Wladimir Lyra (New Mexico State University): What are the Kuiper Belt objects telling us about planet formation?

Thursday 30 Oct 2025, 14:00 → 15:00 Europe/Stockholm

90101

Title: What are the Kuiper Belt objects telling us about planet formation?
Speaker: Wladimir Lyra
Affiliation: New Mexico State University
Room: 90101
Time: 30.10.2025, 14:00  

Abstract:

The Kuiper Belt stands as the only location in the Universe where pristine planetesimals are accessible to local direct study. In deep freeze beyond the orbit of Neptune, Kuiper Belt Objects (KBOs) preserve a record of how solids accumulated in the early Solar System, offering key tests for planet formation theory. One particularly striking trend is seen in the densities of these objects: larger KBOs are up to five times denser than smaller ones, a dichotomy difficult to explain under the usual assumption of constant composition and collisional growth. In this talk, I will show how this trend can be explained by formation models based on streaming instability and pebble accretion. I will present planetesimal formation simulations tracking ice and rock fractions, and a major update on the theory of pebble accretion by including a continuous multi-species (polydisperse) distribution of grain sizes. Accretion rates of low-mass objects rise by up to two orders of magnitude compared to single-species accretion, thus eliminating the need for a long phase of collisional growth. Our model reproduces the observed density–mass relation of KBOs, also constraining the formation of the higher-mass objects to the region around 20AU. Finally, the observed distribution of KBO binaries reveals a gap in the range between 1e19-1e20 kg, coinciding with the truncation in the initial mass function of planetesimals seen in simulations. Together, these results link KBO densities, growth timescales, and architecture into a consistent picture of formation by streaming instability and growth by pebble accretion, mechanisms that likely universally shape planet formation.