Translated material
Pebble giants
American planetary scientists led by Hal Levison from the Southwest Research Institute in Boulder (Colorado) managed to eliminate a number of difficulties in explaining the appearance of the largest giant planets in the Solar System.
Previous models predicted the formation of too many planets such as Jupiter and Saturn, but a new study published in Nature [1] shows that the number of “monsters” born from a rotating protoplanetary cloud of gas and dust is small.
It should be noted that, in accordance with modern views, the Solar system, which was born 4.56 billion years ago, was formed extremely quickly - within several million years. During this time, the Sun had already managed to destroy most of the remaining gas and dust disk, so Jupiter and Saturn had to hurry. And it was the gas giants that appeared first, and not the rocky terrestrial planets, as was once thought.
The formation of the cores of giant planets is best described by the theory of so-called pebble accretion, developed in 2012. In it, tiny dust particles gradually grow, colliding and sticking together thanks to static electricity. The idea of planetary embryos of kilometer-sized planetesimals capable of holding small gas atmospheres was not as successful as the sticking together of much smaller particles (less than a meter in size). The fact is that large blocks do not tend to unite during collisions; they split and fly apart.
The difficulty remained that pebble accretion turned out to be too effective. In many computer simulations, this results in the birth of tens or even hundreds of Earth-sized objects. Fortunately, Levison’s group was able to select the initial conditions in such a way that the process of formation of planetary cores from small rocks is stretched out, allowing large planetary embryos to interact with each other gravitationally and leave the ecliptic plane, making room for the few remaining “pebbles” that form the cores of gas giants. The simulation, which took several weeks on a cluster of five computers, predicts the appearance of one to four gas giants the size of Jupiter and Saturn in orbits between 5 and 15 astronomical units (AU) from the Sun (Jupiter is actually 5.2 AU). AU, and Saturn is 9.6 AU from the Sun). In addition, the simulation creates several ice giants like Uranus and Neptune in the desired range of orbits. According to the forecasts of the scientific team, similarly large planets formed in the Kuiper belt (beyond the orbit of Pluto) should not be observed. Now we have to refine the modeling of the formation of the inner planets, which appeared much later.
Maxim Borisov
1. Levison H. F., Kretke K. A., Duncan J. Growing the gas-giant planets by the gradual accumulation of pebbles // Nature. 524. 322–324 (20 August 2015) — http://dx.doi.org/10.1038/nature14675
2. www.nature.com/news/small-rocks-build-big-planets-1.18200
3. http://news.sciencemag.org/space/2015/08/how-jupiter-and-saturn-were-born-pebbles