Panel Reports—New Worlds, New Horizons in Astronomy and Astrophysics (2011) / Chapter Skim
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4 Report of the Panel on Planetary Systems and Star Formation
4 Report of the Panel on Planetary Systems and Star Formation
Pages 151-206
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From page 151... ...
4 Report of the Panel on Planetary Systems and Star Formation Only one generation in the history of the human species is privileged to live during the time those great discoveries are first made; that generation is ours. -- Carl Sagan The Pale Blue Dot: Earth, as seen in 1990 from a distance of 40.6 AU, by Voyager 1.
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From page 152... ...
The Astro2010 Science Frontiers Panel on Planetary Systems and Star Forma tion was charged to consider science opportunities in the domain of planetary systems and star formation -- including the perspectives of astrochemistry and exobiology -- spanning studies of molecular clouds, protoplanetary and debris disks, and extrasolar planets, and the implications for such investigations that can be gained from ground-based studies of solar system bodies other than the Sun.1 The panel identifies four central questions that are ripe for answering and one area of unusual discovery potential, and it offers recommendations for implementing the technological advances that can speed us on our way. The questions and the area of unusual discovery potential are these: • How do stars form?
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From page 153... ...
In the next stage of star formation, the dense structures in molecular clouds fragment into self-gravitating "cores" that are the direct progenitors of stars. There is mounting evidence from nearby star-forming regions that the distribution of core masses may be directly related to the resulting distribution of stellar masses, although some subsequent fragmentation likely produces binaries and very low mass objects.
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From page 154... ...
Major advances were made over the past decade in characterizing evolutionary timescales of pro toplanetary disks, but their masses and structure are much less certain. In the coming decade, improved angular resolution will routinely yield resolved images of disks, providing keys to their mass, physical and chemical structure, and mass and angular momentum transport mechanisms, crucial to the understanding of both star and planet formation.
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From page 155... ...
To achieve these goals, the panel recommends the following: • Studies of protoplanetary disks in nearby star-forming regions at resolutions below 100 milliarcsec, with every effort to achieve 10-milliarcsec resolution, at millimeter, submillimeter, infrared, and optical wavelengths, in order to map disk structure on spatial scales of approximately 1-10 AU; • Searches for infant planets in disk gaps using JWST, extreme-AO nearinfrared (near-IR) imaging on 8- to 10-m-class telescopes, and eventually extremeAO imaging with 30-m-class telescopes.
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From page 156... ...
Therefore, the panel recommends the following: • Both a substantial expansion of the telescope time available to pursue radial velocity work, and the development of advanced radial-velocity techniques with a target precision sufficient to detect an Earth-mass planet orbiting a Sun-like star at a distance of 1 AU. This investment in radial-velocity precision will also augment the understand ing of more massive worlds located at distances of 1-10 AU from their stars, which is the region of giant planets in our own solar system.
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From page 157... ...
in order to image planets with dynamical mass estimates and to calibrate models predicting emission from planets as a function of mass and age. Do Habitable Worlds Exist Around Other Stars, and Can We Identify the Telltale Signs of Life on an Exoplanet?
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From page 158... ...
Discovery Area: Identification and Characterization of Nearby Habitable Exoplanets An exciting possibility in the coming decade is the detection of possibly habit able, large, rocky planets (super-Earths) orbiting the abundant and nearby stars that are much less massive than the Sun (less than 0.3 solar masses)
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From page 159... ...
TABLE 4.1 Summary of Conclusions of the Panel on Planetary Systems and Star Formation Question 2: Question 4: Discovery Area: How Do Disks Question 3: Can We Identify Identification and Question 1: Evolve and How Diverse the Telltale Signs Characterization of How Do Stars Form Planetary Are Planetary of Life on an Nearby Habitable Form? Systems?
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From page 160... ...
Following its charge, the Panel on Planetary Systems and Star Formation identified four questions that it considers as ripe for answering in the coming decade, as well as one discovery area: • How do stars form? • How do circumstellar disks evolve and form planetary systems?
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From page 161... ...
And finally, is the initial distribution of stellar masses universal or a function of environment? What Determines Star-Formation Rates and Efficiencies in Molecular Clouds?
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From page 162... ...
Local molecular clouds convert only a few percent of their mass into stars before they are dispersed, probably due to stellar energy input; efficiencies may be still lower in the massive giant molecular clouds (GMCs) that contain most of the galaxy's molecular mass.
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From page 163... ...
will also contribute by measuring parallaxes for those young stars having strong nonthermal emission, and will be able to map out the locations of star-forming regions throughout the galaxy using water and methanol masers. Energy input from young stars -- ionizing radiation, winds, radiation pressure, and supernova explosions -- generates dynamically important turbulence, and clearly limits star-formation efficiencies and rates by disrupting and dispersing molecular clouds.
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From page 164... ...
Cores represent the mass reservoirs from which stars form, and their angular momentum content is responsible for the formation of protoplanetary disks and probably fragmenta tion into multiple star systems. Developing a quantitative theory of star formation requires an understanding of how cores form, which in turn necessitates the char acterization of individual core properties and the dependence of these properties on environment.
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From page 165... ...
These studies should be complete for cores of ~0.01 M◉ and extend to regions forming the full range of stellar masses, to encompass those cores with the potential for forming massive planets. As is the case for star-formation efficiencies, it is crucially important to extend the current set of CMF determinations, which are derived mostly from the nearest, low-density star-forming regions, to the more distant sites where massive stars and star clusters are being formed.
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From page 166... ...
Brogan, personal communication. of how molecular clouds fragment to produce stellar clusters and how the dense gas fraction (see the subsection above titled "What Determines Star Formation Rates and Efficiencies in Molecular Clouds?
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From page 167... ...
Chemical simulations will have to be coupled more strongly to heterogeneous and dynamically evolving cloud models. What Is the Origin of the Stellar Mass Function?
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From page 168... ...
To probe the transformation from the CMF to the IMF will require the following: • High-angular-resolution studies of molecular line emission in collapsing cores with ALMA to resolve the transition zones between the infalling envelope of cores and their protostellar, possibly circumbinary, disks on scales of ~100 AU, where further gravitational fragmentation may occur; • Further laboratory studies of molecular spectra in the far-infrared and sub millimeter regions to contribute to the understanding and interpretation of the vast new array of spectral line observations that will form the basis for sophisticated physical and chemical models; and • Improved numerical simulations, including magnetic fields and feedback. In the nearest star-forming clouds, near-IR observations have reached suffi cient angular resolution to distinguish individual stars in crowded regions and to start to resolve wide binaries.
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From page 169... ...
assumption, much larger and more sensitive surveys of the IMF in a wider variety of environments are needed, probing down to substellar masses. In particular, the panel recommends the following: • Measurements of the IMF in the lower-metallicity environment of the Large Magellanic Cloud.
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From page 170... ...
and near-IR integral-field spectrographs (IFUs) ; FIGURE 4.5 Wide fields, high spatial resolution, and high image contrast are needed to cover large areas while separating clusters into individual stars.
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From page 171... ...
, in pace with the depletion of disk gas and the diminution of stellar accretion. By 6 Myr, nearly all inner disks have disappeared; this limits the timescale for planet formation.
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From page 172... ...
masers; Zeeman shifts for typical interferometers, core properties; 0.5″-30″ magnetic fields in star- heterodyne focal plane physics of CMF/ for disks forming clouds arrays on millimeter IMF connection to cores interferometers and large single-dish telescopes ≤20 mas Near-IR Galactic plane K ~ 19, 23 mag for 30-m-class telescope SFR/SFEs; IMF (103 deg2) , stellar census hydrogen burning limit plus AO with multiple studies imaging and Magellanic in the galactic center, object IFU; JWST spectroscopic Clouds LMC, respectively survey 1028 erg s–1 for T > 107 0.1″ X-ray All clusters New X-ray satellite Stellar content imaging of within Local K at galactic center of clusters for clusters Group SFR/SFEs; IMF studies
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From page 173... ...
Muzerolle, and J Stauffer, A Spitzer view of protoplanetary disks in the γ Velorum Cluster, Astrophysical Journal 686:1195-1208, 2008, reproduced by permission of the AAS.
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From page 174... ...
These features are probably sculpted by giant planets, which await direct detection in the coming decade; some may be imaged already. Another revolution in the understanding of protoplanetary and debris disks impends, as significant increases in angular resolution are anticipated from near IR to centimeter wavelengths initially with ALMA, the EVLA, and JWST, and later with a GSMT, along with orders-of-magnitude improvement in the dynamic range, or contrast, of images; the detection and mapping of complex molecules in disks; FIGURE 4.7 Gap in a transitional disk, probably caused by the gravitation of one or more newborn giant planets, around GM Aurigae, a solar-mass star 140 pc away.
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From page 175... ...
. Currently the role of self-gravity in disk evolution is a controversial topic, but if density waves are discovered, disk masses could be derived independently of uncertain gas-to-dust ratios and grainsize distributions, and the role of such instabilities in giant-planet formation could be assessed.
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From page 176... ...
Boss, and C.K. Walker, Molecular line emission from gravitationally unstable protoplanetary disks, Astrophysical Journal 647:1426-1436, 2006, reproduced by permission of the AAS.
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From page 177... ...
These observations should also yield measurements of the disk ionization fraction, which is crucial for assessing the role of magnetic fields in disk transport and distinguishing actively accreting gas from dead zones. The latter may promote the survival of giant planets by acting as barriers to planet migration.
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From page 178... ...
. In the lower panels the disk has a 4-AU-wide gap at 10 AU, created by a 100-Earth-mass planet, showing a contrast ratio of about 0.1 in scattered light and 0.5 in thermal emission with respect to the rest of the disk.
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From page 179... ...
Ground-based near-IR interferometry also has a vital role in the study of protoplanetary disks, probing the inner 0.1 AU, where the stellar magnetosphere and radiation from stellar accretion spots can greatly affect the disk structure. In the coming decade it will be important to extend the sensitivity of this technique so that more than just the few brightest systems can be observed.
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From page 180... ...
SOURCE: Sensitivities for the Gemini Planet Imager (GPI) and the proposed Thirty Meter Telescope's Planet Formation Instrument (PFI)
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From page 181... ...
-- clumps, warps, and eccentricities -- are attributed to sculpting by planets. Planets at stellocentric distances of 20-200 AU, where a handful of debris disks have been resolved and where highcontrast imagers operate best, could be the result of long-range migration, but also might provide long-sought examples of planet formation by gravitational instability.
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From page 182... ...
. Major advances in the theoretical understanding of planet formation, migra tion, and interactions with their disks -- both gaseous and debris -- will be required to take advantage of the wealth of observational data that will be obtained in the coming decade.
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From page 183... ...
A more secure understanding of the dynamical history of our own solar system will come from an unbiased characterization, both kinematic and physical, of the Kuiper belt. Evidence that the orbits of the ice giants Uranus and Neptune have undergone dramatic changes, involving migration and/or violent planet-planet scatterings, is imprinted in the distribution of orbits of known Kuiper belt objects (KBOs)
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From page 184... ...
Conclusions: Disk Evolution and Planet Formation Table 4.3 summarizes the panel's conclusions about activities to address its second science question.
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From page 185... ...
Wide-field synoptic surveys Visible and near-IR wavelengths: interval ≤ weeks, duration ≥ years Obtain a de-biased Limiting magnitude census of Kuiper belt R = 24 objects. Studies of planet formation, planet-disk interactions, using new algorithms, Theory methods; investment in large-scale computational resources Laboratory Identification of rotational molecular spectra in terahertz range
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From page 186... ...
have each yielded a handful of detections. Toward an Understanding of Planet Formation A mere 15 years ago, the knowledge of "normal" planetary systems was re stricted to the eight planets orbiting the Sun.
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From page 187... ...
It also appears that gas and ice giants of our own solar system have moved significantly from their birth place. Such results have underlined the importance of studies of planetary migration and re-opened questions of the basic mechanisms of the formation of giant planets (see the subsection on giant-planet accretion)
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From page 188... ...
Salpeter, The mass-radius relation for cold spheres of low mass, Astrophysical Journal 158:809-813, 1969, reproduced by permission of the AAS. Right: J
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From page 189... ...
If implemented on next-generation large-aperture telescopes (needed to gather a sufficient number of photons) , this could permit a survey of the closest 100 Sun-like stars to find the nearest Earth-like planets orbiting within their stellar habitable zones.
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From page 190... ...
This is in contrast to ground-based microlensing surveys, which must generally rely on HST imaging to accomplish this, and there is insufficient time available to follow up a significant number of detections. To obtain the fundamental data set to test the current picture of how planetary systems form, how their properties depend on the properties of the central star and, by inference, the conditions of the circumstellar disk, the panel recommends a combination of the following: • A space-based microlensing survey, which will provide a statistical picture of planetary architectures, and • The Kepler findings on terrestrial planet frequency within 1.5 AU of their central stars, augmented by • Expanded radial-velocity measurements on 4- to 10-m-class telescopes to provide masses.
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From page 191... ...
The expected diversity of observations will thus be traceable to the diversity of conditions and environments of planet formation. • The tremendously exciting opportunity to make informed estimates of the compositions of perhaps hundreds of Earth-like planets detected by Kepler serves as a compelling motivation for increased radial-velocity precision and the expansion of available observatory time to undertake these measurements.
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From page 192... ...
the star. Since the planetary emission is not observed at phase 0.5, the dashed line indicates the stellar flux, and any flux in excess of this value at other phases originates from the planet.
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From page 193... ...
A particularly compelling avenue would be to image planets for which dynamical mass estimates can be obtained through radial velocities or astrometry, providing an essential calibration of models that predict the emission from planets as a function of mass and age. The current range of these theoretical tracks varies greatly, depending upon differing assumptions of the starting conditions and the rate of cooling, making it currently unreliable to estimate masses when only brightness measurements are available.
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From page 194... ...
DO HABITABLE WORLDS EXIST AROUND OTHER STARS, AND CAN WE IDENTIFY THE TELLTALE SIGNS OF LIFE ON AN EXOPLANET? The gleaming jewel in the crown of exoplanet discoveries would be a world that could sustain life, which would initiate the study of astrobiology on habitable exoplanets as an experimental science.
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From page 195... ...
Substantial expansion of currently available observing time on 4- to 10-m-class telescopes for Kepler follow-up and other RV survey Microlensing surveys: long-duration near-IR coverage of at least 108 stars, with resolution Precision photometry/ <0.3 arcsec, and PSF stability <10% of FWHM over >1 month; JWST primary/secondary transit spectroscopy, 10–4-10–5 of host-stellar signal spectroscopy Image contrast ~10–9 in optical, 30 mas -- 1 arcsec from host star using Gaia; other space mission Direct imaging astrometry with 0.1 mas -- mission relative positional accuracy Theory, Advances in planet/brown dwarf atmospheric models permitting inclusion of dynamics (clouds, numerics zonal flows, etc.) and chemistry Studies of planetary dynamics, resonances; major computational resources required NOTE: Acronyms are defined in Appendix C
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From page 196... ...
The labels for Kuiper and asteroid belts indicate the approximate level of the zodiacal light. SOURCE: Exoplanet Task Force, Worlds Beyond: A Strategy for the Detection and Characterization of Exoplanets, Washington, D.C., 2008.
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From page 197... ...
The Astronomy and Astrophysics Advisory Committee's Exoplanet Task Force (ExoPTF) identified contingent strategies to achieve characterization of Earth analogs based on results of η⊕ and exozody measurements.
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From page 198... ...
4 Exoplanet Task Force, Worlds Beyond: A Strategy for the Detection and Characterization of Exo planets, Washington, D.C., 2008. 5 National Research Council, The Limits of Organic Life in Planetary Systems, The National Acad emies Press, Washington, D.C., 2007.
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From page 199... ...
Conclusions: Discovering Habitable Exoplanets Table 4.5 summarizes the panel's conclusions about activities to address its fourth science question.
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From page 200... ...
In its discussion above of science question PSF 4, the panel outlines a strategy to search for life on Earth-like planets orbiting in the habitable zones of Sun-like stars. That plan will require a dedicated effort spanning at least two decades.
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From page 201... ...
They would be feasible if applied to nearby M-dwarf stars. It is noted that this opportunity was highlighted in the recent report of the ExoPlanet Task Force to the Astronomy and Astrophysics Advisory Committee.7 This discovery opportunity differs from that related to science question PSF 4 above in several respects.
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From page 202... ...
. As a result of the small planet-star separation, small stellar radius, and low stellar mass, the discovery of a large terrestrial planet orbit ing within the stellar habitable zone could be achieved using only current radial velocity and photometric precision, provided that this precision, demonstrated for Sun-like stars, can be obtained on these M-dwarfs.
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From page 203... ...
Latham, and K Ennico, Discovery and characterization of transiting super Earths using an all-sky transit survey and follow-up by the James Webb Space Telescope, Publications of the Astronomical Society of the Pacific 121:952-967, 2009.
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From page 204... ...
orbiting within the stellar habitable zone. This may require the following: • Large-scale ground-based synoptic surveys, or a space-based survey, but the survey by necessity will need to cover a large fraction of the sky; 8 Internal precision of 10 m/sec has recently been reported for near-infrared radial velocity mea surements of a very low mass star.
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From page 205... ...
Because the spectroscopic studies envisioned here will likely extend over several years, it is crucial that the target planets be identified prior to or soon after the launch of the observatory. There is considerable expertise in the exoplanet community with respect to what needs be accomplished to extend the current radialvelocity and transit-survey precision to a sufficient number of M-dwarfs stars to enable this very exciting path toward detecting the first habitable worlds and undertaking a spectroscopic study of its atmosphere to search for biomarkers.
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From page 206... ...
(RV) Substantial expansion of currently available observing time on 4- to 10-m-class telescopes for M dwarf survey Transit survey (precision 10–4-10–5 of host-stellar signal)
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