Stars whizzing by our solar system could cause more havoc than astronomers previously thought, from sending Pluto鈥檚 orbit haywire to forcing Mercury to fly into the sun 鈥� or even catastrophically altering Earth鈥檚 orbit and climate. The overall risk of these events is still low, but the greater influence of passing stars means that events like these might be commonplace in other planetary systems.
While the orbits of the planets were once thought to be as predictable and unchanging as clockwork, modern astronomers have found that on long timescales, they are anything but. This internal unpredictability, or chaos, means it is hard to know exactly what will happen to the solar system in the next few billion years, with some simulations showing it could tear itself apart. The most likely way for this to happen is if Mercury鈥檚 orbit unluckily lines up with Jupiter鈥檚. This could see the smaller planet sent hurtling away, either crashing into the sun or Venus, or setting Earth and Mars on a collision course.
Another source of mayhem could come from stars passing close to the sun, within a few solar system widths, but these events are extremely unlikely, with only a 1 per cent chance every billion years.
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Now, at the University of Bordeaux in France and at the Planetary Science Institute in Arizona have found that it isn鈥檛 just rare events like these that can upset our equilibrium 鈥� even everyday stellar traffic risks throwing the solar system out of kilter. 鈥淲e looked at the typical, run-of-the-mill flybys,鈥� says Raymond. 鈥淭hese are the stars that really do pass by the sun all the time, cosmically speaking.鈥�
Raymond and Kaib created five simulations, each with different strengths of stellar flybys, and ran each of them 1000 times. Each run had a slightly different starting position for the solar system鈥檚 eight planets, plus Pluto, and thousands of stars flying about based on what we know of the local stellar population.
The pair found that the overall odds of a planet in the solar system being knocked off course was 50 per cent higher than from internal chaos alone. 鈥淎 flyby-driven instability can happen anytime, whereas an internally-driven instability is far more likely to happen 4 to 5 billion years in the future, making flybys the biggest threat to solar system stability for the next 4-plus billion years,鈥� says Raymond.
The most surprising potential victim of stellar intruders was Pluto, which Raymond and Kaib found had a 5 per cent chance of becoming unstable. 鈥淚n no previous study was Pluto ever thought to have any chance at all of becoming unstable,鈥� says Raymond, thanks to the calming influence of Neptune鈥檚 gravity.
They also found that the chances of Mercury being flung into a chaotic orbit, which could have knock-on effects for other planets, was between 50 to 80 per cent higher than previous estimates, though the absolute risk remained small, at 0.6 per cent over the next few billion years. The same was true for Mars, Venus and Earth, with their absolute risks rising to 0.3, 0.2 and 0.05 per cent. Earth鈥檚 orbit also has a 0.3 per cent chance of being changed in such a way as to cause significant climate heating, says Raymond. 鈥淭he diversity of ways in which the solar system could fall apart is much higher than previously thought.鈥�
Though the chances of a star upsetting our solar system equilibrium in the next few billion years remain small, rising to around a 1.5 per cent from around a 1 per cent chance of an orbit going haywire, the fact that stars exert more of an influence that we thought could have implications for other planetary systems, he says.
鈥�1-in-1000 seems like nothing, but we know of more than 1000 exoplanets, so even if it didn鈥檛 happen to us, when we interpret what鈥檚 out there, this matters,鈥� says Raymond.
at the University of Warwick, UK, points out that we know that the solar system is definitely due a shakeup in 5 billion years, when the sun will become a white dwarf and potentially destroy the inner planets. 鈥淲e currently see over 1700 white dwarf planetary systems that contain planetary debris in the white dwarf atmospheres themselves,鈥� he says. By understanding how planetary systems like the sun might evolve, we can also learn more about what happened to the ones that have already been transformed, says Raymond.
arXiv
