A neighbouring star getting too close to our solar system could drive it into pandemonium. Simulations suggest that a fly-by star would only need to nudge Neptune鈥檚 position by three times the distance between the Earth and the sun to cause the planets to go haywire.
It may seem obvious that any significant shifts to a planet鈥檚 position could have a large effect on our solar system, but the new simulations suggest that this could happen even if a stellar fly-by only has a small effect, which could later snowball into major instability.
Garett Brown and Hanno Rein at the University of Toronto, Canada, performed 2880 simulations of what would happen if a star flew past the solar system, changing the strength of perturbations to the system depending on the star鈥檚 mass and distance. Each simulation covered 4.8 billion years after the fly-by, or stopped early if a planet escaped the solar system or was destroyed.
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They found that Neptune鈥檚 position has to be changed by about three times the distance between Earth and the sun 鈥� a shift in Neptune鈥檚 distance from the sun of just 0.1 per cent 鈥� to cause the planets to go haywire. This could be caused by a solar-mass star passing by about 37 billion kilometres away.
鈥淭hese weak perturbations don鈥檛 destroy the solar system immediately, they just wiggle it around a little bit, and over the next millions or billions of years something goes unstable,鈥� says Rein.
Of the 2880 simulations, 960 caused perturbations too small to be measured. In that group, four models ended when Mercury smashed into Venus. Among the remaining 1920 simulations, instabilities were more than three times more likely. Of that group, 26 ended in disarray, mostly with collisions between Mercury and Venus, but one with a collision between Earth and Mars, and a few where Uranus, Neptune or Mercury was thrown out of the solar system. Because the simulations stopped when a planet was lost or destroyed, we can鈥檛 know for sure whether there would be further problems after the first, but it is possible, says Brown.
These planetary shake-ups wouldn鈥檛 occur immediately, but millions or billions of years after the star has passed by. 鈥淓ven from Pluto鈥檚 distance, the sun looks basically like another star in the sky, so even a close stellar fly-by wouldn鈥檛 look very dramatic,鈥� says Brown. 鈥淎fter it passes by and destroys everything, that鈥檚 dramatic.鈥�
The researchers calculated that because the solar system is in a relatively empty area of space, perturbations this large are only expected to happen about once every 100 billion years, so we are probably safe for now.
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