Jupiter turns in just ten hours on itself, which seems spectacularly fast for its size but it is abnormally slow according to the scenarios explaining its formation. Indeed, a new theory has just been proposed to solve this enigma: like the young Sun (and other stars), the gas giant would have seen its rotation slowed down by magnetic fields.
The astronomer Konstantin Batygin, known for his work on the possible existence of a ninth planet in the Solar System, is again creating a buzz with a publication on arXiv. He proposes a solution to the enigma of the abnormal rotation speed of Jupiter, which also concerns Saturn and other exoplanets. All these celestial bodies turn more slowly on themselves than their formation models predict.
Astrophysicists and astronomers had encountered a similar problem with the Sun more than a century ago and this had contributed to the temporary abandonment of the Kant-Laplace protoplanetary nebula theory. Indeed, this slowness seems to contradict the law of conservation of the kinetic moment, linked to the speed of rotation and the moment of inertia of a body.
Kant-Laplace’s theory assumes that the Sun and the planets were formed by gravitational collapse of a coarse spherical and slowly rotating cloud of gas and dust. Just as the rotation of a skater accelerates when he brings his arms closer to his body, because of the conservation of the kinetic moment, the speed of the contracting cloud also had to increase. The Sun, representing the majority of the material of the nebula, should turn very fast on itself to retain most of the kinetic moment of the Solar System. However, this is not the case. The planets, especially Jupiter and Saturn, because of their orbit around the Sun, represent 99% of the solar system’s kinetic moment with the Sun accounting for only 1 percent.
Jupiter, just like Saturn, would have begun to form by accretion of small rocky and especially icy bodies, creating a solid nucleus containing several times the mass of the Earth. Then, rather quickly, there occurred a phase of rapid collapse of the gas, composed mainly of hydrogen and helium, which led to the two gas giants that we know today. Calculations indicate that this formation would have lasted a few million years at the most, but also that the rotational speed of these planets should have been the limit velocity beyond which the centrifugal force is greater than the gravitational force.
Jupiter should turn on itself in a little less than three hours and Saturn in about four hours. This is not the case: the rotational speeds of these planets are respectively ten and eleven hours.
The problem seems to be solved for the Sun and the stars with the discovery of material jets of the young T-Tauri stars and also involving magnetic braking forces, as proposed in the 1940s by the Nobel Prize winner Hannes Alfvén. The initial protoplanetary disk was indeed sufficiently hot and close to the young Sun for the matter to be ionized, forming a plasma sheltering a magnetic field.
The lines of this field would have coupled the rotation of the Sun to the disk, which would have acted as a brake. The ejection of matter, along the jets and by the solar wind, would have taken away a part of the kinetic moment, slowing down the proto-Sun.
It was not easy to apply the same mechanism to the case of the young Jupiter in formation since it did not generate the equivalent of the solar wind and jets of matter. But the article by Konstantin Batygin proposes some reasoning and analytical calculations in this sense, which would still require to be confirmed and specified by numerical simulations based on magnetohydrodynamics, the theory of the mechanics of charged fluids.
In the scenario proposed by Batygin, already partly supported by numerical simulations, the proto-Jupiter would add matter to the cold protoplanetary disk (in blue) via material currents falling on the poles as shown in the diagram above. . Some of this matter is absorbed by Jupiter but another forms a second accretion disk, hot and ionized, around the giant (the orange disk). Alfvén’s magnetic braking mechanism can then operate in this disk because material is transported to the outside, thus returning to the protoplanetary disk of the Sun. Finally, this kinetic moment transport reduces the speed of rotation initially acquired by Jupiter.
If Batygin is right, the mystery of Jupiter’s rotation and Jovian exoplanets is solved.