Planetary Exploration Horizon 2061: A long-term perspective for planetary exploration

Kurzfassung

With the discovery of thousands of extrasolar planetary systems it becomes more and more evident that a large variety of planetary system architectures, including very different types of planets, have been realized in nature. Our solar system is just one among many. We do not know yet whether the evolution of the planets and moons in the solar system is typical for such objects in similar environments, or not. This includes in particular the capability to develop habitable surface conditions, or even life. Planets orbiting host stars different to our Sun can experience very different environmental conditions such as stellar spectral energy distributions and harsh cosmic rays impacting the orbiting planets. The dynamical evolution of planetary systems depends on the formation processes and interactions with the protoplanetary disk as well as migration processes. Looking at extrasolar planets in the sky today, we see systems in different astrophysical environments, at different ages and with different evolutionary histories. As outlined above, the number of processes shaping the characteristics of planets is large. Yet, for extrasolar planets the number of observables is small. Observational constraints are usually limited to orbital parameters, planetary masses, radii, and some of the atmospheric constituents. In fortunate cases additional constraints like magnetic fields and Love numbers will become accessible in the future. Additional constraints are given by the host star characteristics (metallicity, composition, age, temperature, etc.), but the link between stellar properties and planetary characteristics is complex and not fully understood yet. In view of this limited achievable data set, it becomes vital to better understand how we can learn from our detailed knowledge of the bodies in the solar system to better understand the planets and moons in extrasolar systems. Vice versa, extrasolar systems show us the possible variety of planetary systems, which helps us in particular to better understand planet formation processes. Furthermore, with the increasing number of terrestrial planets found orbiting in the habitable zone of their hosts, the number of potential targets to search for life significantly increases. Finding clear evidence for life will, however, require a very good understanding of the biogenic processes on planets, their interaction with the atmosphere, and a careful study of nonbiogenetic processes to avoid false alarms. So far, we know life only in our solar system, on Earth. It is therefore crucial to bring together the knowledge we have from both the detailed solar system view and the statistical view of a large number of extrasolar planets. After a brief review of the missions planned to study solar system and extrasolar planets in the future, our knowledge of planets and planetary systems is reviewed and prospects for synergies of solar system and exoplanet research discussed. The discussion includes planet formation processes as well as the geophysical evolution of planets and moons. Particular emphasis is given to the understanding of habitats and search for biosignatures in extrasolar planets and lessons learned from the Earth. Future large telescope facilities will allow us to search for biosignatures outside the solar system, enhancing the prospects to answer the long-standing question whether life has developed elsewhere in our galaxy, at least within our neighborhood. The contents presented in this chapter draw heavily on the presentations, discussions, and final report of the forum “Solar System/Exoplanet Science Synergies in a multi-decadal Perspective” jointly organized by the Europlanet Research Infrastructure and the International Space Science Institute in Bern, Switzerland, on February 19 and 20, 2019.