Firstly, the probes indicated that Jupiter is, in all ways, a very special planet, and not only because it is not solid. One of the definitions coined by astronomers says that a planet is an opaque body that receives light and energy from a star. But Jupiter completely contradicts this statement. In 1973 and 1974, as they approached the planet, Pioneer 10 and Pioneer 11 were interrogated by the Earth, and the incredible answer was that Jupiter irradiated two and a half times the energy it received from the Sun. Where does all this energy come from? The experts do not agree about the answer. It was first thought that what happens in the Sun was being repeated, on a reduced scale, inside Jupiter. In fact, Jupiter's composition is very similar to that of our star: it consists mainly of hydrogen and helium (the first one ten times more abundant than the second one), even if it contains traces of other substances such as ammonia, methane, water, ethane and acetylene. Thus, it was thought that even the interior of Jupiter was the site of nuclear reactions such as those which generate energy in the Sun. However, this hypothesis was immediately rejected because it was calculated that Jupiter, though enormous compared to the Earth, does not have enough interior mass to generate the pressure and temperature conditions needed to trigger nuclear reactions: in other words it is an unsuccessful star. A second theory states that Jupiter is contracting because of the gravitational attraction that it itself exerts on the matter of which it is formed. Whoever has tried to fill a cigarette lighter certainly knows that when a certain amount of gas escapes from the spray can (it expands going from the greater internal pressure to the lower environmental pressure), a thin film of ice forms on the material around it: the expansion absorbs heat from the exterior. Contraction is the opposite mechanism, in other words it gives up energy. Thus, the explanation was: Jupiter is contracting and thus gives up energy. After making the necessary calculations now it seems that the question should be reversed: Jupiter emits energy not because it is contracting. On the contrary, it contracts because it is emitting energy, i.e. it is slowly cooling. In fact, it seems that the energy that comes from Jupiter today is that which the planet has accumulated at the time it was formed, starting from the primordial solar nebula. It is the same energy that even the Earth, in the same period, accumulated, and then lost, or at least most of it, about four billion years ago, when the Earth's crust was first created. Thus, the interior of Jupiter is being rearranged in the same way that on our planet, in that remote past, caused the heavier materials, such as iron and nickel, to concentrate in the center of the sphere. However, the percentage of relatively heavy materials must be very modest on Jupiter considering that the overall density of the planet is slightly more a fifth of that on Earth; in other words just slightly greater than that of water. The low density and huge dimensions may be the reason behind Jupiter's geological "delay" compared to the Earth.
Firstly, the probes indicated that Jupiter is, in all ways, a very special planet, and not only because it is not solid. One of the definitions coined by astronomers says that a planet is an opaque body that receives light and energy from a star. But Jupiter completely contradicts this statement. In 1973 and 1974, as they approached the planet, Pioneer 10 and Pioneer 11 were interrogated by the Earth, and the incredible answer was that Jupiter irradiated two and a half times the energy it received from the Sun. Where does all this energy come from? The experts do not agree about the answer. It was first thought that what happens in the Sun was being repeated, on a reduced scale, inside Jupiter. In fact, Jupiter's composition is very similar to that of our star: it consists mainly of hydrogen and helium (the first one ten times more abundant than the second one), even if it contains traces of other substances such as ammonia, methane, water, ethane and acetylene. Thus, it was thought that even the interior of Jupiter was the site of nuclear reactions such as those which generate energy in the Sun. However, this hypothesis was immediately rejected because it was calculated that Jupiter, though enormous compared to the Earth, does not have enough interior mass to generate the pressure and temperature conditions needed to trigger nuclear reactions: in other words it is an unsuccessful star. A second theory states that Jupiter is contracting because of the gravitational attraction that it itself exerts on the matter of which it is formed. Whoever has tried to fill a cigarette lighter certainly knows that when a certain amount of gas escapes from the spray can (it expands going from the greater internal pressure to the lower environmental pressure), a thin film of ice forms on the material around it: the expansion absorbs heat from the exterior. Contraction is the opposite mechanism, in other words it gives up energy. Thus, the explanation was: Jupiter is contracting and thus gives up energy. After making the necessary calculations now it seems that the question should be reversed: Jupiter emits energy not because it is contracting. On the contrary, it contracts because it is emitting energy, i.e. it is slowly cooling. In fact, it seems that the energy that comes from Jupiter today is that which the planet has accumulated at the time it was formed, starting from the primordial solar nebula. It is the same energy that even the Earth, in the same period, accumulated, and then lost, or at least most of it, about four billion years ago, when the Earth's crust was first created. Thus, the interior of Jupiter is being rearranged in the same way that on our planet, in that remote past, caused the heavier materials, such as iron and nickel, to concentrate in the center of the sphere. However, the percentage of relatively heavy materials must be very modest on Jupiter considering that the overall density of the planet is slightly more a fifth of that on Earth; in other words just slightly greater than that of water. The low density and huge dimensions may be the reason behind Jupiter's geological "delay" compared to the Earth.
