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orbits. Together these techniques will determine the probability of planets with certain orbital characteristics around different types of stars. To complete the planetary census, it will be necessary to use techniques that are sensitive to Earth-mass planets on large orbits. One such technique is called gravitational microlensing, whereby the presence of planets is inferred 7 through the tiny deflections that they impose on passing light rays from background stars. A survey for such events is one of the two main tasks of the proposed WFIRST satellite. As microlensing is sensitive to planets of all masses having orbits larger than about half of Earth’s, WFIRST would be able to complement and complete the statistical task underway with Kepler, resulting in an unbiased survey of the properties of distant planetary systems. The results from this survey will constrain theoretical models of the formation of planetary systems, enabling extrapolation of current understanding to systems that will still remain below the threshold of detectability. However, in addition to determining just the planetary statistics, a critical element of the committee’s exoplanet strategy is to continue to build the inventory of planetary systems around specific nearby stars. Therefore, this survey strongly supports a vigorous program of exoplanet science that takes advantage of the observational capabilities that can be achieved from the ground and in space. The first task on the ground is to improve the precision radial velocity method by which the majority of the close to 500 known exoplanets have been discovered. The measured velocity amplitude of a star depends on the ratio of the planetary to the stellar mass, and on the distance from the star, with a Jupiter-mass body at 5 times the Earth-Sun distance from a Sun-like star producing a 12-meter-per-second signal and an Earth at the Earth-Sun location just a 6-centimeter-per-second signal. Improving the velocity precision will allow researchers to measure the masses of smaller planets orbiting nearby stars. Using existing large ground-based or new dedicated mid-size ground-based telescopes equipped with a new generation of high-resolution spectrometers in the optical and near-infrared, a velocity goal of 10 to 20 centimeters per second is realistic. This could allow detection of bodies twice or three times the mass of the Earth around stars the mass of the Sun, and truly Earth mass planets around stars a factor of two or three less massive than the Sun. The radial velocity technique is also of high value when paired with complementary techniques. For example, transits can determine planet sizes and, in combination with the mass found from another technique, yield clues regarding the bulk planetary compositions—just as we know that Earth is mostly rock and iron from its mass and size and a calculation of the average density. Improved precision astrometry and interferometric techniques that are sensitive to planets at larger separations could not only detect new Jupiter-class planets but also study known planetary systems in combination with radial velocity methods so as to resolve the ambiguity regarding true mass as distinct from the inferred minimum mass. Success with endeavors to determine the solar neighborhood planetary census will be very important because knowing that Earth-mass planets exist around nearby stars will give much higher confidence that a future space mission to investigate the atmospheres of extrasolar earths will succeed. A critical step along the way is a better understanding of the dusty disks surrounding stars, analogous to zodiacal dust found near Earth. Reflected diffuse exozodiacal light from these disks can make detection of the faint light from small Earth-like planets difficult. It is, therefore, important to quantify the prevalence and character of these dusty “debris” disks, and the period 2010-2015 will see the completion of groundbased mid-infrared interferometric instrumentation designed to study these phenomena. It is also important to understand planetary systems in the process of formation to enrich and complement observations of the mature exoplanets. ALMA will revolutionize the imaging of protoplanetary disks at millimeter and submillimeter wavelengths and reveal important clues to the formation and evolution of their constituent planets. JWST and ground-based infrared telescopes equipped with adaptive optics to remove the twinkling due to Earth’s atmosphere will provide spatially resolved multiwavelength images and spectra of light scattered from these disks with spatial resolution comparable to that of ALMA. 7 Neither the planet nor the planet host star is detected outside of the microlensing event. PREPUBLICATION COPY—SUBJECT TO FURTHER EDITORIAL CORRECTION 7-8
JWST, with its superb mid-infrared capability, will also use imaging and spectroscopy transit techniques to study the atmospheres of exoplanets, a science capability that has been amply demonstrated by the currently operating Spitzer Space Telescope. JWST will be a premier tool for studying planets orbiting stars that are smaller and cooler than the Sun. Also promising are improved techniques on the ground for direct imaging of planets using adaptive optics. New instrumentation is required as well as significant amounts of observing time (for example, on the Gemini telescopes and the privately operated facilities accessible through NSF’s Telescope System Instrumentation Program). The proposed GSMTs could also play a crucial role in direct imaging studies with instruments suitably designed for this type of work. In addition, enhancements to NASA’a suborbital and Explorer programs could provide testbeds for the development of occulter techniques such as star shades and coronagraphy, which are both immature, and technology development of astrometry and interferometry from space, so as to set the stage for an ambitious direct detection mission in the 2020s. The scientific contributions and technology development in these various areas are described in detail elsewhere. 8 The culmination of the quest for nearby, habitable planets is a dedicated space mission. The committee concluded that it is too early to determine what the design of that space mission should be, or even which planet-detection techniques should be employed. 9 It is not even clear whether searches are best carried out at infrared, optical, or even ultraviolet wavelengths. This choice awaits the results of the observational studies just described, alongside a vigorous and adaptive program of theoretical and laboratory astrophysics investigations that will inform scientists about the diversity of exoplanet atmospheres. Although the case is compelling for technology development for a future space mission beginning early, its emphasis may shift as new discoveries from ground and space materialize. If progress is sufficiently rapid by mid-decade, then a decadal survey independent advice committee (as discussed in Chapter 3) could determine whether a more aggressive program of technology development should be undertaken, possibly including steps toward a technology down-select and a focus on key elements. Either way, decisions on significant, mission-specific funding of a major space mission should be deferred until the 2020 decadal survey, by which time the scientific path forward should be well determined. In summary, exoplanet astronomy is one of the most rapidly developing and unpredictable fields in modern astronomy. Both the statistical investigations of Kepler and WFIRST, and the location of specific, nearby, potentially habitable Earths under a strong yet flexible program of ground-based research, are recommended. This combined approach will allow new techniques to be devised and surprising discoveries to be made during the coming decade; see Box 7-2. 8 AAAC’s Exoplanet Task Force, 2008; JPL’s Exoplanet Community Report, 2009. 9 In considering possible exoplanet missions for the next decade, the committee gave serious consideration to SIMLite but decided against recommending it. SIMLite is technically mature and would provide an important new capability (interferometry). Through precision astrometry it could characterize the architectures of 50 or so nearby planetary systems, provide targets for future imaging missions, and carry out other interesting astrophysics measurements. However, the committee considered that its large cost (appraised by the CATE process at $1.9 billion) and long time to launch (estimated at 8.5 years) make it uncompetitive in the rapidly changing field of exoplanet science. The planetary architecture science can be more efficiently carried out by the committee’s exoplanet strategy involving Kepler, WFIRST, and the ground-based program. The role of target-finding for future direct-detection missions, one not universally accepted as essential, can be done at least partially by pushing groundbased radial-velocity capabilities to a challenging but achievable precision below 10 centimeters per second. Finally, the ancillary astrophysics promised by SIMLite was not judged to be competitive. PREPUBLICATION COPY—SUBJECT TO FURTHER EDITORIAL CORRECTION 7-9
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PREPUBLICATION COPY
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THE NATIONAL ACADEMIES PRESS 500 Fi
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COMMITTEE FOR A DECADAL SURVEY OF A
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Panel on Stars and Stellar Evolutio
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DOUGLAS FINKBEINER, Harvard Univers
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SPACE STUDIES BOARD CHARLES F. KENN
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activities 2 that have emerged from
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In addition to the 27 panel meeting
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Acknowledgment of Reviewers This re
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The Accelerating Universe 2-26 The
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APPENDIXES A Summary of Science Fro
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light up the universe, marking the
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RECOMMENDED PROGRAM Maintaining a b
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TABLE ES.4 Space: Recommended Activ
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Cosmic Dawn: Searching for the Firs
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this imprint would both probe funda
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important outcome will be to open u
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estimated to be on the order of $20
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observatories. For NASA an annual b
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surveys of large fields, enabling s
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RECOMMENDATION: U.S. investors in a
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departments and the community as a
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dominate spending; (2) private-publ
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us was certainly evident during the
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BOX 2-1 Other Worlds Around Other S
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FIGURE 2‐2 The source 3C 75, show
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FIGURE 2‐3 Numerical simulation o
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FIGURE 2‐4 Left panel: Image of t
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FIGURE 2‐6 Top: Schematic of the
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Most stars with masses smaller than
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BOX 2-2 The Origin of Planets After
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send material raining into the cent
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BOX 2-4 Lifecycles in Galaxies One
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Stars Stars are the most observable
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ubiquitous high-energy charged-part
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The Nature of Inflation As describe
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FIGURE 2‐11 Pie chart showing the
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An important clue to the nature of
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Studying the properties of neutron
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FIGURE 2‐14 A possible pathway is
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partners and makes recommendations
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As astronomy research has blossomed
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are increasingly relevant. The Euro
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TABLE 3-1 Currently Operating OIR F
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29% 1990 44% US Priv US Fed Other E
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CONCLUSION: Complex and high-cost f
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Space Observatories The Laser Inter
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Physics Division (PHY); the Mathema
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4 Astronomy in Society Astronomy of
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FIGURE 4‐2 The dust sculptures of
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FIGURE 4‐5 President Obama takes
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Astronomy Inspires in the Classroom
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teaching the scientific method. The
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where the fraction of astronomers h
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Percentage of AAS Membership by Fie
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0.4 0.3 0.2 0.1 PL SO IM AG SF IN S
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Page 121: assistant and associate professor p
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Page 126 and 127: THEORY Emerging Trends in Theoretic
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Page 130 and 131: TABLE 5‐2 Support for astrophysic
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Page 134 and 135: FIGURE 5‐7 Highly cited HST publi
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Page 138 and 139: FIGURE 5‐8 Number of PhDs per yea
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Page 154 and 155: TOWARD FUTURE PROJECTS, MISSIONS, A
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Page 173 and 174: Box 7.3 Implementing the Physics of
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Page 179 and 180: FIGURE 7.4 Science accomplishments
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Page 187 and 188: Laboratory Astrophysics As describe
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Page 207 and 208: A Summary of Science Frontiers Pane
Page 209 and 210: PLANETARY SYSTEMS AND STAR FORMATIO
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$12 MILLION TO $40 MILLION RANGE CM
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decadal research strategy with reco
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CFP CHIPSat CIP CGRO CMB CMU CoBRA
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NRC NSB NSF NSF-AGS NSF-ARC NSF-AST
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