First stars that could significantly perturb comet motion are finally
  found
  Since 1950 when Oort published his paper on the structure of the cloud of
comets it is believed that stars passing near this hypothetical cometary
reservoir play an important role in the dynamical evolution of long period
comets and injecting them into the observability region of the Solar System.
The aim of this paper is to discuss two cases in which the data obtained from
observations were used and stellar perturbations (of different intensity,
strong case of C/2002 A3 LINEAR and weaker case of C/2013 F3 PANSTARRS) on
cometary motion were detected. Using the best available data from the Gaia DR2
catalogue and some other sources we searched for close stellar passages near
the Sun. Our study took into account that some of the stars are parts of
multiple systems. Over 600 stars or systems that approached or will approach
the Sun closer than 4.0 pc were found. Having the list of perturbers completed
we studied their influence on a sample of 277 Oort spike comets that were
observed since 1901 and discovered that two comets might have their orbits
fundamentally changed due to a close stellar encounter. Our results show how
much different the dynamical evolution of comets would have looked when their
motion was considered only in the Galactic potential. Uncertainties both in
stellar and cometary data were carefully taken into account. Our analysis
indicates that the occurrence of stellar perturbations on cometary motions is
very rare and the uncertainties of these effects are hard to estimate.
