Stars are gravitationally fastened to their galaxies and move in concert with their surroundings. But sometimes, something breaks the bond. If a star gets too close to a supermassive black hole, for example, the black hole can expel it out into space as a rogue star. <\/p>\n
What would happen to Earth if one of these stellar interlopers got too close?<\/p>\n
<\/p>\n It\u2019s not a very likely occurrence, but the chance is not zero. <\/p>\n After several billion years, our Solar System has evolved into sedentary predictability. The planets move as they move, and the Sun sits stolidly in the middle of it all. <\/p>\n But if another star came too close, the invisible gravitational bonds that keep everything going the way it is would be stretched or broken. Earth is a tiny planet, containing only about three millionths the mass of the Sun. Our planet exists at the whims of the Sun and its powerful gravity, and if another star shoulders its way into our tidy arrangement, Earth will be entirely at the mercy of the new gravitational paradigm. <\/p>\n A new paper examines what would happen if a rogue star comes to within 100 AU of the Sun. The paper\u2019s title is \u201cFuture Trajectories of the Solar System: Dynamical Simulations of Stellar Encounters Within 100 au.\u201d It\u2019ll be published in Monthly Notices of the Royal Astronomical Society. The lead author is Sean Raymond, an astronomer at the Laboratoire d\u2019Astrophysique de Bordeaux, CNRS (National Center for Scientific Research) and the Universit\u00e9 de Bordeaux. <\/p>\n We know that the stable predictability in our Solar System will not last. The Sun will continue to evolve and, over the next billion years, will become more luminous. Earth is awfully close to the inner edge of the habitable zone. Only a little closer to the Sun and the delicate balance that allows liquid water to persist on the surface will be disrupted. <\/p>\n In that same one billion year range, there\u2019s about a 1% chance for an encounter with a rogue star. What will happen to Earth if that happens? Will Earth be nudged out of the habitable zone?<\/p>\n \u201cEarth has about a billion years of habitable surface conditions remaining,\u201d the authors write. That\u2019s in a closed system, which, for the most part, our Solar System is. \u201cWhile the orbital evolution of the planets is largely determined by secular and resonant perturbations,\u201d the authors explain, \u201cpassing stars can have a consequential influence on the planets\u2019 orbits.\u201d<\/p>\n If a passing star comes to close, then our Solar System is no longer a closed system. <\/p>\n Most rogue stars, also called intergalactic stars or hypervelocity stars because their trajectories will take them out of the Milky Way, come nowhere near Earth. Kappa Cassiopeiae, for example, is 4,000 light-years away and will never approach. Others, like the 675 rogue stars astronomers at Vanderbilt University discovered in 2012, were ejected after tangling with the Milky Way\u2019s supermassive black hole, and their trajectories brought them nowhere near Earth. <\/p>\n Even in the Milky Way, space is mostly empty, and most stellar flybys will never approach another solar system. \u201cStatistically speaking, flybys closer than 100 au, which would strongly affect the planets\u2019 orbits, only take place roughly once per 100 Gyr in the current Galactic neighbourhood,\u201d the researchers explain. <\/p>\n Though the odds are low, it\u2019s a possibility. When you look at the galaxy as a whole, it\u2019s almost certain that a stellar flyby sometime somewhere in the galaxy will come within 100 AU of another star. If that star is our Sun, what will happen to Earth? <\/p>\n The team performed N-body simulations to try to determine the potential outcomes for Earth. They started with the Solar System\u2019s eight planets and added a single rogue star. They matched the masses of the simulated rogue stars to the masses of stars in our stellar neighbourhood. They also matched the rogue stars\u2019 velocities to the neighbourhood. They simulated different velocities and trajectories for the star to see what the range of outcomes for Earth looks like. In total, the researchers ran 12,000 simulations.<\/p>\n \u201cIf a star passes within 100 au of the Sun, there is still a very high chance that all 8 Solar System planets will survive,\u201d the authors write. There\u2019s over a 95% chance that no planets will be lost. <\/p>\n The angular momentum deficit (AMD) as a result of the flyby largely determines what happens next. AMD is a measure of a planetary system\u2019s orbital excitation and its long-term stability. It\u2019s the difference between an \u201cidealized system with the same planets of the real system orbiting at the same semimajor axes from the star on circular and planar orbits and the norm of the angular momentum of the real planetary system,\u201d according to this definition. <\/p>\n But what does it look like when one of our Solar System\u2019s planets is lost?<\/p>\n The simulation produced diverse outcomes. Mercury is the most vulnerable and is sometimes lost when it collides with the Sun. Other results include Earth colliding with Venus, ejection of the ice giants Uranus and Neptune, only Earth and Jupiter surviving, or only Jupiter surviving. In one apocalyptic outcome, all eight planets are ejected. <\/p>\n Other results are less dramatic. All eight planets are unperturbed, all eight are slightly perturbed, or all eight are highly perturbed. <\/p>\n Though all eight planets survive in most of the simulations, survival can mean different things. Even though they remain in the Solar System and remain gravitationally bound to the Sun, their orbits can be wildly disrupted. Some can even be shoved way out into the Oort Cloud. <\/p>\n The researchers also tabulated the ten most likely outcomes where planets are destroyed. \u201cWe determined the most common pathways through which planets may be lost, keeping in mind that there is a greater than or equal to 95% chance that no planet will be lost if a star passes within 100 au,\u201d they write.<\/p>\n When it comes to ejected planets, Uranus and Neptune face the worst odds. That\u2019s not surprising since they\u2019re furthest from the Sun and most weakly bound to it gravitationally. It\u2019s also not surprising that Mercury has the highest odds of colliding with the Sun. As the least massive planet, it faces a greater risk of perturbation due to a stellar flyby. <\/p>\n When it comes to Earth, there are a wide variety of potential outcomes. In the list above, Earth has a 0.48 % chance of colliding with another planet. But another potential fate awaits Earth, and it\u2019s not pleasant to contemplate: banishment to the Oort Cloud. <\/p>\n \u201cThe long-term survival of Earth in the Oort cloud is not guaranteed,\u201d the authors deadpanned. <\/p>\n Another exotic outcome of the simulations is worth considering: Earth\u2019s capture by the passing star. That simulation had a star slightly less massive than the Sun and travelling at a relatively low speed approaching our Solar System closely. The outcome was a devastating annihilation of the Solar System as we know it. Earth abandoned the Sun and ran off with the star, while six of the other planets crashed into the Sun. The lone surviving planet was Jupiter. No surprise there since it\u2019s the most massive planet. <\/p>\n The paper presents a wide range of outcomes, including the Moon impacting Earth, both the Earth and Moon being captured by the passing star, and even all of the planets and their moons being destroyed. But the odds of any of this happening are extremely low. <\/p>\n But how likely is it that Earth would remain habitable in such an encounter? If Earth\u2019s orbit is changed, then the planet will be warmer or cooler as a result. <\/p>\n There are yet more potential fates. Earth could survive as a rogue planet for a million years or so until the surface froze over. Or maybe if it did get captured by the rogue star, it would somehow be habitable in some new arrangement. <\/p>\n Ultimately, the odds of a 100 AU stellar flyby are infinitesimally small. And the simulations show that if it did happen, the most likely outcome by far is that all eight planets survive, albeit in orbits slightly different than the ones they follow now. <\/p>\n \u201cDespite the diversity of potential evolutionary pathways, the odds are high that our Solar System\u2019s current situation will not change,\u201d the authors conclude. <\/p>\n![\"This](\"https:\/\/www.universetoday.com\/wp-content\/uploads\/2023\/11\/Rogue-Star-Surviving-Planets.png\")
![\"\"](\"https:\/\/www.universetoday.com\/wp-content\/uploads\/2023\/11\/Rogue-Star-Planets-with-legend-1024x238.png\")
![\"This](\"https:\/\/www.universetoday.com\/wp-content\/uploads\/2023\/11\/final-orbits-rogue-star-planets.png\")
planets with semimajor axes of 104?5<\/sup> au are those trapped in the Oort cloud. Image Credit: Raymond et al. 2023.<\/figcaption><\/figure>\n\n
![\"One](\"https:\/\/www.universetoday.com\/wp-content\/uploads\/2023\/11\/Earth-in-the-Oort-Cloud.png\")
large orbital radius, the Galactic tide increased the Earth\u2019s perihelion distance on a ~ 100 Myr timescale,\u201d the authors write. \u201cEarth finished the simulation on a stable orbit in the Oort cloud with an orbital semimajor axis of 54977 au,\u201d they explain. Image Credit: Raymond et al. 2023.<\/figcaption><\/figure>\n![\"This](\"https:\/\/www.universetoday.com\/wp-content\/uploads\/2023\/11\/Earth-climate-flyby-survival.png\")
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