Our solar system has had a chaotic past. The Earth and other planets are now in stable orbits, but there were drastic changes during their formation. Jupiter was probably much closer to the Sun than it is now, and its displacement not only moved other planets, but also cleared the solar system of debris, throwing most of it into the Oort cloud.
The Oort cloud is at the gravitational edge of the solar system. While most of the debris from the Oort Cloud is likely relatively small, it is possible that planet-sized objects even more distant than the hypothetical Planet X are lurking there. In recent years, astronomers have begun to actively study the Oort cloud in search of the mysterious “planet X”, hints of the existence of which were discovered in the middle of the last decade.
Some of the debris ejected from the inner solar system was likely thrown even further away. They escaped the sun’s gravity and were left to drift through interstellar space. We know this is possible because we have had at least two cometary guests from other star systems. We have also observed rogue planets that have broken gravitational bonds with their parent star. There are many celestial orphans in the galaxy.
But it also raises an interesting question. If a young planetary system can push back comets and planets, could other star systems take over some of these worlds? This is the subject of a new paper accepted for publication in the Monthly Notices of the Royal Astronomical Society and available on the arXiv preprint server.
The team of scientists ran a series of computer simulations, studying how planetary systems kick off large planets and how a planetary system can catch one. In order to be ejected, a planet must gain enough kinetic energy to get out of its star’s gravitational pull. But it also means that the planet has so much kinetic energy that it would be difficult for another star system to hold on to. As with the visits of ‘Oumuamua and Borisov, most celestial encounters with the rogue planet would have been fleeting.
But the team found that the gravitational pull of the galaxy itself can dampen the motion of the rogue planet, and thus, in a small fraction of celestial collisions, the star can trap the planet. The best chance for this is not when the planet passes close to the star, but rather when it drifts directly inside the Oort cloud. Most of the planets captured by the star will be on the outer edge of the system.
“Our calculations show that planets that formed inside the chaotically arranged Oort cloud or fell into it from the outside can exist inside it for billions of years. This suggests that large exoworlds should often be found in such regions of planetary systems. If such a planet exists inside Solar system, then it is removed from the hypothetical “planet X” by tens or hundreds of times,” the researchers write.
According to calculations, up to 10% of the star’s original planets could have been ejected into deep space. Given the dynamics of the galaxy and the early solar system, there is about a 7% chance that the solar system has a trapped icy giant planet in the Oort cloud. But the probability that an ice giant formed in the solar system was forced into the Oort cloud is about 1 chance in 200.
Thus, if there is a planet at the edge of the solar system, then most likely it is an adopted child, and not one from the offspring of the sun. Of course, it is most likely that there are no large worlds in the Oort cloud, but this study shows that planetary systems do not always form in isolation, and there are many star systems that are likely to be mixed families.
