The Way Forward

Space Sci Rev DOI 10.1007/s11214-016-0247-2 The Way Forward Malcolm Fridlund1,2,3 · Artie Hatzes4 · René Liseau3 Received: 11 September 2015 / Accepted: 14 March 2016 © The Author(s) 2016. This article is published with open access at Springerlink.com Abstract For the last few decades the study of disks around stars young and old and of different types have progressed significantly. During the same time a completely new discipline—the study of exoplanets, planets orbiting stars other than our Sun—have emerged. Both these fields, which are interconnected, have benefited from the development of new instrumentation, and especially by telescopes and detectors deployed in space. In this chapter we are describing the state of the art of such instruments and make an inventory of what is being currently developed. We also state some of the requirements of the next steps and what type of instruments will lead the way forward. Keywords Exoplanets · Starforming disks · Debris disks · Instrumentation: photometers · Instrumentation: space 1 Introduction Our Solar System has a complex architecture that contains (1) Planets of several types, from giant gas giants to rocky terrestrial planets, (2) Debris disks, e.g. the zodiacal dust belt and the Kuiper-Edgeworth belt in the Solar System or disks of the β Pic or AU Mic type, and (3) Planetesimals or remnants thereof—e.g. Comets, Asteroids or Dwarf planets. There is also the Oort cloud of comets stretching tens of thousands of astronomical units from the Sun, but the understanding of the structure of the Oort cloud in the context of star formation is a separate issue and will not be discussed here. One of the ultimate goals of the research into both exoplanets and circumstellar disks is to be able to place this complex picture of our own Solar System into a proper context. B M. Fridlund [email protected] 1 Max-Planck-Institute für Astronomie, Königsthul 17, 69117 Heidelberg, Germany 2 Leiden Observatory, P.O. Box 9513, 2300 RA, Leiden, The Netherlands 3 Department of Earth and Space Sciences, Chalmers University of Technology, Onsala Space Observatory, 439 92, Onsala, Sweden 4 Thüringer Landessternwarte Tautenburg, Sternwarte 5, 07778 Tautenburg, Germany M. Fridlund et al. Answering questions like “Is our system a run-of-the-mill kind of system” or is it of a “rarer” kind—or even “unique” is of course central to this research. The answer to such questions is related to the overarching issue of whether we are alone in the Universe. The question of whether life exists elsewhere, than on the Earth, has been discussed by mankind since the beginning of recorded history, or even before, and it is still a fundamental question among scientists and laypersons alike today.1 The understanding of how disks and planetary bodies relate to each other is the key to how planetary systems form and evolve. What kind of planets is the more common? What is the architecture of these planetary systems? And under what conditions were they formed? It must be a consequence of the formation- and evolution-processes that lead to the structure and physics of a specific system or an individual planet. This is very likely due to the conditions leading up to the formation of an accreting star and its associated disk. We can be fairly certain that the existence of planetary systems is a consequence of the process that forms stars. This understanding has not changed significantly since the time when we only had knowledge about only one such system—our own. Our own solar system is of course very important since it is the only place in the Universe where we know that life exists. It is also the only system where we can study individual objects in situ and in exquisite detail. This is evidenced by the fact that we have more than half a dozen working spacecrafts in orbit or on the surface of Mars. As of this writing the New Horizons mission had just made its flyby of the dwarf planet Pluto on its way out into the Solar Systems outer debris disk—the Kuiper-Edgeworth belt. The pioneering Voyager 2 spacecraft has already crossed the heliopause and has become the first human spacecraft to be traveling in interstellar space. And, for more than a year the Rosetta spacecraft has been flying in tandem with Comet 67P/Churyumov-Gerasimenko, giving us valuable, in situ knowledge of a comet/planetesimal. These are indeed exciting times for solar system research. The understanding of star forming and debris disks of different structures and sizes is thus the key to understanding planet formation, specifically the types and masses of planets and the range of orbital distances that can form. Not only must a planet be located in the Habitable Zone (HZ, usually defined as the range of distance from the host star within liquid water can exist). Once such planets have formed, their orbits must be stable for a long enough time to provide the physical conditions for the formation of life. Eventually, the central issue we wish to address is whether we are indeed alone in the Universe. The uniqueness or commonality of the Earth is fundamental to a proper understanding of such central issues as the properties needed by a planet in order for life to arise, the origin and evolution of life, and of course how common life is in the Universe. To cor- rectly understand these issues we must also understand the context within which life on Earth exists and our place in the Universe. This thought, which has been with us throughout recorded history, can now be explored within the structure of a scientific inquiry. Is the Earth the only one of uncountable planets where life has arisen and evolved to the point that one of its life forms has the ability to formulate such a question? Or are the Earth, and its humans unique? Possibly the Earth formed out of random processes that were quite rare and very special circumstances were needed for higher life forms to evolve. The conditions may have been extremely rare and unlikely, but given the shear number of stars in our Galaxy perhaps it can occur at least once every several billions of years. It could also be that we may be alone in our galaxy, but certainly not alone in the vast Universe. Or.... 1As demonstrated by e.g. The recent announcement that “the Breakthrough Prize Foundation”, an organiza- tion created by investor Yuri Milner, has announced that it will commit $100 million over the next ten years to a new, large-scale SETI initiative. The Way Forward The answer to such questions should be based on sound scientific inquiry and not just speculation. 1.1 The Study of Disks Interstellar disks and the process of stellar and planetary formation and evolution have now been studied in detail for more than 60 years. Through advances in Infrared and mm/sub- mm astronomy a consistent picture has slowly been built up. The Milky Way Galaxy, like all galaxies, is made up of stars, stellar remnants, planets, and an interstellar medium. This Inter Stellar Medium (ISM) is composed of atomic and molecular gas as well as solid matter in the form of dust grains. The densest part of the ISM is mainly located in the plane of our Galaxy and it has different components, one of which is (giant) molecular clouds with a much higher (103–104 times higher) density than the “general” diffuse medium. These molecular clouds consist mainly of H2, but with a significant enrichment of metals that supposedly originated in stars during the later stages of stellar evolution. These were then ejected into interstellar space through mass loss and supernova explosions to form the parental environment benevolent to the forming of stars and planets. While elements of the clouds collapse into protostars, some of the molecular material forms a protoplanetary disk, due to the conser- vation of angular momentum. This momentum preserves an enormous amount of rotational energy that the protostar has to lose before it can settle into a stable configuration. This is done through bipolar outflows (atomic and molecular), or jets. Although most of the disk mass and its angular momentum is lost from the system through these processes, a large amount of material is nevertheless left behind in remnant disks. This remaining material disperses slowly during the stellar lifetime either through collisions that form dust and debris disks or through ejections via interactions with the planetary bodies. However some of these debris disks remain as evidenced by the dust and large number of small bodies in the Solar System. We have begun to observe more and more of this material in other stellar systems (Eiroa et al. 2013) and essentially all of the stages outlined above have been detected either from the ground or space. Most recently the ESA space observatory Herschel has observed many aspects of these processes for the first time and in greater detail than ever before. On the ground the “first light” of the Atacama Large MM/sub-mm Array (ALMA) have detected the gaps in protoplanetary disks presumably resulting from the “sweeping up” of material by planets being formed (Partnership ALMA 2015). While many of the details in these processes remain to be observed and clarified it appears that at least the broad elements of the picture are understood. One important remaining step is to connect the understanding of the distribution of exoplanets (type of planets, type of orbits, type of host stars) with this picture of the early stages of a planetary system.

Load more