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.

Dynamics of the Oort Cloud In the Gaia Era I: Close Encounters

Comets in the Oort cloud evolve under the influence of internal and external perturbations from giant planets to stellar passages, the Galactic tides, and the interstellar medium.Using the positions, parallaxes and proper motions from TGAS in Gaia DR1 and combining them with the radial velocities from the RAVE-DR5, Geneva-Copenhagen and Pulkovo catalogues, we calculated the closest encounters the Sun has had with other stars in the recent past and will have in the near future. We find that the stars with high proper motions near to the present time are missing in the Gaia-TGAS, and those to tend to be the closest ones. The quality of the data allows putting better constraints on the encounter parameters, compared to previous surveys.

Comet Showers

Variations in the influx rate of incoming Oort cloud comets, leading to the occurrence of comet showers, are reviewed with special emphasis on the dynamical processes that produce them. It is found that comet showers as intense as 10-100 times the background comet flux may occur at average intervals of several 107 years, being very close stellar passages at distances ≈ 104AU and, perhaps, penetrating encounters with intermediate-size molecular clouds the trigger mechanisms. Penetrating encounters with giant molecular clouds and stellar passages at distances of a few 103AU may trigger comet showers about 102 − 103 times more intense than the background comet flux at average intervals of a few 108 years. The impact cratering record on the Earth does not show any clear evidence of past comet showers. Some orbital properties of new and dynamically young comets suggest that the influx rate of Oort cloud comets is currently near its background level.