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NASA NTRS2004

NASA's Terrestrial Planet Finder Missions

NASA has decided to move forward with two complementary Terrestrial Planet Finder (TPF) missions, a visible coronagraph and an infrared formation flying interferometer. These missions are major missions in the NASA Office of Space Science Origins Theme. The primary science objectives of the TPF missions are to search for, detect, and characterize planets and planetary systems beyond our own Solar System, including specifically Earth-like planets.

Terrestrial Planet Finder (TPF)↗

NASA NTRS2023

Thermal Emission from the Earth-sized Exoplanet TRAPPIST-1 b using JWST

The TRAPPIST-1 system is remarkable for its seven planets that are similar in size, mass, density, and stellar heating to the rocky planets Venus, Earth, and Mars in our own Solar System [1]. All TRAPPIST-1 planets have been observed with the transmission spectroscopy technique using the Hubble or Spitzer Space Telescopes, but no atmospheric features have been detected or strongly constrained [2–5]. TRAPPIST-1 b is the closest planet to the system’s M dwarf star, and it receives 4 times as much irradiation as Earth receives from the Sun. This relatively large amount of stellar heating suggests that its thermal emission may be measurable. Here we present photometric secondary eclipse observations of the Earth-sized TRAPPIST1 b exoplanet using the F1500W filter of the MIRI instrument on JWST. We detect the secondary eclipse in each of five separate observations with 8.7-sigma confidence when all data are combined. These measurements are most consistent with re-radiation of the TRAPPIST-1 star’s incident flux from only the dayside hemisphere of the planet. The most straightforward interpretation is that there is little or no planetary atmosphere redistributing radiation from the host star and also no detectable atmospheric absorption from carbon dioxide (CO 2 ) or other species.

Thermal Emission↗

NASA NTRS2009

The Kepler Project: Mission Update

Kepler is a Discovery-class mission designed to determine the frequency of Earth-size planets in and near the habitable zone of solar-like stars. The instrument consists of a 0.95 m aperture photometer designed to obtain high precision photometric measurement of > 100,000 stars to search for patterns of transits. The focal plane of the Schmidt-telescope contains 42 CCDs with at total of 95 mega pixels that cover 116 square degrees of sky. The photometer was launched into an Earth-trailing heliocentric orbit on March 6, 2009, finished its commissioning on May 12, and is now in the science operations mode. During the commissioning of the Kepler photometer, data were obtained at a 30 minute cadence for 53,000 stars for 9.7 days. Although the data have not yet been corrected for the presence of systematic errors and artifacts, the data show the presence of hundreds of eclipsing binary stars and variable stars of amazing variety. To provide some estimate of the capability of the photometer, a quick analysis of the photometric precision was made. Analysis of the commissioning data also show transits, occultations and light emitted from the known exoplanet HAT-P7b. The data show a smooth rise and fall of light: from the planet as it orbits its star, punctuated by a drop of 130 +/- 11 ppm in flux when the planet passes behind its star. We interpret this as the phase variation of the dayside thermal emission plus reflected light from the planet as it orbits its star and is occulted. The depth of the occultation is similar in amplitude to that expected from a transiting Earth-size planet and demonstrates that the Mission has the precision necessary to detect such planets.

Borucki, William J.↗

NASA NTRS2000

Autonomous Agents and Intelligent Assistants for Exploration Operations

Human exploration of space will involve remote autonomous crew and systems in long missions. Data to earth will be delayed and limited. Earth control centers will not receive continuous real-time telemetry data, and there will be communication round trips of up to one hour. There will be reduced human monitoring on the planet and earth. When crews are present on the planet, they will be occupied with other activities, and system management will be a low priority task. Earth control centers will use multi-tasking "night shift" and on-call specialists. A new project at Johnson Space Center is developing software to support teamwork between distributed human and software agents in future interplanetary work environments. The Engineering and Mission Operations Directorates at Johnson Space Center (JSC) are combining laboratories and expertise to carry out this project, by establishing a testbed for hWl1an centered design, development and evaluation of intelligent autonomous and assistant systems. Intelligent autonomous systems for managing systems on planetary bases will commuicate their knowledge to support distributed multi-agent mixed-initiative operations. Intelligent assistant agents will respond to events by developing briefings and responses according to instructions from human agents on earth and in space.

Malin, Jane T.↗

NASA NTRS2015

Comparative Habitability of Transiting Exoplanets

Exoplanet habitability is traditionally assessed by comparing a planet’s semimajor axis to the location of its host star’s “habitable zone,” the shell around a star for which Earth-like planets can possess liquid surface water. The Kepler space telescope has discovered numerous planet candidates near the habitable zone, and many more are expected from missions such as K2, TESS, and PLATO. These candidates often require significant follow-up observations for validation, so prioritizing planets for habitability from transit data has become an important aspect of the search for life in the universe. We propose a method to compare transiting planets for their potential to support life based on transit data, stellar properties and previously reported limits on planetary emitted flux. For a planet in radiative equilibrium, the emitted flux increases with eccentricity, but decreases with albedo. As these parameters are often unconstrained, there is an “eccentricity-albedo degeneracy” for the habitability of transiting exoplanets. Our method mitigates this degeneracy, includes a penalty for large-radius planets, uses terrestrial mass–radius relationships, and, when available, constraints on eccentricity to compute a number we call the “habitability index for transiting exoplanets” that represents the relative probability that an exoplanet could support liquid surface water. We calculate it for Kepler objects of interest and find that planets that receive between 60% and 90% of the Earth’s incident radiation, assuming circular orbits, are most likely to be habitable. Finally, we make predictions for the upcoming TESS and James Webb Space Telescope missions.

Planet and satellite surfaces↗

DOE OSTI2021

National Academy of Science, Engineering, and Medicine, NASA Planetary Science and Astrobiology Decadal Survey (Chapter 3)

Our inner Solar System has the four terrestrial planets, Mercury, Venus, Earth, and Mars, along with Earth’s large Moon, two small moons of Mars, dwarf planet Ceres, numerous 100-km class asteroids, and a multitude of small bodies that populate the asteroid belt and the inner planet region. To what extent does this structure reflect a deterministic outcome of general Solar System formation processes, and how much of it is instead a consequence of stochastic events that would change in unpredictable ways if the system’s formation could be rerun? This fundamental question motivates scientific investigations of the inner Solar System, with implications not only for understanding our own planet’s origin but also for the origin of planetary systems and Earth-like planets beyond our own.

79 ASTRONOMY AND ASTROPHYSICS↗

NASA NTRS2019

Impact of Clouds and Hazes on the Simulated JWST Transmission Spectra of Habitable Zone Planets in the TRAPPIST-1 System

The TRAPPIST-1 system, consisting of an ultracool host star having seven known Earth-sized planets, will be a prime target for atmospheric characterization with the James Webb Space Telescope (JWST). However, the detectability of atmospheric molecular species may be severely impacted by the presence of clouds and/or hazes. In this work, we perform 3D general circulation model (GCM) simulations with the LMD-G model supplemented by 1D photochemistry simulations at the terminator with the Atmos model to simulate several possible atmospheres for TRAPPIST-1e, 1f, and 1g: (1) modern Earth, (2) Archean Earth, and (3) CO2-rich atmospheres. The JWST synthetic transit spectra were computed using the GSFC Planetary Spectrum Generator. We find that the TRAPPIST-1e, 1f, and 1g atmospheres, with clouds and/or hazes, could be detected using JWST's NIRSpec Prism from the CO2 absorption line at 4.3 μm in less than 15 transits at 3σ or less than 35 transits at 5σ. However, our analysis suggests that other gases would require hundreds (or thousands) of transits to be detectable. We also find that H2O, mostly confined in the lower atmosphere, is very challenging to detect for these planets or similar systems if the planets' atmospheres are not in a moist greenhouse state. This result demonstrates that the use of GCMs, self-consistently taking into account the effect of clouds and subsaturation, is crucial to evaluate the detectability of atmospheric molecules of interest, as well as for interpreting future detections in a more global (and thus robust and relevant) approach.

Thomas J. Fauchez↗

DOE OSTI2020

Photometry as a Proxy for Stellar Activity in Radial Velocity Analyses

Stellar activity remains a limiting factor in measuring precise planet parameters from radial velocity spectroscopy, not least in the search for Earth-mass planets orbiting in the habitable zones of Sun-like stars. One approach to mitigate stellar activity is to use combined analyses of both radial velocity and time-series photometry. We present an analysis of simultaneous disk-integrated photometry and radial velocity data of the Sun in order to determine the useful limits of a combined analysis. We find that simple periodogram or autocorrelation analysis of solar photometry give the correct rotation period <50% of the time. We therefore use a Gaussian process to investigate the time variability of solar photometry and to directly compare simultaneous photometry with radial velocity data. We find that the hyperparameter posteriors are relatively stable over 70 yr of solar photometry and the amplitude tracks the solar cycle. We observe good agreement between the hyperparameter posteriors for the simultaneous photometry and radial velocity data. Our primary conclusion is a recommendation to include an additional prior in Gaussian process fits to constrain the evolutionary timescale to be greater than the recurrence timescale (i.e., the rotation period) to recover more physically plausible and useful results. Our results indicate that such simultaneous monitoring may be a useful tool in enhancing the precision of radial velocity surveys.

79 ASTRONOMY AND ASTROPHYSICS↗

NASA NTRS2004

Pointing Control System Design and Performance Evaluation for TPF Coronagraph

The Terrestrial Planet Finder (TPF) program aims to detect and characterize extra-solar Earth-like planets. The coronagraph telescope is one of the four mission concepts being studied. In order to reject the star flux and detect the planet flux in the visible light range, the coronagraph telescope must achieve a rejection ratio on the order of a billion to one. Dynamic jitter, introduced by environmental and on-board mechanical disturbances, degrades the optical performance, as characterized primarily by contrast ratio. The feasibility of using passive vibration isolation combined with active attitude and line-of- sight control systems to stabilize the spacecraft and the optical components to the requisite level is being studied. The telescope is also required to slew between targets. The slew mode control law must be designed to balance the need for efficient large-angle maneuvers while simultaneously avoiding the excitation of flexible modes in order to minimize settling time.

Liu, Kuo-Chia↗

NASA NTRS2004

The Terrestrial Planet Finder and Darwin Missions

Both in the United States and in Europe, teams of scientists and engineers are exploring the feasibility of the Terrestrial Planet Finder (TPF) and Darwin missions, which are designed to search for Earth-like planets in the habitable zone of nearby stars. In the US, the TPF Science Working Group is studying four options - small (4m by 6 m primary mirror) and large (4m by 10 m primary mirror) coronagraphs for planet detection at visible wavelengths, and structurally connected and free-flyer interferometers at thermal infrared wavelengths. The US TPF-SWG is charged with selecting an option for NASA by the end of 2006. In Europe the Darwin Terrestrial Exo-planet Advisory Team (TE- SAT) is exploring the free-flyer interferometer option only at this time. I will discuss the vurtures and difficulties of detecting and characterizing extra-solar planets in both wavelength regions as well as some of the technical challenges and progress in the past year.

Danchi, William C.↗

NASA NTRS2006

Status of the Terrestrial Planet Finder Interferometer (TPF-I)

The interferometric version of the Terrestrial Planet Finder (TPF-I) has the potential to find and characterize earth-sized planets in the habitable zones of over 250 nearby stars and to search for life using biomarkers in the atmospheres of any planets found. The scientific case for such a mission continues to be strengthened by on-going progress in the detection of planets via indirect means. This paper summarizes the status of TPF-I, illustrative scientific requirements for the mission, and its enabling technologies.

Terrestrial Planet Finder (TPF)↗

NASA NTRS2015

Lower Limits on Aperture Size for an ExoEarth Detecting Coronagraphic Mission

The yield of Earth-like planets will likely be a primary science metric for future space-based missions that will drive telescope aperture size. Maximizing the exoEarth candidate yield is therefore critical to minimizing the required aperture. Here we describe a method for exoEarth candidate yield maximization that simultaneously optimizes, for the first time, the targets chosen for observation, the number of visits to each target, the delay time between visits, and the exposure time of every observation. This code calculates both the detection time and multiwavelength spectral characterization time required for planets. We also refine the astrophysical assumptions used as inputs to these calculations, relying on published estimates of planetary occurrence rates as well as theoretical and observational constraints on terrestrial planet sizes and classical habitable zones. Given these astrophysical assumptions, optimistic telescope and instrument assumptions, and our new completeness code that produces the highest yields to date, we suggest lower limits on the aperture size required to detect and characterize a statistically motivated sample of exoEarths.

Earth-like planets↗

NASA NTRS2002

Eccentricity Evolution of Migrating Planets

We examine the eccentricity evolution of a system of two planets locked in a mean motion resonance, in which either the outer or both planets lose energy and angular momentum. The sink of energy and angular momentum could be a gas or planetesimal disk. We analytically calculate the eccentricity damping rate in the case of a single planet migrating through a planetesimal disk. When the planetesimal disk is cold (the average eccentricity is much less than 1), the circularization time is comparable to the inward migration time, as previous calculations have found for the case of a gas disk. If the planetesimal disk is hot, the migration time can be an order of magnitude shorter. We show that the eccentricity of both planetary bodies can grow to large values, particularly if the inner body does not directly exchange energy or angular momentum with the disk. We present the results of numerical integrations of two migrating resonant planets showing rapid growth of eccentricity. We also present integrations in which a Jupiter-mass planet is forced to migrate inward through a system of 5-10 roughly Earth-mass planets. The migrating planets can eject or accrete the smaller bodies; roughly 5% of the mass (averaged over all the integrations) accretes onto the central star. The results are discussed in the context of the currently known extrasolar planetary systems.

Murray, N.↗

NASA NTRS2004

Formation Algorithms and Simulation Testbed

Formation flying for spacecraft is a rapidly developing field that will enable a new era of space science. For one of its missions, the Terrestrial Planet Finder (TPF) project has selected a formation flying interferometer design to detect earth-like planets orbiting distant stars. In order to advance technology needed for the TPF formation flying interferometer, the TPF project has been developing a distributed real-time testbed to demonstrate end-to-end operation of formation flying with TPF-like functionality and precision. This is the Formation Algorithms and Simulation Testbed (FAST) . This FAST was conceived to bring out issues in timing, data fusion, inter-spacecraft communication, inter-spacecraft sensing and system-wide formation robustness. In this paper we describe the FAST and show results from a two-spacecraft formation scenario. The two-spacecraft simulation is the first time that precision end-to-end formation flying operation has been demonstrated in a distributed real-time simulation environment.

Wette, Matthew↗

NASA NTRS2019

Breakthroughs in Picometer Ultra-Stable Spatial Metrology Systems for Next Generation Telescopes

Goal is to discover and characterize habitable planet candidates around Sun-like stars. Need 10(exp -10) contrast between reflected earth like planet and sun like stars. Options are internal Coronagraphs (LUVOIR and Habex) and large starshades (Habex) which each have pros and cons. Coronagraphs require 10(exp -10)contrast stability which means the primary mirror must be stable to roughly 10 picometers RMS wavefront over an exposure(minutes). Simplistically: Primary mirror instabilities of 10 picometers in certain spatial frequencies look like planets!

Feinberg, Lee↗

NASA NTRS2004

The formation algorithms and simulation testbed

Formation flying for spacecraft is a rapidly developing field that will enable a new era of space science. For one of its missions, the Terrestrial Planet Finder (TPF) project has selected a formation flying interferometer design to detect earth-like planets orbiting distant stars. In order to advance technology needed for the TPF formation flying interferometer, the TPF project has been developing a distributed real-time testbed to demonstrate end-toend operation of formation flying with TPF-like functionality and precision. This is the Formation Algorithms and Simulation Testbed (FAST) . This FAST was conceived to bring out issues in timing, data fusion, inter-spacecraft communication, inter-spacecraft sensing and system-wide formation robustness. In this paper we describe the FAST and show results from a two-spacecraft formation scenario. The two-spacecraft simulation is the first time that precision end-to-end formation flying operation has been demonstrated in a distributed real-time simulation environment.

Benowitz, Edward↗

NASA NTRS2011

Views from EPOXI: Colors in Our Solar System as an Analog for Extrasolar Planets

The first visible-light studies of Earth-sized extrasolar planets will employ photometry or low-resolution spectroscopy. This work uses EPOCh medium-hand filter photometry between 150 and 950 nm obtained with the Deep Impact (DI) High Resolution Instrument (HRI) of Earth, the Moon, and Mars in addition to previous full-disk observations of the other six solar system planets and Titan to analyze the limitations of using photometric colors to characterize extrasolar planets. We determined that the HRI 350, 550, and 850 nm filters are optimal for distinguishing Earth from the other planets and separating planets to first order based on their atmospheric and surface properties. Detailed conclusions that can be drawn about exoplanet atmospheres simply from a color-color plot are limited due to potentially competing physical processes in the atmosphere. The presence of a Rayleigh scattering atmosphere can be detected by an increase in the 350-550 nm brightness ratio, but the absence of Rayleigh scattering cannot be confirmed due to the existence of atmospheric and surface absorbing species in the UV. Methane and ammonia are the only species responsible for strong absorption in the 850 nm filter in our solar system. The combination of physical processes present on extrasolar planets may differ from those we see locally. Nevertheless, a generation of telescopes capable of collecting such photometric observations can serve a critical role in first-order characterization and constraining the population of Earth-like extrasolar planets.

Crow, Carolyn A.↗

NASA NTRS2002

Detection by Transit Photometry

A periodic sequence of planetary transits provides a valid detection of an orbiting planet and provides the relative size of the planet and its orbital period. Ancillary measurements of the stellar spectrum and the variations of the star's radial velocity or position combined with stellar models allow the absolute size of the planet and its mass to be obtained. The results of this approach have already shown that the planet orbiting HD209458 has only 70% of the mass of Jupiter, but is nearly 50% larger in radius. Based on models of planetary structure, these results imply that the planet must have spent most of its lifetime so close to the star that it has not been able to cool and contract as have the giant planets in our Solar System. Thus its density is much less than Jupiter and Saturn and is actually less than that of water; i.e., about 0.4 gr/cu cm. If more sensitive measurements of the light curve of stars with closely orbiting planets can be made that provide the varying amplitude of the light reflected by the planet at various phases in its orbit, then characteristics of the planetary atmosphere can be obtained. Potentially, these data can identify major molecular species present in the atmosphere and tell us if clouds are present and yield the phase function of the aerosols. Although such detail cannot be obtained for Earth-size planets because their signal amplitudes are too small, it is possible to get data critical to the determination of the structure of extrasolar planetary systems. In particular, the size distributions and their orbital distributions can be measured by the transit photometry missions now in development. The COROT mission should be able to find large terrestrial planets in short-period orbits while the more ambitious Kepler and Eddington missions should be able to detect planets even smaller than the Earth and at orbital distances that place them in the habitable zone of their stars.

Borucki, William J.↗