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An ancient passerby may have visited the Sun and inadvertently helped shape the Solar System into what it is today. It happened billions of years ago when a stellar drifter came to within 110 astronomical units (AU) of our Sun. The effects were long-lasting and we can see evidence of the visitor\u2019s fleeting encounter throughout the Solar System.<\/p>\n
<\/p>\n Neptune is the outermost planet in the Solar System, and by a simple definition, that can mark the edge of the Solar System. There\u2019s an entire realm of other objects beyond Neptune called the Kuiper Belt. It\u2019s the home of Pluto, most of the dwarf planets, and some comets. Astronomers aren\u2019t certain how large the Kuiper Belt population is, but it could contain tens of thousands of objects larger than 100 km in diameter.<\/p>\n Some of these objects have unusual orbits and are called Trans-Neptunian objects (TNO). In new research, a team of astronomers suggest that these orbits, and some other evidence in the Solar System, support the idea that another star passed by our Solar System and drove these objects into their current orbits. The star may have disturbed some objects so strongly that they were driven into the inner Solar System and took up residence as moons around the giant planets. <\/p>\n These results are in two new papers. One is published in the journal Nature and is titled \u201cTrajectory of the Stellar Flyby Shaping the Outer Solar System.\u201d The second is published in the Astrophysical Journal Letters and is titled \u201cIrregular moons possibly injected from the outer solar system by a stellar flyby.\u201d Susanne Pfalzner, the lead author of both, is from J\u00fclich Supercomputing Centre, Forschungszentrum (Research Center) J\u00fclich, J\u00fclich, Germany.<\/p>\n \u201cThe beauty of this model lies in its simplicity. It answers several open questions about our solar system with just a single cause.\u201d<\/p>\n Susanne Pfalzner, J\u00fclich Supercomputing Centre, Forschungszentrum J\u00fclich, Germany<\/cite><\/p><\/blockquote>\n<\/figure>\n While Neptune marks the outermost boundary of planets in our Solar System, an entire population of objects exists beyond it. \u201cHowever, several thousand celestial bodies are known to move beyond the orbit of Neptune,\u201d said Pfalzner. \u201cSurprisingly, many of these so-called trans-Neptunian objects move on eccentric orbits that are inclined relative to the common orbital plane of the planets in the solar system. \u201c<\/p>\n Pluto is the most well-known TNO because it used to be considered a planet. Its orbit is inclined by 17 degrees relative to the ecliptic, an imaginary plane that Earth follows as it orbits the Sun. In the ecliptic, Earth is considered to orbit the Sun at zero degrees, and none of the other planets are inclined by more than only seven degrees. <\/p>\n Pfalzner and her co-researchers used simulations to try to understand how some objects are inclined. They ran more than 3,000 supercomputer simulations in their effort. They wanted to investigate the idea that a passing star could be responsible, and their work showed that it could.<\/p>\n \u201cOur exhaustive numerical parameter study consists of over 3,000 individual simulations modelling the effect of a stellar flyby on a planetesimal disk surrounding the Sun extending to 150?au and 300?au, respectively,\u201d the authors write in their research. <\/p>\n There are three distinct populations of TNOs: <\/p>\n Any theory on the formation of the Solar System has to explain these three groups, according to the authors. \u201cWhile only three Sedna-like objects and few highly inclined TNOs are known so far, they are the make-or-break test for any outer Solar System formation theory,\u201d they write. <\/p>\n This isn\u2019t the first time scientists have wondered if a stellar flyby can explain these puzzling parts of our Solar System. But this question has been dismissed because stellar flybys were thought to be rare. However, as we get more powerful telescopes, we\u2019re discovering that they\u2019re more commonplace. \u201cHowever, recent Atacama Large Millimeter Array observations reveal that close stellar flybys seem to be relatively common,\u201d the authors write.<\/p>\n The flyby hypothesis has gained renewed interest, but it\u2019s difficult to study because the flyby parameter space is so large, and predictions are vague. <\/p>\n These researchers have made important progress, though, and their simulations can explain a lot.<\/p>\n \u201cEven the orbits of very distant objects can be deduced, such as that of the dwarf planet Sedna in the outermost reaches of the solar system, which was discovered in 2003. And also objects that move in orbits almost perpendicular to the planetary orbits,\u201d Pfalzner said. Sedna has an extremely wide orbit and takes 11,400 years to complete one orbit around the Sun. Its orbit is also wildly eccentric. <\/p>\n According to Pfalzner and her colleagues, a stellar flyby can also explain two Solar System objects with very oddball orbits. 2008 KV42 has a retrograde orbit, meaning it orbits in the opposite direction than the planets. 2011 KT19\u2018s orbit is tilted 110 degrees, meaning it effectively follows a polar retrograde orbit.<\/p>\n What kind of star could\u2019ve shaped these objects\u2019 orbits?<\/p>\n \u201cThe best match for today\u2019s outer solar system that we found with our simulations is a star that was slightly lighter than our Sun \u2013 about 0.8 solar masses, \u201cexplained Pfalzner\u2019s colleague Amith Govind. \u201cThis star flew past our sun at a distance of around 16.5 billion kilometres. That\u2019s about 110 times the distance between Earth and the Sun, a little less than four times the distance of the outermost planet Neptune.\u201d<\/p>\n \n\n
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