Alessandro Morbidelli

Chaire Chaire Formation planétaire : de la Terre aux exoplanètes

Collège de France

Année 2025-2026

David Nesvorný

Conférence - David Nesvorný : Dynamical Origins and Properties of the Near-Earth Object Population

David Nesvorný

Southwest Research Institute, USA

Résumé

Near-Earth Objects (NEOs) are a transient population of small bodies with orbits in or near the terrestrial planet region. They represent an intermediate stage in the dynamical evolution of asteroids and comets - originating in the main belt or trans-Neptunian scattered disk - and ending with planetary impacts, solar disintegration, or ejection from the Solar System. We present an accurate physical model of NEOs developed by tracking the dynamical evolution of main-belt asteroids and comets onto NEO orbits. The model is calibrated using data from the Catalina Sky Survey (CSS) and Wide-field Infrared Survey Explorer (WISE). Results show a size-dependent contribution from the main belt, with the ν6 and 3:1 resonances accounting for ~80% of faint NEOs. The debiased NEO albedo distribution is well fit by a sum of two Rayleigh distributions with scale parameters of 3% and 17%. We estimate 830±60 NEOs with diameters >1 km and 20,000±2,000 with diameters >140 m. The Earth impact probability for a >140 m object over the next millennium is ~5%. Compared to a direct-delivery model from the main belt, the CSS-detected small NEOs show an excess of low-eccentricity orbits between 1–1.6 au. This excess may result from tidal disruption of large NEOs during close encounters with the Earth.

Alessandro Morbidelli

Chaire Chaire Formation planétaire : de la Terre aux exoplanètes

Collège de France

Année 2025-2026

09 - Les astéroïdes et leurs échantillons : les météorites – témoins des premières phases de la formation planétaire : Formation et évolution de la ceinture des astéroïdes

Alessandro Morbidelli

Chaire Chaire Formation planétaire : de la Terre aux exoplanètes

Collège de France

Année 2025-2026

David Nesvorný

Conférence - David Nesvorný : Collisional Families in the Asteroid Belt and Sources of Meteorites

David Nesvorný

Southwest Research Institute, USA

Résumé

Main-belt asteroids originally formed in a dynamically quiet disk, but their orbits were later stirred by Jupiter's gravity, leading to high-speed collisions. Over the age of the Solar System, dozens of large asteroids have been disrupted by such impacts, producing groups of fragments known as asteroid families. Here we explain how asteroid families are identified, review the current inventory, and discuss how they provide insight into the collisional and dynamical evolution of the asteroid belt. Currently, about 360 asteroid families have been cataloged, including ~100 that formed within the past 10 million years. Studies of these families shed light on the physics of large-scale collisions - a key process in the formation of Earth and other terrestrial planets - and place important constraints on the belt's collisional history. Some findings suggest that much of the asteroid belt may be composed of fragments from early, unresolved breakups. Asteroid families also play a crucial role in understanding orbital evolution, including the influence of radiative forces like the Yarkovsky and YORP effects, as well as resonant interactions - processes that underpin the delivery of near-Earth asteroids and meteorites.

Alessandro Morbidelli

Chaire Chaire Formation planétaire : de la Terre aux exoplanètes

Collège de France

Année 2025-2026

Sébastien Charnoz

Université Paris-Cité, IPGP, Paris

Séminaire - Sébastien Charnoz : Condensation hors-équilibre et oxydation dans le Système solaire

Alessandro Morbidelli

Chaire Chaire Formation planétaire : de la Terre aux exoplanètes

Collège de France

Année 2025-2026

08 - Les astéroïdes et leurs échantillons : les météorites – témoins des premières phases de la formation planétaire : Une vision globale (partielle) de l'évolution initiale du Système solaire interne

Alessandro Morbidelli

Chaire Chaire Formation planétaire : de la Terre aux exoplanètes

Collège de France

Année 2025-2026

David Nesvorný

Conférence - David Nesvorný : Following Comets to Their Distant Source Reservoirs and Back

David Nesvorný

Southwest Research Institute, USA

Résumé

Comets are icy bodies that originate in the trans-Neptunian region and evolve into the inner Solar System, where they become active due to the sublimation of water ice triggered by solar heating. We describe dynamical models that trace the implantation of comets into their distant source reservoirs during the early Solar System, approximately 4.6 billion years ago. These models are used to predict the present-day orbital distribution and number of active comets, which are then compared to observational data. The results indicate that the scattered disk and the Oort cloud are the primary source reservoirs for ecliptic comets (short-period) and isotropic comets (long-period and Halley-type), respectively. The observed orbital distribution of ecliptic comets is well reproduced if their physical lifetimes are limited to a few hundred perihelion passages within 2.5 astronomical units of the Sun. In contrast, small Oort-cloud comets tend to be more active and typically disintegrate after only a few such passages. Halley-type comets appear to represent an extension of the population of large, returning Oort-cloud comets to shorter orbital periods. We explore how the dynamical structure of cometary reservoirs is affected by the Galactic tide and Sun's birth environment within a stellar cluster.

Alessandro Morbidelli

Chaire Chaire Formation planétaire : de la Terre aux exoplanètes

Collège de France

Année 2025-2026

07 - Les astéroïdes et leurs échantillons : les météorites – témoins des premières phases de la formation planétaire : La dynamique des poussière et la formation des planétésimaux: théorie vs. observations

Alessandro Morbidelli

Chaire Formation planétaire : de la Terre aux exoplanètes

Collège de France

Année 2025-2026

David Nesvorný

Conférence - David Nesvorný : Formation of Equal-Size Binaries in the Kuiper Belt

David Nesvorný

Southwest Research Institute, USA

Résumé

A critical step in the emergence of planets within a protoplanetary disk is the accretion of planetesimals - bodies ranging from 1 to 1,000 kilometers in size - formed from smaller solid constituents. However, this process remains poorly understood, largely due to limited observational constraints on the complex physical mechanisms involved in planetesimal formation. In the outer Solar System, the best place to look for clues is the Kuiper belt, where a population of icy planetesimals survived to this day. We present evidence that Kuiper Belt planetesimals formed via the streaming instability, a process in which aerodynamically concentrated clumps of pebbles undergo gravitational collapse to form 100-kilometer-class bodies. This mechanism explains the high prevalence of equal-sized binaries observed in the Kuiper Belt. The model predicts a broad inclination distribution and a predominance of prograde binary orbits, consistent with observations of trans-Neptunian binaries. Given its robustness across a wide range of protoplanetary disk conditions, the streaming instability is likely to have played a central role in seeding planetesimal formation throughout the Solar System and in other planetary systems as well.

Alessandro Morbidelli

Chaire Formation planétaire : de la Terre aux exoplanètes

Collège de France

Année 2025-2026

06 - Les astéroïdes et leurs échantillons : les météorites – témoins des premières phases de la formation planétaire : La dichotomie isotopique

Alessandro Morbidelli

Chaire Formation planétaire : de la Terre aux exoplanètes

Collège de France

Année 2025-2026

Séminaire - Clara Maurel : Le paléomagnétisme des météorites : de l'évolution du disque protoplanétaire à la structure interne des corps parents

Clara Maurel

CNRS, CEREGE, Aix-en Provence