Exploring other worlds: Science questions for future direct imaging missions

SAG15 Team

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The SAG15 team has solicited, collected, and organized community input on high-level science questions that could be addressed with future direct imaging exoplanet missions and the type and quality of data answering these questions will require (see Appendix A: SAG15 Charter for details). Input was solicited through a variety of forums and the report draft was shared with the exoplanet community continuously during the period of the report development (Nov 2015 - May 2017). The report benefitted from the input of over 50 exoplanet scientists and from multiple open-forum discussions at exoplanet and astrobiology meetings. The report considered the expected science yield of current and approved major space- and ground-based telescopes and instruments within the timeframe relevant for future direct imaging missions. In particular, Gaia, JWST, WFIRST, Plato, and the 30m-class ground-based telescopes will provide important answers to a variety of exoplanet science questions. The authors agreed that some science questions that are important today will be partly or fully answered by the time new direct imaging exoplanet missions may fly; while some questions that cannot be addressed with existing technology are expected to emerge as central questions in the next decades. The SAG15 team has identified three group of questions, those that focus on the properties of planetary systems (Questions A1-A2), those that focus on the properties of individual planets (Questions B1-B4), and questions that relate to planetary processes (Questions C1-C4). The questions in categories A, B, and C require different target samples and often different observational approaches. Figure 2 provides a visual summary of the key questions, the type of targets, and the types of data required for answering them. The two set of questions on the properties of planetary systems aim to explore the architecture and diversity of exoplanets (massive and low-mass, detected and undetectable) and planetesimals and planetesimal belts. Specifically, Question A1 seek to determine the diversity of planetary architectures, and to identify if there are typical classes of planetary architectures. Answering these questions will also naturally establish the Solar System's relation to the multitude of planetary systems. Question A2 focuses on the distribution and properties of planetesimal belts and exo-zodiacal disks, tracers of the planetesimal population and”fossils” from the planet formation process. Questions in the second group focus on the properties of individual planets. Question B1 explores how rotational periods, planetary obliquity, orbital elements, and planet mass/composition are connected. Identifying correlations between these sets of parameters will provide constraints on the formation and evolution of individual planets, and determining these parameters is also important for atmospheric modeling of the climate and atmospheric evolution of habitable planets. Question B2 seeks to understand which rocky planets harbor liquid water on their surface. Question B3 explores the origins and composition of clouds and hazes in ice and gas giant exoplanets and their dependence on the fundamental atmospheric and orbital parameters. Question B4 focuses on rocky planets (habitable and non-habitable) and aims to understand the interplay of photochemistry, transport chemistry, surface chemistry, and mantle outgassing, and their effects on the atmospheric compositions of these planets. Questions in the third group focus on the evolution of exoplanets and on processes that drive the evolution; questions in this group often require observations that are likely to partly or completely exceed the capabilities of next-generational direct imaging telescopes but may nevertheless be important for missions on the longer-term horizon. Specifically, Question C1 asks what processes and properties set the modes of atmospheric circulation and heat transport in exoplanets and how do these vary with system parameters. Question C2 focuses on rocky planets and seeks to understand the types of evolutionary pathways that are possible for these bodies and what factors determine which pathway will be followed by a given planet. Question C3 seeks understanding of geophysical/geological activity and interior processes in rocky planets, in part to probe the presence of plate tectonics and continent forming/resurfacing processes. For each questions we summarize the current body of knowledge, the available and future observational approaches that can directly or indirectly contribute to answering the question, and provide examples and general considerations for the target sample required. The questions identified in this report suggest a trend in which questions will increasingly aim to understand complex processes that occur in (or set the properties of) planetary atmospheres and, to some extent, interiors, rather than only exploring the planets' physical properties (mass, density, orbital elements). The community also identified the need for developing a contextual understanding of rocky planets as a prerequisite to correctly interpreting biosignatures (covered in the SAG16 report). Our report also provides guidelines and examples for the types and quality of observations required for addressing the individual questions, but a comprehensive and detailed study of the set of required instrumental capabilities is beyond the scope of the current study. Furthermore, we discuss the importance of auxiliary datasets for individual questions and the impact of the independent measurements of planetary mass. The wide-ranging questions discussed in our report demonstrate how little we know about extrasolar planets, planetary systems, and habitable worlds; and they also highlight the enormous science gains future direct imaging missions will lead to, as well as the fact that comprehensive, multi-mission/multi-instrument studies are often required to provide a thorough understanding of other planets.

Original languageEnglish (US)
JournalUnknown Journal
StatePublished - Aug 9 2017

ASJC Scopus subject areas

  • General

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