Solar System Physics for Exoplanet Research Horner, J.1*, Kane, S. R.2, Marshall, J. P.3, Dalba, P. A.2†, Holt, T. R.1,4, Wood, J.1,5, Maynard- Casely, H. E.6, Wittenmyer, R.1, Lykawka, P. S.7, Hill, M.2, Salmeron, R.8,9,1, Bailey, J.10, Löhne, T.11, Agnew, M.12, Carter, B. D.1, Tylor, C. C. E.1 1 Centre for Astrophysics, University of Southern Queensland, Toowoomba, QLD 4350, Australia 2 Department of Earth and Planetary Sciences, University of California, Riverside, CA 92521, USA 3 Academia Sinica, Institute of Astronomy and Astrophysics, 11F Astronomy-Mathematics Building, NTU/AS campus, No. 1, Section 4, Roosevelt Rd., Taipei 10617, Taiwan 4 Southwest Research Institute, Department of Space Studies, Boulder, CO. USA 5 Hazard Community and Technical College, 1 Community College Drive, Hazard, Ky, USA 41701 6 Australian Nuclear Science and Technology Organisation, Locked Bag 2001, Kirrawee DC, NSW, Australia 7 Kindai University, 3–4–1 Kowakae, Higashiosaka City, Osaka, 577–8502, Japan 8 Research School of Astronomy and Astrophysics, Australian National University, Canberra, ACT 2611, Australia 9 Mathematical Sciences Institute, Australian National University, Canberra, ACT 2601, Australia 10 School of Physics, University of New South Wales, Sydney, New South Wales, Australia 11 Astrophysikalisches Institut, Friedrich-Schiller-Universität Jena, Schillergäßchen 2–3, 07745 Jena, Germany 12 Centre for Astrophysics and Supercomputing, Swinburne University of Technology, Hawthorn, Victoria 3122, Australia * e-mail: [email protected] †: National Science Foundation Astronomy & Astrophysics Postdoctoral Fellow 1 ABSTRACT Over the past three decades, we have witnessed one of the great revolutions in our understanding of the cosmos - the dawn of the Exoplanet Era. Where once we knew of just one planetary system (the Solar system), we now know of thousands, with new systems being announced on a weekly basis. Of the thousands of planetary systems we have found to date, however, there is only one that we can study up- close and personal - the Solar system. In this review, we describe our current understanding of the Solar system for the exoplanetary science community - with a focus on the processes thought to have shaped the system we see today. In section one, we introduce the Solar system as a single well studied example of the many planetary systems now observed. In section two, we describe the Solar system’s small body populations as we know them today - from the two hundred and five known planetary satellites to the various populations of small bodies that serve as a reminder of the system's formation and early evolution. In section three, we consider our current knowledge of the Solar system’s planets, as physical bodies. In section four we discuss the research that has been carried out into the Solar system's formation and evolution, with a focus on the information gleaned as a result of detailed studies of the system's small body populations. In section five, we discuss our current knowledge of planetary systems beyond our own - both in terms of the planets they host, and in terms of the debris that we observe orbiting their host stars. As we learn ever more about the diversity and ubiquity of other planetary systems, our Solar system will remain the key touchstone that facilitates our understanding and modelling of those newly found systems, and we finish section five with a discussion of the future surveys that will further expand that knowledge. Keywords: comets: general; minor planets, asteroids: general; Kuiper belt: general; Oort Cloud; planets and satellites: general; celestial mechanics; meteorites, meteors, meteoroids 2 1. INTRODUCTION Prior to the discovery of the first planets around other stars (Gamma Cephei Ab and HD 114762b, initially thought to be brown dwarfs (Campbell et al., 1988; Latham et al., 1989); the planets orbiting pulsars PSR 1257+12 (Wolszczan & Frail, 1992) and PSR B1620-26 (Thorsett, Arzoumanian & Taylor, 1993); and 51 Pegasi, the first planet around a Sun-like star (Mayor & Queloz, 1995)), our ideas on how planetary systems formed and evolved were based purely on observations of the Solar system. A wide range of scenarios had been proposed, of varying complexity, all of which attempted to explain the minutiae of our Solar system. For many years, it was believed that the Solar system was formed as a result of an encounter between the Sun and a passing star, which was thought to have pulled a tongue of material either from our star, or that which it encountered, that subsequently fragmented to form the planets (e.g. Lyttleton, 1936; Woolfson, 1964). Such a scenario would suggest that the formation of planetary systems should be an incredibly rare and violent process, since the likelihood of passing stars experiencing such close encounters (nearly colliding with one another) is almost vanishingly small. The theory which held sway at the start of the epoch of exoplanetary discovery argued instead that the planets had formed from a primordial circumstellar disc around the Sun, and that their formation was essentially a gentle, leisurely process (e.g. Swedenborg, 1734; Laplace, 1796; Edgeworth, 1949; Safronov, 19691; Lissauer, 1993). The expectation was that, if we were we to discover planets around other stars, those planetary systems would strongly resemble our own. The fact that those first detected systems (and, indeed, the vast majority of those discovered since) were dramatically different to our own led to a thorough re-examination of how planetary systems could form. Since the discovery of the first planets orbiting other stars, the fields of exoplanetary and Solar system science have remained largely separate – one dealing with a small amount of information about a large number of systems, the other dealing with our almost overwhelmingly detailed knowledge of our own Solar system. In an attempt to help remedy the disconnect between the two fields, in this work, we present a review of our current knowledge and understanding of the Solar system. In the sections that follow, we detail the various populations of objects known in the Solar system and describe the theories that attempt to explain how they arrived in their current locations. We include some of the more speculative ideas within Solar system astronomy, in order to ensure that this review is as clear and comprehensive as possible. Once all these concepts have been discussed, we move on to a discussion of our burgeoning knowledge of exoplanetary systems, with a particular focus on highlighting the similarities and differences between those systems and our own. Since this is by nature a lengthy review, spanning a wide variety of topics, we here present a short bulleted summary of the various topics described in this work, together with pointers to the relevant section, and indications of any exoplanetary systems to which those ideas may apply (where appropriate). ● In section 2, we describe the Solar system in its entirety, as known today. ● In section 3, we introduce the Solar system’s planets, describing them as physical objects – from their atmospheres to our current understanding of their interiors and compositions. ● In section 4, we give a brief overview of the key processes thought to have occurred in the Solar system's formation. That section comprises the following subsections: o In section 4-1, we describe the pivotal role played by giant impacts in the formation of the Solar system we observe today. 1 For an English language translation of Safronov (1969), we direct the interested reader to Safronov (1972). 3 o In section 4-2, we discuss the impact rate within our Solar system in more general terms. o In section 4-3, we consider the migration, both inward and outward, of the giant planets of the Solar system. o In section 4-4, we discuss the ongoing debate over the origin of water on the Earth. o In section 4-5, we discuss theories that invoke the presence of planet-mass objects moving sufficiently far from the Sun that they have, to date, escaped detection. o In section 4-6, we consider the formation and evolution of the Oort cloud, which might provide evidence on the type of cluster environment in which the Sun formed. o In section 4-7, we detail the formation and distribution of dust in the Solar system. o In section 4-8, we discuss the recent discoveries of rings orbiting some of the Solar system's small bodies o In section 4-9, we describe the various non-gravitational effects that can significantly affect the orbits of both dust and kilometre-sized bodies, before moving on to detail the various mechanisms by which material is lost from the Solar system's small body reservoirs. ● In section 5, we turn our attention to exoplanetary science, highlighting how our knowledge of the Solar system can (and does) inform our understanding of other planetary systems. That section comprises the following subsections: o In section 5.1, we describe the known demographics of exoplanets. o In section 5.2, we discuss how our Solar system would look as an exoplanetary system. o In section 5.3, we discuss debris and dust in exoplanetary systems. o In section 5.4, we discuss the comparative analysis of planetary systems. o In section 5.5, we discuss super-Earths and sub-Neptune - two classes of planet that are not found within the Solar system. o In section 5.6, we discuss the atmospheres of exoplanets, and their relation to those in the Solar system. o In section 5.7, we describe research into the dynamics of exoplanetary systems. o In section 5.8, we describe current and future exoplanet surveys and missions. ● Finally, in section 6, we draw together our conclusions.