Special Department Lecture - Darryl Seligman [Chicago]

Darryl Seligman [Chicago]
Wednesday, March 17, 2021 - 4:00pm to 5:00pm
Virtual via Zoom


A Galactic Census of Minor Bodies: What Are They, How Do They Form, and Where Do They Come From?


Minor bodies opened windows into our understanding of the solar system’s formation and evolution. The Kirkwood gaps in the asteroid belt led to the discovery that seemingly immutable solar system bodies were chaotic, and the discovery of the Kuiper belt led to the realization that the giant planets underwent a transient period of violent instability. `Oumuamua (I1 2017) was the first macroscopic body observed to traverse the inner solar system on an unbound hyperbolic orbit, making it the first interstellar object ever detected up close. `Oumuamua’s light curve displayed strong periodic variation, and it showed no hint of a coma or emission from molecular outgassing. Astrometric measurements indicate that `Oumuamua experienced non-gravitational acceleration on its outbound trajectory, but energy balance arguments indicate this acceleration is inconsistent with a water ice sublimation-driven jet of the type exhibited by solar system comets. In this talk, I will show that all of `Oumaumua's observed properties can be explained if it contained a significant fraction of molecular hydrogen ice. H2 sublimation at a rate proportional to the incident solar flux generates a surface-covering jet that reproduces the observed acceleration while maintaining the P ~8h photometric period. Mass wasting from sublimation leads to monotonic increase in the body axis ratio, explaining `Oumuamua's shape. Back-tracing `Oumuamua's trajectory through the solar system permits calculation of its mass and aspect ratio prior to encountering the Sun. I show that H2-rich bodies plausibly form in the coldest dense cores of Giant Molecular Clouds, where number densities are of order n~105, and temperatures approach the T=3 K background. Post-formation exposure to galactic cosmic rays implies a 40 Myr age, explaining the kinematics of `Oumuamua's inbound trajectory. If the hypotheses raised in my recent papers are correct, it appears likely that there exist two distinct and roughly equally numerous populations of interstellar objects: `Oumuamua-like, dark, hydrogen icebergs that form in GMC cores and Borisov-like interstellar comets that are ejected by giant exoplanets.  I assess the near-term prospects for detecting and observing (both remotely and in-situ) future solar system visitors of this type. I conclude that, similar to the time period after the discovery of 15760 Albion in 1992 when thousands of trans-Neptunian objects were detected and characterized, we are on the precipice of a wave of detections of interstellar objects, and they may offer a similar window into the formation of extrasolar planets and stars throughout the galaxy.

About this Series

Weekly talks given by leading thinkers in the areas of geology, geophysics, geobiology, geochemistry, atmospheric science, oceanography, climatology, and planetary science. Lectures take place on Wednesdays from 4pm EST unless otherwise noted. For more information please contact: Maggie Cedarstrom, maggie84@mit.edu.