If a passing star came near the solar system, Earth would probably be fine – but there is a small chance our entire world could get thrown to the outer reaches of the solar system, crash into another planet or even get stolen by the wandering star. The other planets in the solar system could suffer similar fates, and Mercury in particular would be at risk of falling into the sun.
About a billion years from now, the sun’s evolution will have proceeded enough that our solar system’s habitable zone will have moved out beyond Earth’s orbit, meaning our planet’s surface will broil. But over that same billion years, there is about a 1 per cent chance that another star will fly within 100 astronomical units (or AU, the distance between Earth and the sun) of our solar system. at University of Bordeaux in France and his colleagues performed a set of 12,000 simulations to determine what effect such a flyby would have on the solar system – and whether it could save Earth.
If such a flyby happens while we are still around to see it, it will look spectacular. “Over about a thousand years, we would see an object about as bright as Venus slowly moving across the sky until it became as bright as the full moon,” says at the University of Toronto in Canada, who was not involved in this research. “It would be very bright, but even at 100 AU it would still only look half the size of Jupiter.”
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The simulations suggest there is a 92 per cent chance all the planets would be fine, surviving the flyby on orbits similar to the ones they are on now. “I thought more would happen,” says Raymond. “It’s really not that easy to mess with planets’ orbits – a star has to get really into another star’s business to mess with its planets. But that means we shouldn’t hold our breath for a star to come and rescue Earth from the sun.”
In fact, the researchers found there is only a 0.28 per cent probability, should a close stellar flyby happen, of Earth ending up in a cooler, and therefore more habitable, location: generally either getting tossed out into the Oort cloud – a region of icy objects beyond Pluto – or captured by the other star. The odds are about the same that Earth would fall into the sun, and slamming into another planet is nearly twice as likely as either scenario.
Even if Earth does end up in a cooler orbit, the disruption to the whole solar system could change the moon’s orbit as well. “The cases when Earth ends up on a cooler orbit are going to be, quite frequently, the cases where the moon falls on us,” says Raymond. “That’s not good for life.”
In a close stellar flyby, the second most likely outcome – after everything turning out fine – is that Mercury would smash into the sun, which has a probability of about 2.54 per cent. Mars falling into the sun is the next most likely, followed by Venus hitting another planet, and there is about 2 per cent chance that either Uranus or Neptune would get ejected from the solar system entirely.
So if another star does pass by, we are probably better off hoping that we stay in the 92 per cent of scenarios where nothing major changes, rather than planning for cosmic salvation.
Monthly Notices of the Royal Astronomical Society
