Direct contact binary planetesimal formation from gravitational collapse

Abstract

Bilobate contact binaries comprise a significant fraction of the relict Kuiper Belt, which includes the exemplary contact binary (486958) Arrokoth. The surfaces of its lobes contain similar amounts of highly volatile chemical species and few craters, indicating formation in a homogeneous and gentle environment. Arrokoth's bilobate shape was initially hypothesized to have formed via the direct gravitational collapse of a pebble cloud in the solar system's protoplanetary disk. However, alternative hypotheses have proposed that Arrokoth may be the result of binary planetesimal formation and the subsequent dynamical evolution of the binary components into contact through external perturbations over long timescales. Here, we show that contact binary planetesimals like Arrokoth can form directly from the gravitational collapse of pebble clouds. We used a soft-sphere discrete element method (SSDEM) to discover that planetesimals form a wide variety of shapes, including bilobate contact binaries. This method creates planetesimals as particle-aggregates with particles resting upon each other's surfaces via mutual surface penetration. The formation of contact binaries in our simulations strengthens the hypothesis that Arrokoth, and perhaps many other contact binaries in the Kuiper Belt, formed directly as bilobate objects from gravitational collapse, and so their shapes and surfaces record the era of planet formation.

Figures (4)

• Figure 1: Several examples of contact binary planetesimals created using the PKDGRAV SSDEM (panels a--d and f--i) as well as two shape models of (486958) Arrokoth from Keane2022 (panel e, left) and Porter2024 (panel e, right). Each contact binary is shown as a pair of images from two perspectives, equatorial and polar, with associated scale bars included for reference. The black-colored lobes have the most mass prior to impact and are therefore considered the primary lobes, while the blue-colored lobes have less mass and so are the secondary lobes. Red-colored particles accreted onto their respective contact binary planetesimal after the lobes made contact.
• Figure 2: Contact binary spin rates from simulated and observed populations as a function of the collision velocities of the mutually orbiting lobes. SSDEM contact binaries are shown as red solid and dotted circles. The dotted circles are simulated planetesimals that did not have as clear a bilobate shape as indicated by red-solid circles, but may be considered contact binaries because they are each the result of two mutually orbiting planetesimals that made contact to create a near-bilobate elongated shape. The spin rates of suspected contact binaries from the Kuiper Belt are displayed as gray dashed lines Kern2006Spencer2006Lacerda2011Thirouin2017aThirouin2017bThirouin2018Thirouin2019aRabinowitz2020Thirouin2022Thirouin2024Porter2025, and the spin rate of the confirmed contact binary Arrokoth is displayed as a black dashed line Buie2020. One value of contact velocity, for the planetesimal in Fig. \ref{['fig:cb_composite']}f, resides outside the current axes; it has a contact velocity of $\sim$ 16.9 m s$^{-1}$ with a spin rate of 2.65 rev/day. Theorized permissible contact velocities for the Arrokoth lobes are shown as a red vertical band McKinnon2020.
• Figure 3: Contact binary lobe accretion efficiencies $\left(m_1+m_2 \right)/M_{cloud}$, where $M_{cloud}$ is the initial pebble cloud mass, are shown as a function of their lobe mass ratios $m_2/m_1$. The main figure shows the full scope of mass ratios and accretion efficiencies of binary planetesimal systems from the SSDEM simulations. The gray dotted line indicates the region displayed in the left sub-figure, which is a comparison of all simulated and observed contact binary planetesimals. SSDEM contact binaries are displayed as circles (solid and dotted), SSDEM binary planetesimal systems Barnes2025 are displayed as squares, the mass ratios of suspected contact binaries from the Kuiper Belt are displayed as gray vertically dashed lines, Lacerda2007Thirouin2017aThirouin2017bThirouin2018Thirouin2019aFarkas-Takacs2020Rabinowitz2020Thirouin2022Thirouin2024, and the mass ratios of the confirmed contact binary Arrokoth are displayed as pink vertically dashed Keane2022 and dotted Porter2024 lines. As in Fig. \ref{['fig:contact_vel']}, the dotted circles are simulated planetesimals that did not have as clear a bilobate shape, but may be considered contact binaries because they are each the result of two mutually orbiting planetesimals that made contact to create a near-bilobate elongated shape.
• Figure 4: The prolateness $\left(1 - b/a\right)$ and oblateness $\left(1 - c/a\right)$ of simulated and observed contact binary planetesimals. Simulated contact binary planetesimals are shown as solid and dotted red circles as in Figs. \ref{['fig:contact_vel']} and \ref{['fig:mass_ratios']}. As in Fig. \ref{['fig:contact_vel']}, the red-dotted circles are simulated planetesimals that did not have as clear a bilobate shape as indicated by red-solid circles, but may be considered contact binaries because they are each the result of two mutually orbiting planetesimals that made contact to create a near-bilobate elongated shape. SSDEM contact binaries are compared to suspected contact binaries in the asteroid belt (201 Penelope, Shepard2015; 216 Kleopatra, Shepard2018, 413 Edburga, Shepard2015) and Trojan (624 Hektor, Descamps2015) populations (black pentagons), suspected contact binaries in the Kuiper Belt (2004 TT357, Thirouin2017a; 2014 JL80, 2014 JO80, 2014 JQ80, Thirouin2018; and 2004 VC131, Thirouin2019a; gray pentagons), and two models of 486958 Arrokoth from Keane2022 (red pentagon) and Porter2024PDSPorter2024 (red pentagon with dashed line).