The origin of (47171) Lempo-like Kuiper belt triple systems during binary-binary interactions

Abstract

The trans-Neptunian triple Lempo-Paha-Hiisi is composed of a tight inner binary with components of similar size and an outer companion about half their size orbiting 10 times further away. Large trans-Neptunian objects like Pluto also have multiple small moons, but Lempo's structure is unique in the Solar system, and the place and timing of its origin is still a subject of debate. We propose a new formation mechanism able to form a large number of systems like Lempo-Paha-Hiisi, which involves binary-binary close encounters in the primordial planetesimal disc at 30-40 au. Some of these systems were then implanted in different populations of the trans-Neptunian region during Neptune's outward migration. Our results strongly support that the 4:7 resonant multiple object Manwë-Thorondor was once a triple system similar to Lempo-Paha-Hiisi, but the orbit of the inner binary evolved by tides, becoming a contact binary. As with Lempo-Paha-Hiisi, it should have originated in the planetesimal disc below 30-40 au. Triple systems like Lempo-Paha-Hiisi or Manwë-Thorondor could not have formed in situ and the existence of this kind of system is not expected in the cold classical Kuiper belt.