  Given the inexorable increase in the Sun's luminosity, Earth will exit the
habitable zone in ~1 Gyr. There is a negligible chance that Earth's orbit will
change during that time through internal Solar System dynamics. However, there
is a ~1% chance per Gyr that a star will pass within 100 au of the Sun. Here,
we use N-body simulations to evaluate the possible evolutionary pathways of the
planets under the perturbation from a close stellar passage. We find a ~92%
chance that all eight planets will survive on orbits similar to their current
ones if a star passes within 100 au of the Sun. Yet a passing star may disrupt
the Solar System, by directly perturbing the planets' orbits or by triggering a
dynamical instability. Mercury is the most fragile, with a destruction rate
(usually via collision with the Sun) higher than that of the four giant planets
combined. The most probable destructive pathways for Earth are to undergo a
giant impact (with the Moon or Venus) or to collide with the Sun. Each planet
may find itself on a very different orbit than its present-day one, in some
cases with high eccentricities or inclinations. There is a small chance that
Earth could end up on a more distant (colder) orbit, through re-shuffling of
the system's orbital architecture, ejection into interstellar space (or into
the Oort cloud), or capture by the passing star. We quantify plausible outcomes
for the post-flyby Solar System.
