jupiter and the sun -- 2/9/21

Today's selection -- from Jupiter by William Sheehan and Thomas Hockey. Our solar system can be described simply as the Sun and Jupiter with the rest being “mere parings and detritus”:

"We here propound a scale model of the solar system, with the planets arranged according to their distances from the Sun and scaled to volume. Thus if the planet Mercury is represented as a pea, Venus and Earth are grapes, while Mars is either a small grape or a large pea. Then comes Jupiter -- a slightly flattened cantaloupe! Saturn is somewhat smaller, perhaps the size of a grapefruit, while Uranus and Neptune, which are of roughly the same size, are a lemon and a lime.

"From a dynamical point of view -- the way in which one body's gravity influences the motion of another body -- the quantity of stuff, or mass, making up Jupiter matters, not mere size. Jupiter's mass is 318 times that of the Earth, making it by far the most massive planet in the solar system. Its nearest rival, Saturn, is not even close, with only 95 times the mass of the Earth. Indeed, if all the other planets -- giant planets, terrestrial planets and assorted debris such as asteroids and meteoroids -- were thrown together on one side of a balance, Jupiter by itself would still topple the scale to the other side. It is by every measure the dominant planet in the solar system. In fact, the solar system might well be described, as a first approxi­mation, as the Sun and Jupiter, the rest being mere parings and detritus. …

"One might even say that the only real difference between a star like the Sun and a giant planet like Jupiter is the initial mass. If the mass is large enough, the gas will collapse into a star -- that is, it will suc­cessfully initiate thermonuclear reactions in its core. If it is not quite large enough, it will become an almost-star, a giant planet. The next step, once the average composition and density of a planet are known, is to work out the internal structure based on the way that various materials behave under pressure. In general, materials deep below the surface ought to be highly compressed, that is, much denser than those closer to the surface. …

"Thus, in a way, Jupiter resembles a baseball, which consists of wool and cotton yarn tightly wound round a round cork centre. To complete the analogy, the visible cloud layers would resemble the stitched cowhide covering. Jupiter is not only the largest gas giant (and hence planet) in the solar system; it is also almost as large as it can be and still remain a planet. Add more mass to Jupiter and gravity will compress the resulting body into one with a smaller diameter. If the process were to continue, the pressure in Jupiter's core would soon become high enough to trigger thermonuclear reactions, where four hydrogen atoms are fused into one of helium, with the difference in mass released as energy according to the familiar formula E = me2. At this stage, Jupiter would no longer be a planet; it would be a star (in which case the Sun-Jupiter system would represent a typical binary star)."

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William Sheehan and Thomas Hockey




Reaktion Books


Copyright William Sheehan and Thomas Hockey 2018


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