Evolutionary Processes in the Centaur Region

TL;DR

The chapter addresses how Centaurs, a diverse population in the giant-planet region, evolve under thermal, collisional, and tidal processing as they move from outer solar-system reservoirs into orbits influenced by Jupiter and Saturn. It argues that orbital evolution governs the environmental conditions that enable surface modification, with thermal processing as the principal driver, supplemented by rare tidal encounters and occasional impacts; space weathering during the Centaur phase is unlikely to dominate. The authors synthesize activity signatures, surface properties, spectra, albedos, and morphologies to constrain evolution while highlighting substantial gaps and variability across source populations. Looking ahead, JWST, LSST, SPHEREx, next-generation telescopes, and spacecraft missions (e.g., Lucy, Comet Interceptor, Centaur-focused concepts) promise transformative insights into Centaur processing, with implications for understanding the TNO–Centaurs–JFC continuum and the early solar system.

Abstract

Centaurs populate relatively short-lived and rapidly evolving orbits in the giant-planet region and are believed to be one of the solar system's most complex and diverse populations. Most Centaurs are linked to origins in the dynamically excited component of the trans-Neptunian region, and are often considered an intermediate phase in the evolution of Jupiter-family comets (JFCs). Additionally, the Centaur region hosts objects from varied source populations and having different dynamical histories. In this chapter, we focus on the physical processes responsible for the evolution of this heterogeneous population in the giant-planet region. The chapter begins with a brief review on the origin and early evolution that determine Centaurs' properties prior to entering the giant-planet region. Next, we discuss the thermal, collisional, and tidal processes believed to drive the changes Centaurs undergo. We provide a comprehensive review of the evidence for evolutionary changes derived from studies of the activity, physical properties, and surface characteristics of Centaurs and related populations, such as trans-Neptunian objects, JFCs, and Trojans. This chapter reveals a multitude of gaps in the current understanding of the evolution mechanisms acting in the giant-planet region. In light of these open questions, we conclude with an outlook on future telescope and spacecraft observations, detailing how they are expected to elucidate Centaur evolution processes.

Figures (4)

• Figure 1: Schematic representation of the dynamical evolution of Centaurs and the minor-planet populations related to them. This diagram highlights the main pathway of the TNOs-Centaurs-JFCs continuum (blue arrows) and depicts the complex dynamical links of Centaurs with other small-body populations. The approximate range of typical orbits is shown by the horizontal extent of a given box. The blue boxes indicate the populations representing the main stages of the Centaur life cycle. In the cases when the population density and/or the definition of a given population varies across different works the shading of the boxes indicates the uncertainty in the heliocentric range. For simplicity, Hildas, Quasi-Hildas, Trojans and irregular satellites have all been depicted together within the heliocentric distance range of the giant planet orbits.
• Figure 2: Example dynamical and resulting thermal evolution for an object leaving the trans-Neptunian region, reaching the giant planet region, and eventually reaching a JFC orbit nesvorny2017gkotsinas2022. The top panel shows the evolution of the perihelion distance as a function of time for the last Myr of evolution before the object is ejected out of the solar system. Shaded areas illustrate the most likely drivers for Centaurs’ activity: CO$_2$ segregation or sublimation (in blue), amorphous water ice (AWI) to crystalline water ice (CWI) transition (in green). We note that these phase transitions can also occur at lower heliocentric distances. The bottom panel shows the distribution of the internal temperature as a function of depth and time, resulting from the orbital evolution. The depths of two isotherms are given as guides: dotted line for 80 K and dash-dotted line for 110 K.
• Figure 3: Minimum tensile strength required for Centaurs to spin at their observed rotation period without breaking apart. For each object, the line gradient represents a range of possible densities (from light to dark: 500 kg m$^{-3}$ to 2500 kg m$^{-3}$). To preserve figure readability, Centaur 2013 XZ8 is not displayed ($P~=~88$ h, $Y_t < 10^{-3}$ Pa).
• Figure 4: Apparent correlation between the phase coefficient ($\beta$) and geometric albedo ($p_\mathrm{V}$) of JFCs (green squares) and other comets (light green diamonds) from Knight2023. Centaurs are overplotted as circles with colors corresponding to their $B-R$ color index. The phase coefficients of Chiron and Echeclus were derived from observations when they were weakly active Dobson2023. With the exception of the very-red (Elatus and Pholus) and the active (Chiron and Echeclus) Centaurs, other Centaurs appear to follow the correlation of increasing phase coefficient with geometric albedo identified by Kokotanekova2018 and do not agree with the phase-coefficient--albedo correlation found for Main-Belt asteroids by Belskaya2000, plotted as a dashed line.