Is there an exoplanet in the Solar System?
Alexander J. Mustill, Sean N. Raymond, Melvyn B. Davies

TL;DR
This paper explores the possibility that the hypothesized Planet 9 was captured from another star in the Sun's birth cluster, using simulations to evaluate the likelihood and implications of such a scenario.
Contribution
It demonstrates through N-body simulations that Planet 9-like objects can be captured during stellar encounters in the Sun's birth cluster, providing a plausible formation pathway.
Findings
A few percent of slow stellar encounters can result in capture of Planet 9-like orbits.
Many stars in the cluster could host highly-eccentric, wide-orbit planets.
Capture scenarios predict a large population of aligned Trans-Neptunian Objects.
Abstract
We investigate the prospects for the capture of the proposed Planet 9 from other stars in the Sun's birth cluster. Any capture scenario must satisfy three conditions: the encounter must be more distant than ~150 au to avoid perturbing the Kuiper belt; the other star must have a wide-orbit planet (a>~100au); the planet must be captured onto an appropriate orbit to sculpt the orbital distribution of wide-orbit Solar System bodies. Here we use N-body simulations to show that these criteria may be simultaneously satisfied. In a few percent of slow close encounters in a cluster, bodies are captured onto heliocentric, Planet 9-like orbits. During the ~100 Myr cluster phase, many stars are likely to host planets on highly-eccentric orbits with apastron distances beyond 100 au if Neptune-sized planets are common and susceptible to planet--planet scattering. While the existence of Planet 9…
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