Planets, stars and things between – Skywatching

If you want to make a comet, asteroid, planet, star or many other bodies, the recipe is the same. There is just one ingredient: the cosmic clouds of gas and dust.

The procedure is the same, one just gathers some of the material into a lump. What you end up with is entirely determined by how big a lump you are working with. The mass of the lump determines two critical quantities, the pressure and temperature in the core of that body.

The temperature at the core of the Earth is around 5,200 C and the pressure some 3.6 million times the atmospheric pressure at the surface. The pressure comes from the weight of the overlying rock. The internal heat comes from two sources—the energy released by the impacts of incoming objects when the Earth formed, some 4.5 billion years ago, and from the decay of radioactive elements present in the cosmic material.

For a planet the size of ours, the heat escapes very slowly. For smaller worlds, the process is faster.

Imagine that somewhere in a great cloud of cosmic gas and dust, a couple of grains wander into one another, and thanks to static electricity or something else, they stick. The resulting grain is larger, and presents a bigger target for other particles to hit, so it has a higher chance of picking up more particles. Even in these clouds, the density of material is very low, so collisions are rare, but there is lots and lots of time.

As the grain grows it picks up samples of all the chemicals making up the cloud, including hydrogen and other volatiles. Eventually it graduates from being a grain to a lump, and after more time it gets massive enough for a new force to take over in holding the lump together and increasing its growth rate by pulling in more and more of the surrounding material: gravity.

The impacting of new material on the growing lump makes it hot, melting it so that when it gets big enough and its gravity strong enough, it pulls itself into a sphere, maybe a thousand kilometers in diameter. It is now a largish asteroid. Continuing impacts produce more heat. Of course the formation process can stop any time, leaving the object to cool off, and eventually to solidify throughout. However, in our case the growth continues. When it reaches a diameter of several thousand kilometers, it has graduated as a planet.

If our new planet is near enough to a star, the heat from the star will evaporate and drive off most of the gas and other volatile materials, so that we end up with a rocky planet, like Mercury, Venus, Earth or Mars.

On the other hand, if the planet manages to hang onto its gas and volatiles, it can grow into a gas giant planet, like Jupiter, Saturn, Uranus and Neptune. During their formation these planets collected a huge amount of internal heat, so even today, their cores are extremely hot.

Now things get really interesting. If our planet collects material to the point where it exceeds about 20 times the mass of Jupiter, the core pressure and temperature become high enough for some elements, such as deuterium and lithium to undergo nuclear fusion, producing energy. It is no longer a planet and is not yet a star, which obtains energy through hydrogen fusion.

Objects like this one, a not-quite-graduated star, are known as brown dwarfs. These objects show some aspects of star behaviour, such as flaring. Astronomers are very interested in them. If the material keeps coming, and our star reaches 100 or more Jupiter masses of material, we have a new star.

It is amazing what can be done with one recipe, one ingredient, and just changing the amount.

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• Saturn rises soon after sunset, followed a couple of hours later by Jupiter. After another two hours or so, Mars creeps into view, followed, just as the sky starts to brighten for dawn, by Venus.

• The Moon will be full on the 11.

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

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