Jupiter-family comets (JFCs) are among the most intriguing objects in the Solar System. Traditionally, they were considered to be well-preserved planetesimals that have spent billions of years beyond the orbit of Neptune and have therefore retained key evidence from the epoch of planet formation. However, recent studies of JFCs, especially ESA’s Rosetta mission to the Jupiter-family comet 67P/Churyumov–Gerasimenko, have challenged this idea. It is now established that this population of comets have been significantly altered and that tracing their evolution is key for understanding the conditions in the Solar System throughout its history.

Today’s JFCs formed in a progenitor population of planetesimals, beyond the orbits of the giant planets. During the epoch of planetary migration ~4 billion years ago, gravitational interactions with the giant planets scattered many millions of them outward, emplacing them on orbits in the trans-Neptunian region. In the current epoch, some of the Trans Neptunian objects (TNOs) have further gravitational interactions with the outer planets and re-enter the middle Solar System, orbiting between the giant planets: first as Centaurs, and then as active JFCs. As the objects move closer to the Sun during the Centaur phase, their temperatures rise, which causes the onset of their activity. Once they transition to JFCs and reach heliocentric distances of ~3-5 au, their activity becomes dominated by water-ice sublimation, they continuously lose material, and undergo noticeable surface changes with every orbit. Finally, JFCs are believed to gradually lose their activity until they become dormant comets, lacking detectable sublimation-driven activity.

The individual stages of the comet life cycle have been studied extensively over the past few decades. However, the findings about the different populations are rarely combined and a complete model of the changes experienced by comet nuclei from the time they re-enter the inner Solar System until their gradual end is still missing. Our team of experts working on telescope observations, theoretical models and spacecraft data will create a synthesis overview of the current knowledge on the life cycle of comets. We will work towards a complete understanding of cometary evolution by linking together the evidence for the different populations collected and analysed by distinct subfields. This synthesis is ideally timed to take advantage of the “ground truth” results from Rosetta and the recent flyby by the New Horizons mission of the small TNO Arrokoth. For the first time, these two missions enabled a direct comparison of spacecraft-detailed measurements of an evolved JFC nucleus to those of an unaltered planetesimal from the TNOs region.

After reviewing the current understanding of comet evolution, we will shift the team’s focus onto the missing knowledge. We plan to create a roadmap to solve the key questions that should be pursued in the coming decade using 1) data from ongoing and planned space missions (New Horizons and Comet Interceptor); 2) current and future observing facilities (e.g. JWST, ELT); 3) upcoming surveys (e.g. the Vera Rubin Observatory’s Legacy Survey of Space and Time; LSST), and 4) numerical simulations. Finally, we will identify the gaps in our understanding that could be addressed with short-term projects and match the expertise of the team, in order to include them in publications written during the active period of the International Team.