How stable is the solar system? In relative human and historical terms, it is pretty stable but small gravitational influences can cause dramatic effects due to the chaotic and complex nature of the forces involved. Now, two researchers have set out to determine just how easily it might be disrupted. And the answer is fascinating.

For things to go truly wrong for the solar system, you’d only need the average distance between Neptune and the Sun to be altered by 0.1 percent, which would make the chance of the solar system descending into chaos ten times higher.

The work is accepted for publication in the Monthly Notices of the Royal Astronomical Society and can be read on the paper repository ArXiv. 

One of the possible starting points of the instability of the solar system is the smallest of the planets, Mercury. The perihelion – the closest point on a planet’s orbit around the Sun – of Mercury moves by about 1.5 degrees every 1,000 years, a very close rate to Jupiter’s own.

If the two were to fall in sync – resonance – there’s a one percent chance that Mercury would be pulled out of orbit and either ejected from the Solar System or set on a collision course with Venus, the Sun, or even Earth over the next three to four billion years.

Just letting things evolve naturally is all well and good but there could be ways to create such instability and mess up the Solar System. The scientists envision a passing star getting a bit too close for comfort. Mercury is too close to the Sun to feel it, but Neptune would, and the perturbation would spread through the solar system.

The effects of a 0.1 percent perturbation – equivalent to 4.5 million kilometers (2.8 million miles) in Neptune’s semi-major axis – spread to Earth and Mars in just 20 million years. A perturbation of 10 percent could mean catastrophe for us and the Red Planet.

The team ran 2,880 simulations with 960 having perturbations too small to be measured. Still, in four of those, Mercury hit Venus. It’s not all death and destruction in the other 1,920 models, but there are 26 that end with chaos unfolding, a lot of collisions between Mercury and Venus, one with Earth and Mars, slamming into each other, and some where Uranus, Neptune, or Mercury are thrown out completely.

The team also estimated the chance of a star getting close enough to cause all that and we can sleep soundly that there are only about 20 chances over the next 100 billion years. 

Knowing the Sun as it is will stick around for just another five, if there’s anything that will mess with the solar system, it’s likely coming from inside it.