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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 NTRS2021

Two Bright M Dwarfs Hosting Ultra-Short-Period Super-Earths with Earth-like Compositions

We present observations of two bright M dwarfs (TOI-1634 and TOI-1685: J = 9.5–9.6) hosting ultra-short-period (USP) planets identified by the TESS mission. The two stars are similar in temperature, mass, and radius (T eff ≈ 3500 K, M ⋆ ≈ 0.45–0.46 M ⊙ , and R ⋆ ≈ 0.45–0.46 R ⊙ ), and the planets are both super-Earth size (1.25 R ⊕ < R p < 2.0 R ⊕ ). For both systems, light curves from ground-based photometry exhibit planetary transits, whose depths are consistent with those from the TESS photometry. We also refine the transit ephemerides based on the ground-based photometry, finding the orbital periods of P = 0.9893436 ± 0.0000020 days and P = 0.6691416 ± 0.0000019 days for TOI-1634b and TOI-1685b, respectively. Through intensive radial velocity (RV) observations using the InfraRed Doppler (IRD) instrument on the Subaru 8.2 m telescope, we confirm the planetary nature of the TOIs and measure their masses: 10.14 ± 0.95 M ⊕ and 3.43 ± 0.93 M ⊕ for TOI-1634b and TOI-1685b, respectively, when the observed RVs are fitted with a single-planet circular-orbit model. Combining those with the planet radii of R p = 1.749 ± 0.079 R ⊕ (TOI-1634b) and 1.459 ± 0.065 R ⊕ (TOI-1685b), we find that both USP planets have mean densities consistent with an Earth-like internal composition, which is typical for small USP planets. TOI-1634b is currently the most massive USP planet in this category, and it resides near the radius valley, which makes it a benchmark planet in the context of discussing the size limit of rocky planet cores as well as testing the formation scenarios for USP planets. Excess scatter in the RV residuals for TOI-1685 suggests the presence of a possible secondary planet or unknown activity/instrumental noise in the RV data, but further observations are required to check those possibilities.

Teruyuki Hirano↗

DOE OSTI2021

Two Bright M Dwarfs Hosting Ultra-Short-Period Super-Earths with Earth-like Compositions

We present observations of two bright M dwarfs (TOI-1634 and TOI-1685: J = 9.5–9.6) hosting ultra-short-period (USP) planets identified by the TESS mission. The two stars are similar in temperature, mass, and radius (T {sub eff} ≈ 3500 K, M {sub ·} ≈ 0.45–0.46 M {sub ⊙}, and R {sub ·} ≈ 0.45–0.46 R {sub ⊙}), and the planets are both super-Earth size (1.25 R {sub ⊕} < R {sub p} < 2.0 R {sub ⊕}). For both systems, light curves from ground-based photometry exhibit planetary transits, whose depths are consistent with those from the TESS photometry. We also refine the transit ephemerides based on the ground-based photometry, finding the orbital periods of P = 0.9893436 ± 0.0000020 days and P = 0.6691416 ± 0.0000019 days for TOI-1634b and TOI-1685b, respectively. Through intensive radial velocity (RV) observations using the InfraRed Doppler (IRD) instrument on the Subaru 8.2 m telescope, we confirm the planetary nature of the TOIs and measure their masses: 10.14 ± 0.95 M {sub ⊕} and 3.43 ± 0.93 M {sub ⊕} for TOI-1634b and TOI-1685b, respectively, when the observed RVs are fitted with a single-planet circular-orbit model. Combining those with the planet radii of R {sub p} = 1.749 ± 0.079 R {sub ⊕} (TOI-1634b) and 1.459 ± 0.065 R {sub ⊕} (TOI-1685b), we find that both USP planets have mean densities consistent with an Earth-like internal composition, which is typical for small USP planets. TOI-1634b is currently the most massive USP planet in this category, and it resides near the radius valley, which makes it a benchmark planet in the context of discussing the size limit of rocky planet cores as well as testing the formation scenarios for USP planets. Excess scatter in the RV residuals for TOI-1685 suggests the presence of a possible secondary planet or unknown activity/instrumental noise in the RV data, but further observations are required to check those possibilities.

47 OTHER INSTRUMENTATION↗

DOE OSTI2021

New Perspectives on the Exoplanet Radius Gap from a Mathematica Tool and Visualized Water Equation of State

Recent astronomical observations obtained with the Kepler and TESS missions and their related ground-based follow-ups revealed an abundance of exoplanets with a size intermediate between Earth and Neptune (1 R ⊕ ≤ R ≤ 4 R ⊕ ). A low occurrence rate of planets has been identified at around twice the size of Earth (2 × R ⊕ ), known as the exoplanet radius gap or radius valley. We explore the geometry of this gap in the mass–radius diagram, with the help of a Mathematica plotting tool developed with the capability of manipulating exoplanet data in multidimensional parameter space, and with the help of visualized water equations of state in the temperature–density (T–ρ) graph and the entropy–pressure (s–P) graph. We show that the radius valley can be explained by a compositional difference between smaller, predominantly rocky planets (<2 × R ⊕ ) and larger planets (>2 × R ⊕ ) that exhibit greater compositional diversity including cosmic ices (water, ammonia, methane, etc.) and gaseous envelopes. In particular, among the larger planets (>2 × R ⊕ ), when viewed from the perspective of planet equilibrium temperature (T eq ), the hot ones (T eq ≳ 900 K) are consistent with ice-dominated composition without significant gaseous envelopes, while the cold ones (T eq ≲ 900 K) have more diverse compositions, including various amounts of gaseous envelopes.

79 ASTRONOMY AND ASTROPHYSICS↗

NASA NTRS2015

Composition, Structure and Formation of Low-Density Planets Within 0.5 AU of Their Star

Kepler has discovered that tens of percent of sunlike stars possess one or more planets larger than Earth within 0.5 AU. Masses of dozens of small and mid-sized exoplanets have been measured using radial velocity or transit timing variations. Few transiting planets larger than 1.6 Earth radii are rocky. Rather, the planetary mass-radius diagram for planetary radius between 1.6 and 8 times that of Earth shows a tendency for mass to increase slowly with size. The scatter is larger than can be explained by observational errors and correlations with temperature, implying that the population is heterogeneous. Many Neptune-size planets have very low densities and must have a substantial fraction of their volumes occupied by hydrogen and/or helium. Models of the structure, growth and evolution of planets made of mixtures of rock and light gases will be presented and compared with observations of both individual bodies and of the characteristics of the population as a whole.

radial velocity↗

NASA NTRS2022

TOI-712 A System of Adolescent Mini-Neptunes Extending to the Habitable Zone

As an all-sky survey, NASA’s Transiting Exoplanet Survey Satellite (TESS) mission is able to detect the brightest and rarest types of transiting planetary systems, including young planets that enable study of the evolutionary processes that occur within the first billion years. Here we report the discovery of a young, multiplanet system orbiting the bright K4.5V star, TOI-712 ( V = 10.838, M * = 0.733 +0.026 -0.025 M ⨀ , R * = 0.674 ± 0.016 R ⨀ , T eff = 4622 +61 -60 K). From the TESS light curve, we measure a rotation period of 12.48 days and derive an age between about 500 Myr and 1.1 Gyr. The photometric observations reveal three transiting mini-Neptunes ( R b = 2.049 +0.120 -0.080 R ⊕ , R c = 2.701 +0.092 -0.082 R ⊕ , R d = 2.474 +0.090 -0.082 R ⊕ ), with orbital periods of P b = 9.531 days, P c = 51.699 days, and P d = 84.839 days. After modeling the three-planet system, an additional Earth-sized candidate is identified, TOI-712.05 ( P = 4.32 days, R P = 0.81 ± 0.11 R ⊕ ). We calculate that the habitable zone falls between 0.339 and 0.844 au (82.7 and 325.3 days), placing TOI-712 d near its inner edge. Among planetary systems harboring temperate planets, TOI-712 ( T = 9.9) stands out as a relatively young star bright enough to motivate further characterization.

Exoplanets↗

NASA NTRS1994

The symbiosis of photometry and radial-velocity measurements

The FRESIP mission is optimized to detect the inner planets of a planetary system. According to the current paradigm of planet formation, these planets will probably be small Earth-sized objects. Ground-based radial-velocity programs now have the sensitivity to detect Jovian-mass planets in orbit around bright solar-type stars. We expect the more massive planets to form in the outer regions of a proto-stellar nebula. These two types of measurements will very nicely complement each other, as they have highest detection probability for very different types of planets. The combination of FRESIP photometry and ground-based spectra will provide independent confirmation of the existence of planetary systems in orbit around other stars. Such detection of both terrestrial and Jovian planets in orbit around the same star is essential to test our understanding of planet formation.

Cochran, William D.↗

NASA NTRS1976

Observations at the planet Mercury by the plasma electron experiment, Mariner 10

Plasma electron observations made onboard Mariner 10 are reported. Three encounters with the planet Mercury show that the planet interacts with the solar wind to form a bow shock and a permanent magnetosphere. The observations provide a determination of the dimensions and properties of the magnetosphere, independently of and in general agreement with magnetometer observations. The magnetosphere of Mercury appears to be similar in shape to that of the Earth but much smaller in relation to the size of the planet. Electron populations similar to those found in the Earth's magnetotail, within the plasma sheet and adjacent regions, were observed at Mercury; both their spatial location and the electron energy spectra within them bear qualitative and quantitative resemblance to corresponding observations at the Earth. The magnetosphere of Mercury resembles to a marked degree a reduced version of that of the Earth, with no significant differences of structure.

Ogilvie, K. W.↗

NASA NTRS2017

Exploration of Venus' Deep Atmosphere and Surface Environment

Venus formed in the same part of our solar system as Earth, apparently from similar materials. Although both planets are about the same size, their differences are profound. Venus and Earth experienced vastly different evolutionary pathways resulting in unexplained differences in atmospheric composition and dynamics, as well as in geophysical processes of the planetary surfaces and interiors. Understanding when and why the evolutionary pathways of Venus and Earth diverged is key to understanding how terrestrial planets form and how their atmospheres and surfaces evolve. Measurements made in situ, within the near-surface or surface environment, are critical to addressing unanswered questions. We have made substantial progress modernizing and maturing pressure vessel technologies to enable science operations in the high temperature and pressure near-surface/surfaceenvironment of Venus.

Venus Atmosphere↗

NASA NTRS2015

Exo-C: A Space Mission for Direct Imaging and Spectroscopy of Extrasolar Planetary Systems

Exo-C is NASAs first community study of a modest aperture space telescope designed for high contrast observations of exoplanetary systems. The mission will be capable of taking optical spectra of nearby exoplanets in reflected light, discovering previously undetected planets, and imaging structure in a large sample of circumstellar disks. It will obtain unique science results on planets down to super-Earth sizes and serve as a technology pathfinder toward an eventual flagship-class mission to find and characterize habitable Earth-like exoplanets. We present the mission/payload design and highlight steps to reduce mission cost/risk relative to previous mission concepts. Key elements are an unobscured telescope aperture, an internal coronagraph with deformable mirrors for precise wavefront control, and an orbit and observatory design chosen for high thermal stability. Exo-C has a similar telescope aperture, orbit, lifetime, and spacecraft bus requirements to the highly successful Kepler mission (which is our cost reference). The needed technology development is on-course for a possible mission start in 2017. This paper summarizes the study final report completed in January 2015. During 2015 NASA will make a decision on its potential development.

high contrast observations↗

NASA NTRS2014

Exo-C: a Probe-Scale Space Mission to Directly Image and Spectroscopically Characterize Exoplanetary Systems Using an Internal Coronagraph

"Exo-C" is NASA's first community study of a modest aperture space telescope designed for high contrast observations of exoplanetary systems. The mission will be capable of taking optical spectra of nearby exoplanets in reflected light, discover previously undetected planets, and imaging structure in a large sample of circumstellar disks. It will obtain unique science results on planets down to super-Earth sizes and serve as a technology pathfinder toward an eventual flagship-class mission to find and characterize habitable exoplanets. We present the mission/payload design and highlight steps to reduce mission cost/risk relative to previous mission concepts. At the study conclusion in 2015, NASA will evaluate it for potential development at the end of this decade. Keywords: Exoplanets, high contrast imaging, optical astronomy, space mission concepts

high contrast imaging↗

NASA NTRS2019

Exo-C: A Dedicated Probe-scale Space Mission for Coronagraphic Imaging and Spectroscopy of Exoplanetary Systems

"Exo-C", the Exoplanet Coronagraph, is a concept for a dedicated probe-scale (< $1B) space mission optimized for direct imaging of exoplanetary systems. It was the product of a detailed mission study carried in 2013-2015 under the sponsorship of the NASA Astrophysics Division. Exo-C was designed to be capable of spectrally characterizing 1-2 dozen nearby exoplanets in reflected visible light, discovering previously undetected planets, and imaging structure in hundreds of circumstellar disks. It would obtain unique science results on planets down to super-Earth size and serve as a technology pathfinder toward an eventual flagship-class mission to find and characterize habitable Earth-like exoplanets. Key design elements are an unobscured telescope aperture, an internal coronagraph with deformable mirrors for precise wavefront control, and an orbit and observatory design chosen for high thermal stability. ExoC’s telescope aperture, orbit, mission lifetime, and spacecraft bus are all comparable to those of the highly successful Kepler mission. Much of the needed technology development (precision pointing, wavefront sensing and correction, high contrast integral field spectroscopy, detector flight readiness) has been advanced by the WFIRST Coronagraph Instrument (CGI) project over the past several years. Mission performance, mass, power, and cost estimates have also benefited from detailed work by the WFIRST CGI team. A Cost and Technical Evaluation (CATE) was performed in 2015 by the Aerospace Corporation. Today Exo-C is a backup option to WFIRST CGI. It could nominally be implemented within seven years from project start. This white paper summarizes the study final report, with updates on performance and technical readiness as of spring 2019.

Karl R. Stapelfeldt↗

NASA NTRS1985

Primordial magmaspheres and their lasting consequences

The lunar magmasphere is a useful but potentially misleading analog for the earliest evolution of other planetary objects. A significant fraction of the lunar magmasphere became a crust rich in buoyant cumulus plagioclase. Another significant fraction became a series of ultramafic cumulates (the mare basalt sources) complementary to the anorthosite. These events predetermined all subsequent lunar evolution. Empirically, the Moon was big enough to produce a magmasphere. Beyond a depth limit of roughly 200 km, nearly independent of the size of the planet, the remainder of the magmasphere will probably at all times be a single convective, and therefore essentially non-differentiating layer. The fraction of the mantle contained in the outer 200 km is of course inversely related to planet size. Compared to the Moon the Earth's mantle comprises a volume of 40 x greater and a pressure range 30 x greater. Steeper dP/dZ, favoring garnets and pyroxenes, also works to dampen differentiation in larger planets. One long term consequence of the Earth's magmasphere was probably a depletion of H2O in much of the mantle. Because water is a flux for mantle convection, anhydrous parts of the mantle probably had anomalously thick lithospheres and hot asthenospheres.

Warren, P. H.↗

NASA NTRS2005

Kepler Mission to Detect Earth-like Planes

Kepler Mission has been approved and funded for launch into an Earth-trailing orbit in 2007. The satellite observatory is designed to detect Earth-like planets in the Cygnus region of the sky (one hundred square degrees). If all goes as planned, we expect to detect some 60 Earth-like planets, plus a greater number of larger planets. None of the planets that have been discovered so far are not Earth-like; they are mostly Jupiter-sized, large planets.

Kondo, Yoji↗

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 NTRS2023

A JWST Transmission Spectrum of A Nearby Earth-Sized Exoplanet

The critical first step in the search for life on exoplanets over the next decade is to determine whether rocky planets transiting small M-dwarf stars possess atmospheres and, if so, what processes sculpt them over time. Because of its broad wavelength coverage and improved resolution compared with previous instruments, spectroscopy with the James Webb Space Telescope (JWST) offers a new capability to detect and characterize the atmospheres of Earth-sized, M-dwarf planets. Here we use the JWST to independently validate the discovery of LHS 475 b, a warm (586 K), 0.99 Earth-radius exoplanet, interior to the habitable zone, and report a precise 2.9–5.3 μm transmission spectrum using the Near Infrared Spectrograph G395H instrument. With two transit observations, we rule out primordial hydrogen-dominated and cloudless pure methane atmospheres. Thus far, the featureless transmission spectrum remains consistent with a planet that has a high-altitude cloud deck (similar to Venus), a tenuous atmosphere (similar to Mars) or no appreciable atmosphere at all (akin to Mercury). There are no signs of stellar contamination due to spots or faculae. Our observations show that the JWST has the requisite sensitivity to constrain the secondary atmospheres of terrestrial exoplanets with absorption features <50 ppm, and that our current atmospheric constraints speak to the nature of the planet itself, rather than instrumental limits.

JWST↗

NASA NTRS1979

Formation, history and energetics of cores in the terrestrial planets

The size, evolution and energetics of the earth's core and the probable central, metalic cores of Mercury, Venus, the moon and Mars are discussed. The cores of Mercury, Mars and the earth are considered likely to decrease in relative mass and volume with distance from the sun; the moon does not fit this sequence and data from Venus are insufficient. Core formation is concluded to have occurred early (prior to four billion years ago) on the earth and Mercury, while that on the moon would have occurred over a longer interval and the core of Mars would have formed much later. Of the possible energy sources in planetary cores able to maintain a molten state and drive magnetic dynamos, the energy of core formation was probably spent too early in planetary history to provide much present power, and the energy gained from freezing an inner core in the earth and Mercury are considered at best marginally able to match conductive heat loss. Future measurements proposed to improve the quantitative understanding of core properties include a better documentation of the magnetic fields of Venus and Mars and the seismometry of Mars.

Solomon, S. C.↗

NASA NTRS1995

New Observations at the Slate Islands Impact Structure, Lake Superior

Slate Islands, a group of 2 large and several small islands, is located in northern Lake Superior, approximately 10 km south of Terrace Bay. Shatter cones, breccias and shock metamorphic features provide evidence that the Slate Islands Structure was formed as a result of asteroid or comet impact. Most of the island group is believed to represent the central uplift of a complex impact crater. The structure possibly has a diameter of about 32 km. For Sage (1978, 1991) shock metamorphic features, shatter cones and pervasive rock brecciation are the results of diatreme activity. The present investigations represent the second year of a co-operative study of the Lunar and Planetary Institute, Houston, Texas and the Field Services Section (Northwest) of the Ontario Geological Survey. The objective of this investigation is to come to a better understanding of the formation of mid-size impact structures on Earth and the planets of the solar system. Impact processes played a fundamental role in the formation of the planets and the evolution of life on Earth. Meteorite and comet impacts are not a phenomenon of the past. Last year, more than 20 pieces of the Shoemaker-Levy 9 impacted on Jupiter and the Tunguska comet impacted in Siberia in the early years of this century. The study of impact processes is a relatively young part of geoscience and much is still to be learnt by detailed field and laboratory investigations. The State Islands Structure has been selected for the present detailed investigations because of the excellent shoreline outcrops of rock units related to the impact. The structure is a complex impact crater that has been eroded so that important lithological and structural elements are exposed. We know of no other mid-size terrestrial impact structure with equal or better exposures. In this publication we present preliminary results of our 1994 and 1995 field and laboratory investigations. We have tentatively identified a few impact melt and a considerable number of suevite occurrences.

Dressler, B. O.↗