A major ninth planet, which apparently exists far beyond Pluto's orbit, could have formed directly in the solar system, and not be captured by another star.
Almost two years ago - in early 2016 - astronomers showed that the unusual trajectories of celestial bodies in the Kuiper belt, which surrounds the far reaches of the solar system, should indicate the presence here of a very large and as yet unknown object - the mysterious ninth planet. It has not yet been possible to directly observe it, but scientists are trying to find out various features of the potential planet. In particular, its mass is estimated at 20 Earth masses, and its orbit should lie 150–350 times farther than us from the Sun.
Modern models of planet formation cannot explain how such a massive body was formed at such a distance from the parent star. In addition, the orbit of "planet X" is tilted from the plane usual for the solar system by as much as 60 °. These and other factors lead many astronomers to suspect that the body is an exoplanet that appeared in another star system and was subsequently captured by the gravity of the Sun. However, new work by British and Swiss scientists refutes this hypothesis. The article by Richard Parker and his colleagues has been accepted for publication in the MNRAS journal and presented in the arXiv.org online repository.
Indeed, today our Sun is located in a rather deserted region of the Galaxy, but 4.5 billion years ago it was surrounded by a much larger number of stars - it would seem that one of them could well “lose” a newborn planet. This scenario was modeled by Parker and his co-authors, showing that the probability of a star catching a planetary mass object does not actually depend on the total mass of the star-forming region. In other words, the density of the location of young stars did not increase the chances that one of them would catch an alien planet from a distant and unstable orbit. But this was influenced by the internal motion of the group of stars itself - for example, the expansion at a relatively low density of location greatly increased the frequency of such events in the model.
"Exchange" of planets can affect no more than 1-6 percent of them. According to scientists, in the case of the real Sun, the implementation of such a scenario in the past looks almost unbelievable. And if we accept their arguments, then two options remain: either it is an “orphan” planet that accidentally flew by and was captured by the gravity of our star, or “planet X” was formed directly in the solar system. And then we will have to explain how such a large and heavy body could be at such a distance from the star.
Sergey Vasiliev
