If a star flying past our solar system moved Neptune's orbit by just 0.1 per
cent, it could eventually cause the other planets to smash into one another or
get thrown out of the solar system entirely.
A neighbouring star getting too close to our solar system could drive it
into pandemonium. Simulations suggest that a fly-by star would only need to
nudge Neptune’s position by three times the distance between the Earth and
the sun to cause the planets to go haywire.
It may seem obvious that any significant shifts to a planet’s position could
have a large effect on our solar system, but the new simulations suggest
that this could happen even if a stellar fly-by only has a small effect,
which could later snowball into major instability.
Garett Brown and Hanno Rein at the University of Toronto, Canada, performed
2880 simulations of what would happen if a star flew past the solar system,
changing the strength of perturbations to the system depending on the star’s
mass and distance. Each simulation covered 4.8 billion years after the
fly-by, or stopped early if a planet escaped the solar system or was
destroyed.
They found that Neptune’s position has to be changed by about three times
the distance between Earth and the sun – a shift in Neptune’s distance from
the sun of just 0.1 per cent – to cause the planets to go haywire. This
could be caused by a solar-mass star passing by about 37 billion kilometres
away.
“These weak perturbations don’t destroy the solar system immediately, they
just wiggle it around a little bit, and over the next millions or billions
of years something goes unstable,” says Rein.
Of the 2880 simulations, 960 caused perturbations too small to be measured.
In that group, four models ended when Mercury smashed into Venus. Among the
remaining 1920 simulations, instabilities were more than three times more
likely. Of that group, 26 ended in disarray, mostly with collisions between
Mercury and Venus, but one with a collision between Earth and Mars, and a
few where Uranus, Neptune or Mercury was thrown out of the solar system.
Because the simulations stopped when a planet was lost or destroyed, we
can’t know for sure whether there would be further problems after the first,
but it is possible, says Brown.
These planetary shake-ups wouldn’t occur immediately, but millions or
billions of years after the star has passed by. “Even from Pluto’s distance,
the sun looks basically like another star in the sky, so even a close
stellar fly-by wouldn’t look very dramatic,” says Brown. “After it passes by
and destroys everything, that’s dramatic.”
The researchers calculated that because the solar system is in a relatively
empty area of space, perturbations this large are only expected to happen
about once every 100 billion years, so we are probably safe for now.
Reference:
Garett Brown, Hanno Rein, On the long-term stability of the Solar System in the presence of weak perturbations from stellar flybys, Monthly Notices of the Royal Astronomical Society, stac1763, https://doi.org/10.1093/mnras/stac1763, arxiv.org/abs/2206.14240
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Space & Astrophysics