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NASA NTRS2016

Titan-Like Exoplanets: Variations in Geometric Albedo and Effective Transit Height with Haze Production Rate

Extensive studies characterizing Titan present an opportunity to study the atmospheric properties of Titan-like exoplanets. Using an existing model of Titan's atmospheric haze, we computed geometric albedo spectra and effective transit height spectra for six values of the haze production rate (zero haze to twice present) over a wide range of wavelengths (0.2-2 microns). In the geometric albedo spectra, the slope in the UV-visible changes from blue to red when varying the haze production rate values from zero to twice the current Titan value. This spectral feature is the most effective way to characterize the haze production rates. Methane absorption bands in the visible-NIR compete with the absorbing haze, being more prominent for smaller haze production rates. The effective transit heights probe a region of the atmosphere where the haze and gas are optically thin and that is thus not effectively probed by the geometric albedo. The effective transit height decreases smoothly with increasing wavelength, from 376 km to 123 km at 0.2 and 2 microns, respectively. When decreasing the haze production rate, the methane absorption bands become more prominent, and the effective transit height decreases with a steeper slope with increasing wavelength. The slope of the geometric albedo in the UV-visible increases smoothly with increasing haze production rate, while the slope of the effective transit height spectra is not sensitive to the haze production rate other than showing a sharp rise when the haze production rate increases from zero. We conclude that geometric albedo spectra provide the most sensitive indicator of the haze production rate and the background Rayleigh gas. Our results suggest that important and complementary information can be obtained from the geometric albedo and motivates improvements in the technology for direct imaging of nearby exoplanets.

Geometric albedo↗

NASA NTRS2016

Combining Photometry from Kepler and TESS to Improve Short-Period Exoplanet Characterization

Planets emit thermal radiation and reflect incident light that they receive from their host stars. As a planet orbits its host star the photometric variations associated with these two effects produce very similar phase curves. If observed through only a single bandpass, this leads to a degeneracy between certain planetary parameters that hinder the precise characterization of such planets. However, observing the same planet through two different bandpasses gives much more information about the planet. Here we develop a Bayesian methodology for combining photometry from both Kepler and the Transiting Exoplanet Survey Satellite. In addition, we demonstrate via simulations that one can disentangle the reflected and thermally emitted light from the atmosphere of a hot-Jupiter as well as more precisely constrain both the geometric albedo and day-side temperature of the planet. This methodology can further be employed using various combinations of photometry from the James Webb Space Telescope, the Characterizing ExOplanet Satellite, or the PLATO mission.

photometric variations↗

NASA NTRS2016

Stellar Echo Imaging of Exoplanets

All stars exhibit intensity fluctuations over several timescales, from nanoseconds to years. These intensity fluctuations echo off bodies and structures in the star system. We posit that it is possible to take advantage of these echoes to detect, and possibly image, Earth-scale exoplanets. Unlike direct imaging techniques, temporal measurements do not require fringe tracking, maintaining an optically-perfect baseline, or utilizing ultra-contrast coronagraphs. Unlike transit or radial velocity techniques, stellar echo detection is not constrained to any specific orbital inclination. Current results suggest that existing and emerging technology can already enable stellar echo techniques at flare stars, such as Proxima Centauri, including detection, spectroscopic interrogation, and possibly even continent-level imaging of exoplanets in a variety of orbits. Detection of Earth-like planets around Sun-like stars appears to be extremely challenging, but cannot be fully quantified without additional data on micro- and millisecond-scale intensity fluctuations of the Sun. We consider survey missions in the mold of Kepler and place preliminary constraints on the feasibility of producing 3D tomographic maps of other structures in star systems, such as accretion disks. In this report we discuss the theory, limitations, models, and future opportunities for stellar echo imaging.

stellar↗

NASA NTRS2018

Exoplanet Biosignatures: Observational Prospects

Exoplanet hunting efforts have revealed the prevalence of exotic worlds with diverse properties, including Earth-sized bodies, which has fueled our endeavor to search for life beyond the Solar System. Accumulating experiences in astrophysical, chemical, and climatological characterization of uninhabitable planets are paving the way to characterization of potentially habitable planets. In this paper, we review our possibilities and limitations in characterizing temperate terrestrial planets with future observational capabilities through the 2030s and beyond, as a basis of a broad range of discussions on how to advance "astrobiology" with exoplanets. We discuss the observability of not only the proposed biosignature candidates themselves but also of more general planetary properties that provide circumstantial evidence, since the evaluation of any biosignature candidate relies on its context. Characterization of temperate Earth-sized planets in the coming years will focus on those around nearby late-type stars. The James Webb Space Telescope (JWST) and later 30-meter-class ground-based telescopes will empower their chemical investigations. Spectroscopic studies of potentially habitable planets around solar-type stars will likely require a designated spacecraft mission for direct imaging, leveraging technologies that are already being developed and tested as part of the Wide Field InfraRed Survey Telescope (WFIRST) mission. Successful initial characterization of a few nearby targets will be an important touchstone toward a more detailed scrutiny and a larger survey that are envisioned beyond 2030. The broad outlook this paper presents may help develop new observational techniques to detect relevant features as well as frameworks to diagnose planets based on the observables.

Characterization↗

NASA NTRS2016

Concept Study for a Compact Homodyne Astrophysics Spectrometer for Exoplanets (CHASE)

In this concept study, we are targeting to build a new instrument to sequentially observe exoplanet atmospheres and their parent’s stellar spectra over a significant time in NUV and FUV. The Compact Homodyne Astrophysics Spectrometer for Exoplanets (CHASE) offers integrated spectra over a wide field -of-view (FOV~40arcsec) in high spectral resolution (R>10(exp 5)) in a miniaturized architecture using no (or a small < 1m) primary mirror. CHASE’s wide FOV is compatible with the relaxed pointing requirements of current CubeSats and SmallSats which makes it readily qualifiable for space in a compact format and have the potential to enable major scientific breakthroughs.

Interferometry↗

NASA NTRS2006

Earth-Like Exoplanets: The Science of NASA's Navigator Program

This book outlines the exoplanet science content of NASA's Navigator Program, and it identifies the exoplanet research priorities. The goal of Navigator Program missions is to detect and characterize Earth-like planets in the habitable zone of nearby stars and to search for signs of life on those planets.

habitable planets↗

NASA NTRS2008

Observing Exoplanets in the Mid-Ultraviolet

There are good reasons for pushing the spectral range of observation to shorter wavelengths than currently envisaged for terrestrial planet-finding missions utilizing with a 4-m, diffraction-limited, optical telescope: (1) The angular resolution is higher, so the image of an exoplanet is better separated from that of the much brighter star. (2) The exozodiacal background per resolution element is smaller, so exposure times are reduced for the same incident flux. (3) Most importantly, the sensitivity to the ozone biomarker is increased by several hundred-fold by access to the ozone absorption band at 250-300 nm. These benefits must be weighed against challenges arising from the faintness of exoplanets in the mid-UV. We will evaluate both the technical and cost challenges including image quality of large telescopes, advanced mirror coatings and innovative designs for enhanced optical throughput, and CCD detectors optimized for 250-400 nm.

Heap. Sara↗

NASA NTRS2010

Light from Exoplanets: Present and Future

Measurements using the Spitzer Space Telescope have revealed thermal emission from planets orbiting very close to solar-type stars, primarily transiting "hot Jupiter" exoplanets. The thermal emission spectrum of these worlds has been measured by exploiting their secondary eclipse. Also, during transit of the planet, absorption signatures from atoms and molecules in the planet's atmosphere are imprinted onto the spectrum of the star. Results to date from transit and eclipse studies show that the hot Jupiters often have significant haze and cloud components in their atmospheres, and the temperature structure can often be inverted, i.e. temperature is rising with height. New and very strongly irradiated examples of hot Jupiters have been found that are being stripped of their atmospheres by tidal forces from the star. In parallel, transiting superEarth exoplanets are being discovered, and their atmospheres should also be amenable to study using transit techniques. The 2014 launch of the James Webb Space Telescope will clarify the physical nature of hot Jupiters, and will extend transit and eclipse studies to superEarths orbiting in the habitable zones of lower main sequence stars.

Deming, Leo↗

NASA NTRS2011

Exoplanets, Exo-Solar Life, and Human Significance

With the recent detection of over 500 extrasolar planets, the existence of "other worlds", perhaps even other Earths, is no longer in the realm of science fiction. The study of exoplanets rapidly moved from an activity on the fringe of astronomy to one of the highest priorities of the world's astronomical programs. Actual images of extrasolar planets were revealed over the past two years for the first time. NASA's Hubble Space Telescope is already characterizing the atmospheres of Jupiter-like planets, in other systems. And the recent launch of the NASA Kepler space telescope is enabling the first statistical assessment of how common solar systems like our own really are. As we begin to characterize these "other worlds" and assess their habitability, the question of the significance and uniqueness of life on Earth will impact our society as never before. I will provide a comprehensive overview of the techniques and status of exoplanet detection, followed by reflections as to the societal impact of finding out that Earths are common, or rare. Will finding other potentially habitable planets create another "Copernican Revolution"? Will perceptions of the significance of life on Earth change when we find other Earth-like planets? I will discuss the plans of the scientific community for future telescopes that will be abe to survey our solar neighborhood for Earth-like planets, study their atmospheres, and search for biological signs of life.

Wiseman, Jennifer↗

NASA NTRS2011

Non-Detection of L-Band Line Emission from the Exoplanet HD 189733b

We attempt to confirm bright non-local thermodynamic equilibrium (non-LTE) emission from the exoplanet HD 189733b at 3.25 microns, as recently reported by Swain et al. based on observations at low spectral resolving power (lambda/delta lambda approximately equals 30). Non-LTE emission lines from gas in an exoplanet atmosphere will not be significantly broadened by collisions, so the measured emission intensity per resolution element must be substantially brighter when observed at high spectral resolving power. We observed the planet before, during, and after a secondary eclipse event at a resolving power lambda/delta lambda equals 27,000 using the NIRSPEC spectrometer on the Keck II telescope. Our spectra cover a spectral window near the peak found by Swain et al., and we compare emission cases that could account for the magnitude and wavelength dependence of the Swain et al. result with our final spectral residuals. To model the expected line emission, we use a general non-equilibrium formulation to synthesize emission features from all plausible molecules that emit in this spectral region. In every case, we detect no line emission to a high degree of confidence. After considering possible explanations for the Swain et at., results and the disparity with our own data, we conclude that an astrophysical source for the putative non-LTE emission is unlikely. We note that the wavelength dependence of the signal seen by Swain et al. closely matches the 2(sub 1/2) band of water vapor at 300 K, and we suggest that an imperfect correction for telluric water is the source of the feature claimed by Swain et al.

Mandell, Avi M.↗

NASA NTRS2011

Observing Exoplanets with the James Webb Space Telescope

The search for exoplanets and characterization of their properties has seen increasing success over the last few years. In excess of 500 exoplanets are known and Kepler has approx. 1000 additional candidates. Recently, progress has been made in direct imaging planets, both from the ground and in space. This presentation will discuss the history and current state of technology used for such discoveries, and highlight the new capabilities that will be enabled by the James Webb Space Telescope.

Clampin Mark↗

NASA NTRS2010

Visible Nulling Coronagraphy Testbed Development for Exoplanet Detection

Three of the recently completed NASA Astrophysics Strategic Mission Concept (ASMC) studies addressed the feasibility of using a Visible Nulling Coronagraph (VNC) as the prime instrument for exoplanet science. The VNC approach is one of the few approaches that works with filled, segmented and sparse or diluted aperture telescope systems and thus spans the space of potential ASMC exoplanet missions. NASA/Goddard Space Flight Center (GSFC) has a well-established effort to develop VNC technologies and has developed an incremental sequence of VNC testbeds to advance the this approach and the technologies associated with it. Herein we report on the continued development of the vacuum Visible Nulling Coronagraph testbed (VNT). The VNT is an ultra-stable vibration isolated testbed that operates under high bandwidth closed-loop control within a vacuum chamber. It will be used to achieve an incremental sequence of three visible light nulling milestones of sequentially higher contrasts of 10(exp 8) , 10(exp 9) and 10(exp 10) at an inner working angle of 2*lambda/D and ultimately culminate in spectrally broadband (>20%) high contrast imaging. Each of the milestones, one per year, is traceable to one or more of the ASMC studies. The VNT uses a modified Mach-Zehnder nulling interferometer, modified with a modified "W" configuration to accommodate a hex-packed MEMS based deformable mirror, a coherent fiber bundle and achromatic phase shifters. Discussed will be the optical configuration laboratory results, critical technologies and the null sensing and control approach.

Lyon, Richard G.↗

NASA NTRS2012

Hubble/WFC3 Spectroscopy of the Transiting Exoplanets WASP-19b and WASP-17b

Measurements of transiting exoplanets that target extremes in parameter space offer the best chance to disentangle the structure and composition of the atmospheres of hot Jupiters. WASP-19b is one of the hottest exoplanets discovered to date, while WASP-17b has a much lower equilibrium temperature but has one of the largest atmospheric radii of known transiting planets. We discuss results from HST/WFC3 grism 1.1-1.7 micron spectroscopy of these planets during transit. We compare our integrated-light transit depths to previous IR transit photometry, and derive the 1.4-micron water absorption spectrum. We discuss implications for the atmospheric composition and structure of these hot Jupiters, and outline future observations that will further expand on these results.

Mandell, A.↗

NASA NTRS2013

The Ultraviolet Radiation Environment around M Dwarf Exoplanet Host Stars

The spectral and temporal behavior of exoplanet host stars is a critical input to models of the chemistry and evolution of planetary atmospheres. Ultraviolet photons influence the atmospheric temperature profiles and production of potential biomarkers on Earth-like planets around these stars. At present, little observational or theoretical basis exists for understanding the ultraviolet spectra of M dwarfs, despite their critical importance to predicting and interpreting the spectra of potentially habitable planets as they are obtained in the coming decades. Using observations from the Hubble Space Telescope, we present a study of the UV radiation fields around nearby M dwarf planet hosts that covers both far-UV (FUV) and near-UV (NUV) wavelengths. The combined FUV+NUV spectra are publicly available in machine-readable format. We find that all six exoplanet host stars in our sample (GJ 581, GJ 876, GJ 436, GJ 832, GJ 667C, and GJ 1214) exhibit some level of chromospheric and transition region UV emission. No "UV-quiet" M dwarfs are observed. The bright stellar Lyman-alpha emission lines are reconstructed, and we find that the Lyman-alpha line fluxes comprise approximately 37%-75% of the total 1150-3100 A flux from most M dwarfs; approximately greater than 10(exp3) times the solar value. We develop an empirical scaling relation between Lyman-alpha and Mg II emission, to be used when interstellar H I attenuation precludes the direct observation of Lyman-alpha. The intrinsic unreddened flux ratio is F(Lyman-alpha)/F(Mg II) = 10(exp3). The F(FUV)/F(NUV) flux ratio, a driver for abiotic production of the suggested biomarkers O2 and O3, is shown to be approximately 0.5-3 for all M dwarfs in our sample, greather than 10(exp3) times the solar ratio. For the four stars with moderate signal-to-noise Cosmic Origins Spectrograph time-resolved spectra, we find UV emission line variability with amplitudes of 50%.500% on 10(exp2)-10(exp3) s timescales. This effect should be taken into account in future UV transiting planet studies, including searches for O3 on Earth-like planets. Finally, we observe relatively bright H2 fluorescent emission from four of the M dwarf exoplanetary systems (GJ 581, GJ 876, GJ 436, and GJ 832). Additional modeling work is needed to differentiate between a stellar photospheric or possible exoplanetary origin for the hot (T(H2) approximately equal to 2000-4000 K) molecular gas observed in these objects.

France, Kevin↗

NASA NTRS2013

Crossing the Boundaries in Planetary Atmospheres - From Earth to Exoplanets

The past decade has been an especially exciting time to study atmospheres, with a renaissance in fundamental studies of Earths general circulation and hydrological cycle, stimulated by questions about past climates and the urgency of projecting the future impacts of humankinds activities. Long-term spacecraft and Earth-based observation of solar system planets have now reinvigorated the study of comparative planetary climatology. The explosion in discoveries of planets outside our solar system has made atmospheric science integral to understanding the diversity of our solar system and the potential habitability of planets outside it. Thus, the AGU Chapman Conference Crossing the Boundaries in Planetary Atmospheres From Earth to Exoplanets, held in Annapolis, MD from June 24-27, 2013 gathered Earth, solar system, and exoplanet scientists to share experiences, insights, and challenges from their individual disciplines, and discuss areas in which thinking broadly might enhance our fundamental understanding of how atmospheres work.

hydrological cycle↗

NASA NTRS2014

An Introduction to Exoplanets and the Kepler Mission

A quarter century ago, the only planets known to humanity were the familiar objects that orbit our Sun. But improved observational techniques allowed astronomers to begin detecting planets around other stars in the 1990s. The first extrasolar planets (often referred to as exoplanets) to be discovered were quite exotic and unfamiliar objects. Most were giant objects that are hundreds of times as massive as the Earth and orbit so close to their star that they are hotter than pizza ovens. But as observational capabilities improved, smaller and cooler planets were found. The most capable planet-hunting tool developed to date is NASA's Kepler telescope, which was launched in 2009. Kepler has found that planets similar in size to our Earth are quite abundant within our galaxy. Results of Kepler's research will be summarized and placed into context within the new and growing discipline of exoplanet studies.

Earth-like planets↗

NASA NTRS2015

The Orbital Design of Alpha Centauri Exoplanet Satellite (ACESat)

Exoplanet candidates discovered by Kepler are too distant for biomarkers to be detected with foreseeable technology. Alpha Centauri has high separation from other stars and is of close proximity to Earth, which makes the binary star system 'low hanging fruit' for scientists. Alpha Centauri Exoplanet Satellite (ACESat) is a mission proposed to Small Explorer Program (SMEX) that will use a coronagraph to search for an orbiting planet around one of the stars of Alpha Centauri. The trajectory design for this mission is presented here where three different trajectories are considered: Low Earth Orbit (LEO), Geosynchronous Orbit (GEO) and a Heliocentric Orbit. Uninterrupted stare time to Alpha Centauri is desirable for meeting science requirements, or an orbit that provides 90% stare time to the science target. The instrument thermal stability also has stringent requirements for proper function, influencing trajectory design.

Alpha Centauri↗

NASA NTRS2014

Successful Starshade Petal Deployment Tolerance Verification in Support of NASA's Technology Development for Exoplanet Missions

A Starshade is a sunflower-shaped satellite with a large inner disk structure surrounded by petals that flies in formation with a space-borne telescope, creating a deep shadow around the telescope over a broad spectral band to permit nearby exoplanets to be viewed. Removing extraneous starlight before it enters the observatory optics greatly loosens the tolerances on the telescope and instrument that comprise the optical system, but the nature of the Starshade dictates a large deployable structure capable of deploying to a very precise shape. These shape requirements break down into key mechanical requirements, which include the rigid-body position and orientation of each of the petals that ring the periphery of the Starshade. To verify our capability to meet these requirements, we modified an existing flight-like Astromesh reflector, provided by Northrup Grumman, as the base ring to which the petals attach. The integrated system, including 4 of the 30 flight-like subscale petals, truss, connecting spokes and central hub, was deployed tens of times in a flight-like manner using a gravity compensation system. After each deployment, discrete points in prescribed locations covering the petals and truss were measured using a highly-accurate laser tracker system. These measurements were then compared against the mechanical requirements, and the as-measured data shows deployment accuracy well within our milestone requirements and resulting in a contrast ratio consistent with exoplanet detection and characterization.

External occulters↗