see two things immediately. The first is that the edges of the planet are fuzzy. This is because Jupiter is made of gas and so has no clearly defined edge. Effectively we are looking "through" the edge of the planet. The second thing you see are "stripes" or bands. These are caused by clouds in the atmosphere being dragged into bands by Jupiter's rapid rotation. Despite being so large Jupiter rotates on it's axis extremely rapidly and the "day" is only about 10 hours long. It is this incredible rotational speed which creates the banding on the planet's surface, and the brown bands are known to be ammonium hydrosulphide gas, not the sort of thing we would want to be breathing! One of the most famous features on Jupiter is the "Great Red Spot", which has been seen by Earthly observers for more than 300 years. The spot is an oval about 12,000 by 25,000 km, which is 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 spot is a high pressure region whose cloud tops are significantly higher and colder than the surrounding regions. It is not unlike a "storm" in Jupiter's atmosphere, and similar structures have also been seen on Saturn and Neptune. It is not known how such structures can persist for so long, but the colour certainly has changed and it is no longer red, but a pale salmon pink colour. This usually causes disappointment when people observe it through a telescope, because the spot is surprisingly difficult to see, and most people expect to see a clear, red patch.
Through a telescope Jupiter is fascinating, mostly because it is large enough to appear as a disc, even in moderate power binoculars. The cloud bands can be clearly seen in even a small telescope, and also the four largest moons, which are named the "Galilean Moons" after the man who first discovered them, the Italian scientist Galileo Galilei. These moons are particularly fascinating, because they are just like a mini solar system and we can sit and watch their movements around Jupiter. Their names are Io, Europa, Callisto and Ganymede and it is not unusual for one of them to "transit" or move across the face of the planet. When they do so owners of moderate sized telescopes are able to see the tiny shadows which they cast on the surface, and in larger instruments it is even possible to see the moons as discs, highlighted against the background of the planet below. To find Jupiter in the sky you really need to know the stars and also be able to recognize something which is bright, but clearly not a star. This takes some practice, but Jupiter is usually very bright, and shines with a steady light. It also follows an imaginary line drawn on the sky, which we call the Ecliptic. This is the line which the moon and all the planets except Venus and Mercury follow as they trace their paths across the sky, and anyone with only a passing interest in astronomy should be able to quickly become sufficiently familiar with the sky to be able to find Jupiter. Another fascinating scientific fact is that Io, Europa and Ganymede are locked together by tidal forces into 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 and eight times the period of Io. This type of orbital balancing act is well known and there are complicated scientific reasons why it is a stable configuration. Jupiter a very large number of moons and spacecraft are discovering more all the time. One of the most remarkable discoveries of our lifetime could come about when we finally learn the secrets of Europa. It is known that the surface is covered with water ice beneath which lie oceans of liquid water! There is the distinct possibility that life could exist in those oceans and spacecraft to explore those oceans are being prepared for their journey. We could fill several pages with discussion of the Galilean moons themselves, but for now it is time to move on. Now we have to jump back into our spaceship and leave this interesting place - but there is good news! Our next port of call on this voyage of discovery is an equally interesting planet, the one with the wonderful rings - the planet Saturn.
To reach Saturn we again have to travel a long, long way. In fact we have to travel almost as far away from the sun as we have already travelled to reach Jupiter. Saturn lies at an average distance from the sun of 1,430 million km (893 million miles), takes 29.4 years to complete pone orbit and is only slightly smaller than Jupiter at 120,500 km in diameter (75,300 miles). It is also a gas planet and like Jupiter is made up mostly of hydrogen and helium, but in a ratio of 75/25 this time. Saturn is another planet which rotates very quickly, and the "day" is just over 10 hours long. The planet is also very squashed at the poles, more so than is Jupiter, and it's north - south diameter is 10% less than that across the equator. It has a very low density - only 0.7 - which means that if there were a bath large enough it would float! It also has a large number of moons, most small but some larger ones as well. There are cloud bands, but they are much fainter than on Jupiter, and hard. What is not hard to see are the fantastic rings - as in this superb picture of Saturn.
The first sight people get of Saturn through my telescope is usually accompanied by gasps of amazement and disbelief. I am accused of having placed a colour slide over the end of the telescope and am often asked if the image is real. And this is because Saturn does look very unreal, hanging against the black background of space, with the rings clearly defined. At certain times you can even see the shadow of the planet on the rings, and the largest gap in them, called the Cassini Division, named after the astronomer Cassini who first observed the gap. Though they look continuous from the Earth the rings are actually composed of innumerable small particles of water ice and rock, each in an independent orbit. They range in size from a centimetre or so to several meters. A few kilometer sized objects are also likely. The rings are extraordinarily thin - though they are 250,000 km or more in diameter they're no more than 1.5 kilometers thick. Despite their impressive appearance there's really very little material in the rings -- if they were compressed into a single body it would be no more than 100 km across! In fact, Saturn is not the only world to have rings. Since we started sending spacecraft to visit the outer planets faint rings have been discovered around Jupiter, Uranus and Neptune. We don't know why the rings form around these outer planets, but they are basically unstable and are thought to be maintained by the gradual fragmentation of minor moons over time. Saturn's moon Titan is one of the largest in the solar system and was the target of the Cassini space craft, which successfully landed a probe - called the "Huygens Probe" on the surface in 2004. The reason for all the interest in Titan is that alone of all the satellites in the solar system, it possesses a significant atmosphere. At the surface the atmospheric pressure is more than 1.5 bar (50% higher than Earth's). It is composed primarily of molecular nitrogen (as is Earth's) with no more than 6% argon and a few percent methane. Interestingly, there are also trace amounts of at least a dozen other organic compounds (i.e. ethane, hydrogen cyanide, carbon dioxide) and water. The organic compounds are formed when methane, which dominates in Titan's upper atmosphere, is destroyed by sunlight. The result is similar to the smog found over large cities, but much thicker. In many ways, this is similar to the conditions on Earth early in its history when life was first getting started and we might be looking at another contender for extra-terrestrial life! Isn't it fascinating that we are coming across all these potential opportunities for life so very far from our own world? Only time will tell if our guesses were correct. In powerful binoculars (perhaps 18 x 70's) you can see the rings and perhaps Titan, but in even a small telescope it can be a magnificent sight. Do yourself a favour if you have never seen it. Find a telescope which you can look through, and see for yourself perhaps the most impressive object you will ever see in the night sky.
So where do we go next? Well, this is beginning to get a little silly, because to reach the next planet we have to travel almost double the distance from the sun as we have already travelled to reach Saturn. Uranus is an incredible 2,870 million km (1,794 million miles) from the sun and this is why in even a large amateur telescope it only ever shows as a small greenish disk. At this great distance it takes nearly 84 years to orbit the sun once. Even though Uranus is technically a gas "giant", it is less than half the size of Saturn, so couple the smaller size with the doubling of distance and you can quickly see why it is so tough to see. Even so it is bright enough that a keen eye in a very dark sky can see it as a faint star, and it is easy to pick out in binoculars. Uranus was discovered in 1781 by William Herschel. It had been catalogued prior to that date, but had been assumed to be a star. The composition of Uranus is different from Jupiter and Saturn, being composed mostly of rock and 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, but minus the massive hydrogen envelope.
The most fascinating thing about Uranus is that it is rotating on it's axis while lying on it's side, almost as if it has toppled over or been knocked over by something. Nobody really knows the truth behind why this is, and Uranus is the only planet to exhibit this strange behavior. Scientists are still arguing over which end is which - i.e. which is the north and which the south poles. The "toppled over" aspect is clear to see in this spacecraft image which shows the thin rings as well as the planet. Even though in this light the planet looks blue, in a telescope Uranus definitely has a greenish colour. 21 satellites (moons) of Uranus have been listed, but more are being discovered all the time. Only four of them are significant in terms of size or special areas of interest.
As if we weren't far enough out in space, there is still another gas giant beyond Uranus. If we coax our spaceship just a little farther - actually another 1,630 million km - we will come across a planet almost the same size as Uranus, and this one is called Neptune. Neptune is 4,504 million km (2,815 million miles) from the sun and it takes 164 years to go once round. It was discovered in 1846, but history records that it was actually observed by Galileo in 1613! Unfortunately, conditions did not allow him to make a vital follow up observation, and so it was left to wander around the sun for another 233 years, long enough for it to go round more than once. 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 be more or less uniform in composition. 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 is a really pretty blue colour, largely the result of absorption of red light by methane in the atmosphere but there is some additional as-yet-unidentified colouring agent 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, which is odd when you consider that it is so far from the nuclear furnace which we said at the beginning is powering everything which happens in the solar system. Like Jupiter and Saturn, Neptune has an internal heat source -- it radiates more than twice as much energy as it receives from the Sun.
Here is a picture of Neptune clearly showing the pretty blue color and some of the white markings in the cloud structure. Neptune has eight satellites, but only one - Triton - is large enough to bother with.
So that is your tour of our solar system. We've come a very long way from Mercury and as you can see we do have quite a variety of different planets to look at. We've seen solid planets and gaseous planets, large ones and small ones, hot ones and cold ones. But the one thing they all have in common is - they are all visible in a telescope!