Stars whizzing by our solar system could cause more havoc than astronomers previously thought, from sending Pluto’s orbit haywire to forcing Mercury to fly into the sun – or even catastrophically altering Earth’s orbit and climate. “A 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,” says Raymond.