Capture of interstellar objects in the Solar system

University essay from Lunds universitet/Astronomi

Abstract: Research on InterStellar Objects (ISOs) has been done for several decades, however, the first observed ISO in the Solar system was only recently discovered in 2017. It was named 1I/'Oumuamua and was only passing though the Solar system, but this may not be the case for all objects of its kind. For instance, one could gain a bound orbit around the Sun if the right amount of nudge is acquired when passing by a planet. The purpose with this thesis is to determine the capture rate of interstellar objects in the Solar system and the orbital features of captured objects. This has been done by N-body simulations where we include the four giant planets Jupiter, Saturn, Uranus and Neptune, and ISOs flying towards the Solar system from random directions. One set of simulations is done regarding how captures happen, e.g. whether encountering a planet is necessary or not. The results suggest that a close encounter with a planet is essential for a capture to happen. The contribution on capture rate from each planet is also studied by doing separated simulations with that planet. Maybe a bit surprisingly, the capture rate is not additive in that the overall capture rate of the Solar system is lower than the sum of that of each planet, though still higher than any individual planet. We suspect the reason could lie in increased possibility for encounters with the planets, which can cause quick ejections. Then we calculate the actual capture rate assuming a Maxwellian velocity distribution for the ISOs with a dispersion of 30 km/s, and the resulting volume capture rate is 0.0120(3) au^3/yr. Therefore, with a current estimate for number density of 0.1 au^-3 for 'Oumuamua-like objects, we expect to capture 1200 objects per Myr. The orbit of captures are typically very eccentric with large semi-major axis. These captured objects may then encounter planets frequently, and as a result, get ejected from the Solar system within a few Myr.

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