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New paper with @hannorein is submitted and on the arXiv! A substellar flyby that shaped the orbits of the giant planets.
We estimate that there is a 0.1% chance that a flyby has the needed characteristics — mass, distance and velocity — to account for the eccentricities and relative inclinations of the solar system’s giant planets.
https://arxiv.org/abs/2412.04583
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There’s no single process that explains every solar system feature. But we can assess a process’s likelihood. Planet formation theories suggest planets form from circular, coplanar protoplanetary disks. Yet, we observe planets with modest eccentricities and inclinations.
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Consequently, the origin of the eccentricities and inclinations of the giant planets in the solar system remains a puzzle. Previous studies have explained these features using planet-planet interactions. However, our investigation explores the possibility of a flyby event as an explanation.
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The solar system formed in an open star cluster that dispersed over 10–100 Myr. Given stellar density and velocity distribution, we can estimate the likelihood of a flyby within a given distance from the Sun. We can also estimate the flyby object’s mass using an initial mass function (IMF).
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Observation limitations restrict us from detecting objects below the stellar-substellar boundary, as they are too faint to be seen directly. Consequently, the IMF distribution below the boundary is uncertain. However, extending the IMF into the substellar regime is consistent with microlensing surveys.
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In our study, we sample flyby masses from 1 Jupiter mass to 100 solar masses and consider flyby encounters within 20 AU, or the orbit of Uranus.
This is novel. No other study has investigated the impact of substellar flyby objects passing this close to the Sun while the solar system is still in its primordial star cluster.
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We find that exciting the gas giants’ orbits with a substellar flyby is surprisingly efficient and certainly not like trying to find a needle in a haystack. In other words, assuming that Jupiter-mass objects form via collapsing gas and dust, there is a 1 in 100 chance that a flyby within 20 AU will result in a system similar to the solar system.
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While this assumes similar masses and semi-major axes to today’s gas giants, the likelihood of a flyby within 20 AU is at most 1 in 100. Thus, there’s a 1-in-10,000 chance a substellar flyby caused the solar system’s eccentricities and inclinations.
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Given the 10^10 stars in the galaxy, a 1-in-10^4 chance is highly likely. The uniqueness of our solar system lies in forming the gas giants on their observed orbits. Given the likelihood of substellar flyby encounters, no solar system formation scenario should exclude them entirely.
Check out Hanno’s thread as well.
https://mastodon.social/@hannorein/113636973058064895
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