I recall seeing a paper with orbital dynamical simulations in which they found a 2 percent chance that over the remaining lifetime of the sun, the orbital eccentricity of Mercury would increase such that it interacted with Venus, either hitting Venus or getting boosted onto an Earth-interacting trajectory which could lead to a collision or eject it from the solar system altogether, and which puts Venus on a much more closely Earth-interacting orbit. In one simulation out of the 2,500 they did, Mars's eccentricity was perturbed until it became Earth-crossing.
Remember that there is a bit of a selection effect when it comes to looking at exoplanets - we see the compact large planet systems much more easily than systems like ours. The latest work I've seen has suggested that we are a less common class of star system but that stuff like ours might be something like 10% of star systems. Nobody REALLY has a handle on planet formation yet, and it looks like there may be several very different ways that planets and planet-forming material can migrate around the protoplanetary disc during the planetary accretion stage that people argue bitterly over in the literature.
I was tracking these runaway stars for a SF story i had in mind, but this is the closest one i have heard of yet, and the ArXiv paper describes one that also passed thru 2.5 mya.
Gliese 710 will pass the Sun even closer
Close approach parameters recalculated based on the first Gaia data release
http://www.aanda.org/articles/aa/abs/2016/11/aa29835-16/aa29835-16.html
Close encounters of the stellar kind
https://arxiv.org/abs/1412.3648
tl:dr article
http://www.businessinsider.com/star-hurting-towards-solar-system-2016-12\
"Gliese 710 is about half the size of our sun, and it is set to reach Earth in 1.35 million years, according to a paper published in the journal Astronomy & Astrophysics in November.
And when it arrives, the star could end up a mere 77 light-days away from Earth — one light-day being the equivalent of how far light travels in one day, which is about 26 billion kilometers, the researchers worked out.
As far as we know, Gliese 710 isn't set to collide directly with Earth, but it wil be passing through the Oort Cloud, a shell of trillions of icy objects at the furthest reaches of our solar system. "
Seems like a great opportunity to send out some interstellar probes. The star will be trailing lots of ISM, free gas that would help bring a ramjet up to speed, and track till you could curve towards another destination. Likewise, a solar sail probe launched out in front of it by laser could "hitchhike" , and get some deep space ISM , and EM measurements.
Can we think of some other opportunities that this might present ? If we are past the filter by then, then we will already prob have samples of the Oort objects, but looks like they will be delivering then...