<p><span style="background-color: rgb(255, 255, 255); color: rgb(0, 0, 0);">The region of the Solar System immediately beyond the orbit of Neptune -- known as the Edgeworth-Kuiper Belt, or simply Kuiper Belt -- stands as the only location in the Universe where pristine planetesimals, the 100 km-sized building blocks of planets, are accessible to local direct study. In deep freeze since their formation over 4.5 billion years ago, 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 dynamical dust-gas "streaming" instability, and growth by accretion of mm- and cm-sized grains ("pebbles"). 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.</span></p>
