There are many moons

In our solar system, all but two planets have moons, smaller bodies orbiting around them. Pluto, which used to be called the ninth planet out from the Sun, also has a moon.

The only planets orbiting the sun not having any moons, as far as we have yet discovered, are Mercury and Venus. If they turn out to have moons, they must be very small; otherwise we would have seen them.

Our current ideas as to how, stars, planets and moons form are based upon the collapse of a great cloud of gas and dust. Space is full of these. The gravitational balance in these clouds can be precarious, and can be disrupted by a nearby disturbance, such as an exploding star. Since most cosmic clouds are orbiting the centre of our galaxy and churning as they go, they form rotating discs moving in the direction in which the original cloud was moving.

Gradually, the disc shrinks, and a big lump starts to form in the centre. The impact of the falling material and the gravitational squeeze on its core as the body gets bigger and denser eventually raises the temperature to somewhere between 10 and 20 million degrees. At this point nuclear fusion starts, producing huge amounts of energy, and the newly born star starts to shine.

Step back to before this happens. In addition to the big lump at the core of the collapsing disc, other lumps develop at various distances, growing by colliding with the dust along their orbital path, until they sweep up all the material in reach leaving a clear path. Using large radio telescopes to image these collapsing discs at millimetre wavelengths, looking at the radio emissions from the dust, we can see the whole process. We see blank discs, and we see the bullseyes developing as multiple planets form and sweep out clear paths along their orbits. The way the growing planets sweep out their paths is interesting. It is not just a matter of colliding with it. The gravity of the planet pulls at the surrounding dust. However the dust at the inner edge of the cleared range is orbiting closer to the young star and moving faster than the planet, whereas the outside edge is moving slower. The result is that the dust forms a rotating disc around the planet, with the material spiralling in. In this case too, lumps appear in the disc, sweeping up material in the same way as the planet, but on a smaller scale. These lumps become moons. Latest radio images show an example of this actually happening elsewhere in our galaxy. Moons formed in this manner are expected to be much smaller than the planet, in the same way as planets are much smaller than the stars they orbit.

In our solar system, the moons of the planets other than Earth are all less than 6% of the size of the planet they orbit, except for our moon, which is over 27% of the diameter of the Earth. This does not fit the collapsing disc model at all. We got our moon by some other method. Mars has two tiny moons, which are believed to be captured asteroids. So it is possible that when the solar system formed, the four innermost planets: Mercury, Venus, Earth and Mars, did not form moons. Outside the orbit of Mars, Jupiter's gravity prevented a planet from forming, leaving a belt of rubble, the asteroid belt. Mars captured a couple of small asteroids, giving it two tiny moons, smaller than 25 km across. Then, soon after the Earth had formed, a body about the size of Mars is suggested to have collided with it. Some material was blasted off into space; some of it fell back to form the Earth we now live on, and the rest collapsed to form the moon.

Today most of the solar system construction material has been used up. What's left is orbiting way beyond Pluto. Another Earth-shattering impact is unlikely, but cannot be ruled out.

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• Venus lies low in the sunset glow.

• Jupiter and Saturn rise soon after dark.

• The moon will be new on the 8th.

This article is written by or on behalf of an outsourced columnist and does not necessarily reflect the views of Castanet.