Exoplanet Science Strategy (2018) / Chapter Skim
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2 The State of the Field of Exoplanets
2 The State of the Field of Exoplanets
Pages 10-41
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From page 10... ...
METHODS OF DETECTING AND CHARACTERIZING EXOPLANETS: APPLICATIONS, BIASES, AND LIMITATIONS By essentially every physical measure, planets are exceptionally diminutive, in particular in comparison to their host stars. This is the primary reason that it was not until nearly the end of the 20th century that the first definitive detections of exoplanets were made.
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From page 11... ...
For Earth analogues to Sun-like stars, the size ratio is roughly 1 to 100, the mass ratio is roughly 1 to 300,000, and the visible-light flux ratio is roughly 1 part in 2 billion. Since nearly all detection methods rely on either the indirect influence of the exoplanet on its parent star or the direct detection of the planet in the vicinity of its parent star, these ratios make the detection of planets even as large as Jupiter incredibly difficult.
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From page 12... ...
Roughly Earth-size planets on shorter period orbits transiting smaller stars, such as potentially habitable planets orbiting low-mass stars (or M dwarfs) , can and have been detected from the ground.
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From page 13... ...
, the necessary sensitivity is only possible for angular separations beyond a few tenths of an arcsecond from the parent star. Hence almost all the imaged planets are giant planets orbiting at separations of 20 times the distance from Earth to the Sun (also referred to as an astronomical unit, or AU)
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From page 14... ...
Because of the low probability of detecting a stellar-mass microlensing event, and the lower probability of detecting the planetary perturbation even in the case that the microlensing event is detected and assuming the planetary companion exists, microlensing surveys for exoplanets generally require continuously monitoring hundreds of millions of stars on daily time scales to detect the microlensing events, and then monitoring the known microlensing events on hourly to daily time scales to detect the planetary perturbations. Planets detected via microlensing measure the mass ratio between the planet and the star, and the instantaneous projected separation between the planet and host star in units of the Einstein ring radius.
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From page 15... ...
Understanding how planets form is not only interesting in its own right, it informs understanding of the prevalence of potentially habitable worlds -- for example, by providing clues to the dominant processes of water delivery to rocky planets in the habitable zones of their parent stars. Formation models that aim to understand the demographics of mature planetary systems need to ultimately start from realistic physical conditions, and therefore be informed by observations of protoplanetary and debris disks.
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From page 16... ...
. • Low-mass planets at short orbital periods of less than 50 days are more common than giant planets (Howard et al., 2010; Mayor et al., 2011)
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From page 17... ...
The constraints on the frequency of terrestrial planets in the habitable zones of Sun-like stars are less certain than the corresponding planet frequency of such planets orbiting M dwarfs. This is due to several factors, including (1)
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From page 18... ...
Nevertheless, researchers have high confidence that terrestrial planets orbiting in the habitable zones of Sun-like stars are relatively commonplace (Burke et al., 2015)
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From page 19... ...
line, the location in the protoplanetary disks where water ice is stable. Since it is generally believed that terrestrial planets that formed in the habitable zones of their parent stars largely formed without significant amounts of water, it is of great interest to understand how water (or volatiles in general)
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From page 20... ...
The number of spatially resolved disks has increased from a handful to hundreds, thanks to a combination of scattered light observations from the Hubble Space Telescope (HST) and large, adaptively corrected ground-based telescopes, mid-infrared emission resolved with the ground-based telescopes, Spitzer, the Herschel Space Telescope, and far-infrared emission from the Atacama Large Millimeter/ submillimeter Array (ALMA)
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From page 21... ...
, disk masses as estimated from dust and gas tracers such as CO are lower than the Miminum Mass Solar Nebula (Ansdell et al., 2016; Eisner et al., 2018) ; most solids must already be in planetesimals or planets and gas must evolve rapidly.
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From page 22... ...
Their presence signals that planet formation proceeded at least to the point of making planetesimal belts. • The dust mass in debris disks decays with time, such that detectable disks are much more common around
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From page 23... ...
. Exoplanet Atmospheres and Interiors Polluted White Dwarfs One of the only constraints on the compositions of extrasolar planetesimals comes from observations of white dwarfs with metal lines created by the accretion of planetary material that survived the post-main sequence evolution of the star.
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From page 24... ...
. The frequency of small planet sizes and the distribution of mass-radius values together provide a path for homing in on rocky planets, using the measurement of planetary radii (Lopez and Fortney, 2014)
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From page 25... ...
Exoplanet atmosphere observations with time-series techniques have been very successful over the last two decades, despite the fact that none of the instruments that were used was designed specifically for these measurements. Such observations are challenging because planetary signals are 10–3 that of their host stars or smaller.
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From page 26... ...
26 EXOPLANET SCIENCE STRATEGY FIGURE 2.9 An atlas of transmission spectra for 10 hot Jupiters obtained with HST and Spitzer. Absorption from a variety of chemical species including H2O, Na, and K along with scattering from aerosols are common.
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From page 27... ...
will bring transformative data to bear on these issues. Exoplanet Atmospheres from Direct Imaging As discussed above, current direct imaging techniques are limited to young, self-luminous, giant planets.
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From page 28... ...
planet 51 Eri b compared with several models. While general features match, no model fit is particularly compelling -- in addition to significant residuals, all appear to overestimate the planet's effective temperature and underestimate its radius compared with evolutionary models.
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From page 29... ...
. For terrestrial planets, including those in the habitable zones of low-mass stars, this radiation has the potential to strip the planet atmosphere completely, or to generate hazes, two scenarios that result in a flat transmission spectrum of a rocky planet.
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From page 30... ...
the development of exoplanet habitability assessment as an interdisciplinary multiparameter concept including planetary and stellar properties and planet-star-planetary system interactions, and (2) in enhancing confidence in biosignature assessment for more reliable biosignatures like O2, while proposing new exoplanet biosignatures that may broaden the search.
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From page 31... ...
, and JWST and the GSMTs will search for and characterize high-molecular-weight atmospheres, providing the first glimpses into the atmospheres of potentially habitable planets. These new observations will enable the first, albeit challenging, search for signs of life on nearby worlds, and test researchers' understanding and models of habitable planetary environments and processes.
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From page 32... ...
and false positives (abiotic planetary processes that can mimic biosignatures)
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From page 33... ...
. Using the treatment of O2 as a template, the community has started the development of a comprehensive framework that can be used to interpret other potential biosignatures in the context of their environment, and similarly increase confidence that they are indeed due to life, and not abiotic planetary processes.
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From page 34... ...
Monthly Notices of the Royal Astronomical Society: Letters 422(1)
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From page 35... ...
2013. Giant planets orbiting metal-rich stars show signatures of planet-planet interac tions.
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From page 36... ...
Monthly Notices of the Royal Astronomical Society 450(3)
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From page 37... ...
Monthly Notices of the Royal Astronomical Society 285(2)
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From page 38... ...
2017. A direct imaging survey of Spitzer-detected debris disks: Occurrence of giant planets in dusty systems.
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From page 39... ...
Monthly Notices of the Royal Astronomical Society 459(4)
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From page 40... ...
2015. The transiting exoplanet survey satellite: Simulations of planet detections and astrophysical false positives.
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From page 41... ...
Monthly Notices of the Royal Astronomical Society 479(4)
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