Among billions of stars, the sun is one star. It is a star of medium size. It is the star nearest to our earth, and that is why, when we look at it, the sun appears far brighter and bigger than the other stars. The sun, together with the celestial bodies that orbit it, makes up the solar system.
Two sentences in that opening paragraph sit oddly together, and it is worth noticing why. The sun is described as a star of medium size, and yet it looks enormously brighter and bigger than any other star in the sky. Both statements are true and there is no contradiction, because the only thing that makes the sun different from the rest is how close it is. Some of the points of light you can see on a clear night are far larger and far brighter than the sun; they simply happen to be an unimaginable distance away. So when you look up at the night sky, you are not looking at small things. You are looking at other suns, and if the earth went round any one of them instead, that one would be the blinding disc in your sky and this one would be a point of light nobody could pick out.
The group made up of the sun and the planets, satellites, asteroids, meteors, meteorites and comets that go round the sun is called the solar system. In the solar system the sun itself is the largest body. The large celestial bodies in the solar system that go round the sun are called planets, and planets do not have light of their own. There are eight planets in the solar system. A body that orbits a planet is called a satellite; the moon is the satellite of the earth, and there are many satellites in the solar system. Mercury, Venus, Earth and Mars are comparatively near the sun and are called the inner planets; these are made of rock and have a solid surface. Jupiter, Saturn, Uranus and Neptune are comparatively far from the sun and are called the outer planets; these planets are very big and are made of gas.
Look carefully at figure 11.17 and something odd appears. The four planets nearest the sun are small and made of rock. The four beyond them are enormous and made of gas. That is far too neat to be an accident, and the reason is one of the most satisfying explanations in this whole book. The solar system formed from a huge cloud of dust and gas around the young sun. Close in, it was extremely hot, so anything light and easily boiled away, such as gas and ice, was driven outwards; only rock and metal, which need enormous heat to melt, could survive there. Far out it was cold, so gas and ice stayed put, and there was vastly more of that material available than there was rock, which is why the outer planets grew so much bigger. So one fact, how hot it was where each planet formed, explains their size, their composition, their solid or gassy surfaces, and even why the outer ones have so many more moons. Notice too where the asteroids sit: in the gap between the two families, which is exactly the boundary.
The eight planets
| Planet | What the book says about it |
|---|---|
| Mercury | The smallest planet and the nearest to the sun. Because it is very near the sun it is very hot in the daytime. It has no atmosphere and no satellite. |
| Venus | The hottest and brightest planet, second from the sun, and the nearest planet to the earth. It is also known as the Morning star and the Evening star. It has no satellite, and because it matches the earth in size it is called the twin planet of the earth. |
| Earth | The planet that has all the environment favourable for living beings. Third from the sun by distance, and it has only one satellite, which is the moon. |
| Mars | Fourth from the sun, and also called the red planet. Because of rock and dust containing the mineral limonite it appears red. After the earth, scientists are studying it as a planet favourable for living beings. It resembles the earth and has two satellites, Deimos and Phobos. |
The book states both facts one after the other and never explains how they can both be true. Mercury is nearest the sun. Venus is the hottest planet. The answer is in the same two entries: Mercury has no atmosphere, and Venus is wrapped in a very thick one. Heat arrives at Mercury in the daytime and simply leaves again at night, because there is no air to hold it, so the night side is bitterly cold. Venus is covered by a dense blanket of carbon dioxide which lets the sunlight in and then makes it very hard for the heat to escape, so the heat builds up and stays, day and night, until Venus is hot enough to melt lead. This is the same effect that keeps the earth warm enough to live on, since without any atmosphere at all our planet would be frozen; on Venus it has simply gone much too far. So being close to the fire matters, but what you are wearing matters more.
| Planet | What the book says about it |
|---|---|
| Jupiter | The biggest planet, fifth from the sun. Its outer surface is covered by cloud and a big red spot is seen in its middle part. It has the most satellites, 67 of them, and it is also called the king of planets. |
| Saturn | The second biggest planet after Jupiter, sixth from the sun. It is surrounded by three flat rings made of pieces of ice, and is also called the strange planet. It has 62 satellites and is the planet with the least density. |
| Uranus | Seventh by position, made of gas and liquid substances. It has rings like Saturn, but because those rings are made of pieces of carbon they cannot be seen easily. It has 27 satellites. |
| Neptune | The planet farthest from the sun, and because it is farthest it is the coldest planet. It has 14 satellites. |
The comparative table of the eight planets
| Planet | Average distance from the sun (km) | Diameter (km) | Day: one spin on its axis | Year: one orbit of the sun |
|---|---|---|---|---|
| Mercury | 5.76 × 10⁷ | 4851.2 | 58.65 days | 87.97 days |
| Venus | 10.7 × 10⁷ | 12035.2 | 243.02 days | 224.7 days |
| Earth | 14.88 × 10⁷ | 12735 | 23 hours 56 minutes | 365.25 days |
| Mars | 22.56 × 10⁷ | 6742.4 | 24 hours 37 minutes 22 seconds | 686.98 days |
| Jupiter | 76.8 × 10⁷ | 1139040 | 9 hours 55 minutes 30 seconds | 12 years |
| Saturn | 144.0 × 10⁷ | 115811.2 | 10 hours 30 minutes | 29.5 years |
| Uranus | 288 × 10⁷ | 50441.6 | 17 hours 14 minutes 24 seconds | 84 years |
| Neptune | 448 × 10⁷ | 48972.8 | 16 hours 6 minutes 36 seconds | 164 years |
Run your eye along the Venus row and check the last two columns against each other. Its day, one spin on its own axis, is 243 earth days. Its year, one full journey round the sun, is 224.7 earth days. So Venus takes longer to turn round once than it takes to go all the way round the sun, which means a day there is longer than a year. That is not a misprint, and it is one of the exercise questions in this chapter. Two habits are worth building from this. First, always read the two columns as separate things: the day column is about spinning, the year column is about travelling, and a planet can be fast at one and slow at the other. Second, notice that you found something strange by comparing two numbers that were sitting next to each other, which is what a table is for. A table is not a thing to copy; it is a thing to interrogate.
The section on Jupiter ends by saying that it orbits the sun at the fastest speed. Check that against the table two pages later and it cannot be right: Jupiter takes 12 years to go round the sun, while Mercury takes 88 days, so Mercury is by far the fastest and Jupiter is nowhere near. What Jupiter really is fastest at is spinning. Its day is 9 hours 55 minutes, the shortest of all eight planets, and for a planet that size that is a remarkable speed. So the sentence has confused the two motions the table is careful to separate, the daily spin and the yearly orbit, and the correct statement is that Jupiter spins fastest, not that it orbits fastest. This is worth spotting for two reasons: it is the kind of slip that can be caught by reading a table properly, and it shows that a book, like a scientist, has to be checked against evidence rather than believed.
Pluto, and how it stopped being a planet
Until the year 2006 it was believed that there were nine planets in the solar system altogether, counting Pluto. In August 2006 the International Astronomical Union decided that it would stay as a dwarf planet and not as a planet. The reasons for removing Pluto from the planets are these: because when Pluto orbits the sun it comes inside the orbit of Neptune as well; and because its mass is very much less.
It is easy to read this as though Pluto changed, and it did not. Pluto is exactly the same lump of rock and ice today that it was in 2005, in the same orbit, doing the same thing. What changed was a definition, and it changed because astronomers had started finding other objects out there of a similar size, and would soon have had to call twenty or thirty things planets or else admit that the word had never been defined properly. They chose to define it. That is worth pausing on, because it is a rare chance to watch science working. A word people had used for centuries turned out to be vague; the vagueness only became a problem when better telescopes found new evidence; and the response was not to argue about opinions but to agree on a rule and then apply it, even though it meant taking a name away from something generations had grown up with. The rule cost Pluto its place, and a great many people were upset. That is what it looks like when evidence is allowed to overrule habit.
Three more kinds of body
| Satellite | A celestial body that goes round a planet is called a satellite, for example the moon. |
| Asteroid | In the wide region that lies between Mars and Jupiter there are bodies smaller than the planets, in their thousands, and these are called asteroids, for example Ceres, Vesta and Juno. They orbit the sun. |
| Comet | The celestial bodies in the solar system that are made of ice and dust are called comets, for example Halley's comet. |
The book gives Jupiter 67 satellites and Saturn 62, and if you look these up you will find larger numbers, because new small moons are still being discovered as telescopes and spacecraft improve. This is worth understanding rather than treating as an error. A moon a few kilometres across, going round a planet hundreds of millions of kilometres away, is an extremely faint thing to see, and every improvement in equipment finds more of them. So the number in any book is really a statement about how good our instruments were when it was printed. For an examination, give the number your textbook gives. But when you meet a different number somewhere else, you now know why, and you know which kind of fact in science is settled and which kind is still being counted.
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