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Hal Levison Colloquium Abstract (Oct 25, 2017)

The Formation of Planets from the Direct Accretion of Pebbles

In recent years, a radical new scenario has recently been suggested for the formation of planets. This scenario, known as pebble accretion, envisions: 1) Planetesimals form directly from millimeter- to meter-sized objects (the pebbles) that are concentrated by hydrodynamic forces and then gravitationally collapse to form 100 - 1000 km objects. 2) These planetesimals quickly sweep up the remaining pebbles because their capture cross sections are significantly enhanced by aerodynamic drag. Calculations show that in the outer Solar System, a single 1000 km object embedded in a swarm of pebbles can grow to ~10 Earth-masses in less than 10,000 years, thereby solving the long-standing timescale issue for the formation of the cores of Jupiter and Saturn. I will present full-scale simulations of a modified version of this process that result in planetary systems that reproduces the gross structure of the Solar System. This includes explaining why Mars is as small as observed, which has been a long-standing mystery.


Technical talk: The Lucy Mission to the Trojan Asteroids

Jupiter's swarms of Trojan asteroids may be remnants of the primordial material that formed the outer planets, and serve as time capsules from the birth of our Solar System more than 4 billion years ago. As such, these primitive bodies hold vital clues to deciphering the history of the solar system, and perhaps even the origins of life and organic material on Earth.  Lucy will be the first space mission to study the Trojans. The mission takes its name from the fossilized human ancestor (called “Lucy" by her discoverers) whose skeleton provided unique insight into humanity's evolution. Likewise, the Lucy mission will revolutionize our knowledge of planetary origins and the formation of the solar system Lucy will launch in October 2021 and will complete a 12-year journey to seven different asteroids — a Main Belt asteroid and six Trojans, the last two members of a “two-for-the-price-of-one” binary system. Lucy’s complex path will take it to both clusters of Trojans and give us our first close-up view of all three major types of bodies in the swarms (so-called C-, P- and D-types). No other space mission in history has been launched to as many different destinations in independent orbits around our sun. Lucy will show us, for the first time, the diversity of the primordial bodies that built the planets