Evidence for Pebbles in Comets

TL;DR

The paper presents evidence that particles observed around Comet 103P/Hartley 2 are consistent with pebbles, supporting theories that comets and planetesimals form from pebble-sized aggregates in the early Solar System.

Contribution

It provides observational support linking comet particles to pebble-sized building blocks predicted by planet formation models.

Findings

Particles match expected pebble sizes for planetesimal formation.

Supports the pebble accretion model in comet formation.

Indicates formation regions of comets based on particle sizes.

Abstract

When the EPOXI spacecraft flew by Comet 103P/Hartley 2, it observed large particles floating around the comet nucleus. These particles are likely low-density, centimeter- to decimeter-sized clumps of ice and dust. While the origin of these objects remains somewhat mysterious, it is possible that they are giving us important information about the earliest stages of our Solar System's formation. Recent advancements in planet formation theory suggest that planetesimals (or cometestimals) may grow directly from the gravitational collapse of aerodynamically concentrated small particles, often referred to as "pebbles." Here we show that the particles observed in the coma of 103P are consistent with the sizes of pebbles expected to efficiently form planetesimals in the region that this comet likely formed, while smaller pebbles are may be expected in the majority of comets, whose chemistry is often indicative of formation in the colder, outer regions of the protoplanetary disk.