There are eight planets in the solar system. The ninth, Pluto, was demoted in 2001 to “Dwarf Planet” status (discussed later).

Planets of the Solar System
This is a montage of planetary images taken by spacecraft managed by NASA’s Jet Propulsion Laboratory in Pasadena, CA. Included are (from top to bottom) images of Mercury, Venus, Earth (and Moon), Mars, Jupiter, Saturn, Uranus and Neptune.

Most of the planets—all except Mercury—orbit in a flat disc called the ‘ecliptic plane’, and most have a markedly circular orbit. Again, Mercury is the odd one out with more of an oval-shaped orbit.

‘Planet’ means ‘the wanderer of the skies’. They are created through the accretion of cosmic dust and debris left over from the formation of stars. Materials accrete initially by random collision. Only when they get big enough will their increasing gravity start to pull in other material. When these materials finally clump enough matter together to reach a diameter of around 800 kilometres (the critical point) there is enough gravity to form the material into a sphere. Heat then builds up and the materials melt, with the heavier elements dropping down towards the core.

Some 4.5 billion years ago our solar system had 100 planets. They orbited the sun and their paths were strewn with debris and dust, and there were lots of impacts. Often these caused the aggregated clumps to disintegrate. But there were enough to hang on to their start material and further grow through these random impacts. When they became large enough they were able to sweep up the rest of the pre-planetary rocks and debris, grow further and limit future impacts.

The Earth was finally formed from debris in this way, and then a massive object crashed into its initial mass, bringing additional, different elements and, the theory goes, resulted in enough debris that accreted into the Moon. To reach our current state took some 500 million years.

Mercury, Venus, Earth and Mars are the four rocky, or “terrestrial” planets; the innermost of the solar system. They have a solid hot iron core, surrounded by liquid iron, all wrapped in a layer of molten rock, and surrounded by an outer crust. Their primary atmospheres were mainly hydrogen, with a small amount of helium. These are light gases and easily escaped the gravity of these smaller forming planets. Combined with their surface temperatures (they are relatively close to the sun) the atmospheres boiled off to leave a rocky core.

Outgassing from the hot inner cores created the secondary atmospheres of Venus, Earth and Mars. The composition of this outgassing was mainly water (over half), with carbon dioxide, sulphur dioxide and Nitrogen. Oxygen arrived later on Earth from the activity of lifeforms.

It was too hot on Venus for water to form so the carbon dioxide remained undissolved in its atmosphere. Mars also ended up with a carbon dioxide atmosphere, but for a different reason. It was too cold for its water to remain a liquid so again, no way for the CO2 to dissolve. In any event, Mars was too small to retain its atmosphere. It simply lacked sufficient gravity to hold onto it. Venus (very similar in size to Earth) has a very thick, impenetrable atmosphere.

Something hit Mercury and blew off its atmosphere and outer layer, leaving an iron ball. Mercury is the smallest and fastest of the eight planets. It is orbiting at 48 kilometres a second. Its other claim to fame is that it has the greatest orbital inclination at 7° (explained later). Venus is about the same size as the Earth and is covered with thick clouds. Mars is a lot smaller than Venus. It has lost all its atmosphere and surface water and looks an orange-red colour. By coincidence, its landmass is close to that of Earth; remembering that two-thirds of the earth’s surface is covered in water.

Beyond Mars are the four gas planets: Jupiter, Saturn, Uranus and Neptune. They have no solid surfaces Most of them avoided much of the violence as they were further away from the sun. (The average distance of Mars from the sun is about 228 million kilometres. Next in line is Jupiter, 778 million kilometres away. The furthest, Neptune, is 4.5 billion kilometres distant).

Uranus was singled out for violence—one of those random things in nature. It was hit so violently that it was knocked on its side. As a result, its axis of rotation is at right angles to the other seven planets.

Uranus and Neptune are far too big to have been made by neighbouring debris. And it is now thought that they were once orbiting a lot closer to the sun, inside the orbits of Jupiter and Saturn. In fact, Neptune was the closer of the two—and both orbited twice as fast as they do today. The theory goes that they were sent out of kilter by the gravitational influences of Jupiter and Saturn and ended up in their current outermost orbits. It is also posited that the asteroid belt may have been involved. This theory holds that the asteroids slowed the two planets down (this would make any planetary body move away from its central point of gravity—the sun in this case—until their mass and orbital speed rebalanced); and so Uranus and Neptune settled into their orbits we know today. It is further suggested that on their way out to their new homes both Uranus and Neptune barrelled their way through the asteroid belt, and the resultant debris flew out creating the Kuiper Belt (also described later).

As for the really big guys (Jupiter and Saturn), there are two theories lurking about: the first and most widely accepted is that while core accretion works well for the formation of the four rocky planets, there is a problem with giants such as Jupiter. The alternative theory for their formation is the “disc instability method”. Here the action occurs in a disc of gas and dust, which is orbiting around a star. Clumps form, contracting and increasing in density to become gas giant protoplanets; the likes of which we see in Jupiter and Saturn. These clumps of denser gas would form quickly (as quickly as a thousand or even several hundreds of years). This would enable planets as large as Jupiter to develop before the protoplanetary disc disappeared.

Jupiter and Saturn are indeed the true giants of the solar system. Each of them is at least ten times bigger than the Earth. Both planets are huge balls of gas and liquid, with only a relatively small core of rock. Jupiter has about 79 moons and Saturn 53 (at the last count). Though Saturn is distinct for her rings, Uranus and Jupiter also have rings, but they are very faint. Both Uranus and Neptune are made of gas and ice.

Beyond Neptune we have the Kuiper Belt, a doughnut-shaped ring of icy objects, of which the most distinguished member is now Pluto, having been relegated from its planetary status, and no longer belonging to the Solar System. “Sorry, mate, you’re out.”

The Kuiper Belt is between 30 AU – 100 AU from the sun, according to Nasa. An Astronomical Unit (AU) is equal to the Earth-Sun distance, or 149 597 871 kilometres. Beyond this is the Oort Cloud, a ring of ice and debris at between 2 000 AU to 100 000 AU (also according to Nasa). And we’ll also talk about all this later.

The outer edge of the Oort cloud is considered the limit of the Sun’s influence. If it really extends to 100 000 AU then our home, the solar system, is approximately 15 trillion kilometres in diameter.

Measured thoughts

As a further introduction to the planets, perhaps we should pause and consider some ideas about measurements, days and angles. For example, what is a sidereal year compared to a solar year? And what does Orbital Inclination to Earth mean?

When you think of a day, you normally think of it as the usual daytime/night-time routine, 24 hours non-stop, the likes of which you can only escape by taking a holiday. That is called a “solar day” because the sun marks out your routine. This solar day is nominally 24 hours but varies based on the position of the earth’s orbit relative to the sun. However, the orbit of Earth is not a perfect circle. If it were there would be no complication. But instead, it is very slightly elliptical, like the oval outline of an egg. This means that a day on Earth is not always the same length: it varies with the position of Earth in her orbit around the sun. From this point of view, the length of day ranges from a few minutes longer than 24 hours to a few minutes shorter, depending on where we are in our annual orbit. But the average is 24 hours.

Incidentally, the mean solar day is increasing very slightly because of the effect of the moon on our ocean tides. There is a drag effect, presumably because of the way the water bulges out due to the pull of the moon. The upshot is that the earth’s rotation is slowing down, while the lunar orbit and its period of revolution about the earth are increasing. Various calculations suggest the mean distance between the moon and the earth is increasing by 3.8 centimetres a year as a result.

Anyway, that’s the solar day: an average of 24 hours.

The sidereal day, on the other hand, is measured based on the earth’s motion relative to the stars. In other words, with each rotation on her axis, it takes about 23 hours 56 minutes for the earth to return to the same position relative to a fixed star. Why the four-minute discrepancy? This is because, earth has moved in her orbit about the sun, and so the relative position of the sun has changed. For the sun to appear in the same position the Earth has to revolve on her axis approximately 1/365th extra every 24 hours (that’s 3.94 minutes). A full orbit takes 365 days and a bit (accounted for by the leap year), and this is where the 365 comes in.

The ecliptic is an imaginary path across the sky transcribed by the passage of the sun. Rising in the east and setting in the west, the sun makes its way across our sky. Because all the planets lie in the plane of the ecliptic (which is the earth’s orbit) we can look for the planets at night along this path. It is as if the planets are occupying the space of the dinner plate, and the sun is in the middle. That flat disc of the dinner plate is where you will find the planets.

The orbital inclination can be seen as the degrees off angle from our dinner plate, equivalent to the orbit of the Earth. Zero degrees—Earth—would mean the planet orbits precisely in the plane of the dinner plate, neither above nor below its surface—a tight disc, precisely in line. Most planets are very close to this orbital plane, around 2° or less; Venus is 3.4°. Only Mercury is the odd one out, orbiting at 7° compared to the orbital plane of the earth.

And just a final point. Where I use one figure, say, for orbits or temperature and so on, you can infer this as the average or “mean” figure. Sometimes I will give the range. For example, the average distance of Jupiter from the sun is 778.4 million kilometres. In its orbit, the distance varies from 741.6 to 817.4 million kilometres, hence describing an elliptical path.
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By Nigel Benetton, science fiction author of Red Moon Burning and The Wild Sands of Rotar

Last updated: Tuesday, 31 March 2020