We present the discovery and validation of a three-planet system orbiting the nearby (31.1 pc) M2 dwarf star TOI-700 (TIC 150428135). TOI-700 lies in the TESS continuous viewing zone in the Southern Ecliptic Hemisphere; observations spanning 11 sectors reveal three planets with radii ranging from 1 R ? to 2.6 R ? and orbital periods ranging from 9.98 to 37.43 days. Ground-based follow-up combined with diagnostic vetting and validation tests enables us to rule out common astrophysical false-positive scenarios and validate the system of planets. The outermost planet, TOI-700 d, has a radius of 1.19 ± 0.11 R ? and resides within a conservative estimate of the host star’s habitable zone, where it receives a flux from its star that is approximately 86% of Earth’s insolation. In contrast to some other low-mass stars that host Earth-sized planets in their habitable zones, TOI-700 exhibits low levels of stellar activity, presenting a valuable opportunity to study potentially rocky planets over a wide range of conditions affecting atmospheric escape. While atmospheric characterization of TOI-700 d with the James Webb Space Telescope (JWST) will be challenging, the larger sub-Neptune, TOI-700 c (R = 2.63 R ?), will be an excellent target for JWST and future space-based observatories. TESS is scheduled to once again observe the Southern Hemisphere, and it will monitor TOI-700 for an additional 11 sectors in its extended mission. These observations should allow further constraints on the known planet parameters and searches for additional planets and transit timing variations in the system.
We present self-consistent three-dimensional climate simulations of possible habitable states for the newly discovered habitable-zone Earth-sized planet TOI-700 d. We explore a variety of atmospheric compositions, pressures, and rotation states for both ocean-covered and completely desiccated planets in order to assess the planet’s potential for habitability. For all 20 of our simulated cases, we use our climate model outputs to synthesize transmission spectra, combined-light spectra, and integrated broadband phase curves. These climatologically informed observables will help the community assess the technological capabilities necessary for future characterization of this planetas well as similar transiting planets discovered in the futureand will provide a guide for distinguishing possible climate states if one day we do obtain sensitive spectral observations of a habitable planet around an M star. We find that TOI-700 d is a strong candidate for a habitable world and can potentially maintain temperate surface conditions under a wide variety of atmospheric compositions. Unfortunately, the spectral feature depths from the resulting transmission spectra and the peak flux and variations from our synthesized phase curves for TOI-700 d do not exceed 10 ppm. This will likely prohibit the James Webb Space Telescope from characterizing its atmosphere; however, this motivates the community to invest in future instrumentation that perhaps can one day reveal the true nature of TOI-700 d and to continue to search for similar planets around less distant stars.
The origin of life is perhaps the most perplexing unanswered question in science. The main reason for this perplexity is the overlap of four critical components to lifes origin: when and where life emerged, how it formed, and what the earliest life forms looked like. Each of these components has their own set of questions that cross multiple disciplines. For example, insights into the question of when and where life began are predicated on the early Archaean rock record, which is limited due to metamorphism and erosion. With time, continued study of the rock record will reveal more about the…
We observed the 2019 January total lunar eclipse with the Hubble Space Telescope’s STIS spectrograph to obtain the first near-UV (17003200 Å) observation of Earth as a transiting exoplanet. The observatories and instruments that will be able to perform transmission spectroscopy of exo-Earths are beginning to be planned, and characterizing the transmission spectrum of Earth is vital to ensuring that key spectral features (e.g., ozone, or O3) are appropriately captured in mission concept studies. O3 is photochemically produced from O2, a product of the dominant metabolism on Earth today, and it will be sought in future observations as critical evidence for life on exoplanets. Ground-based observations of lunar eclipses have provided the Earth’s transmission spectrum at optical and near-IR wavelengths, but the strongest O3 signatures are in the near-UV. We describe the observations and methods used to extract a transmission spectrum from Hubble lunar eclipse spectra, and identify spectral features of O3 and Rayleigh scattering in the 30005500 Å region in Earth’s transmission spectrum by comparing to Earth models that include refraction effects in the terrestrial atmosphere during a lunar eclipse. Our near-UV spectra are featureless, a consequence of missing the narrow time span during the eclipse when near-UV sunlight is not completely attenuated through Earth’s atmosphere due to extremely strong O3 absorption and when sunlight is transmitted to the lunar surface at altitudes where it passes through the O3 layer rather than above it.
Orbiter and rover data have revealed a complex and intermittent hydrological history in Gale Crater on Mars, where habitable environments appear to have endured for at least thousands of years. The intermittency may be the result of a dominantly cold climate punctuated by geologically brief periods of warmth and active hydrology. However, the time required to establish an integrated hydrological cycle in a warming climate is difficult to ascertain and has not been thoroughly investigated. Here we model the transient evolution of groundwater flow and subsurface temperature, the slowest evolving components of the hydrological cycle, during a warm departure from cold conditions. We find that tens of thousands of years are likely required before groundwater could be a meaningful source for large lakes in Gale. With highly favorable conditions, primarily high permeability, significant flow might develop in thousands of years. This implies that surface water dominates during the beginning of a warm phase. Annual mean surface temperatures in Gale below 290?K would likely leave the nearby highlands frozen at the surface. In that case, deep aquifers beneath a highlands permafrost layer could deliver water to Gale, where low temperatures would have reduced evaporation.
We present a study of the photochemistry of abiotic habitable planets with anoxic CO2–N2 atmospheres. Such worlds are representative of early Earth, Mars, and Venus and analogous exoplanets. Photodissociation of H2O controls the atmospheric photochemistry of these worlds through production of reactive OH, which dominates the removal of atmospheric trace gases.
We report on the validation of two planets orbiting the nearby (36pc) M2 dwarf TOI-1266 observed by the TESS mission. The inner planet is sub-Neptune-sized (R=2.46±0.08R⊕) with an orbital period of 10.9 days. The outer planet has a radius of 1.67+0.09−0.11R⊕ and resides in the exoplanet Radius Valley—the transition region between rocky and gaseous planets. With an orbital period of 18.8 days, the outer planet receives an insolation flux of 2.4 times that of Earth, similar to the insolation of Venus.
A vast diversity of environments exists within and beyond Earth. To the extent life depends on and interacts materially with its environment, it can be expected that life-hosting potential among these environments varies as much as the physical and chemical properties that define them. To understand this potential demands that we first understand lifes requirements in some detail. Discussion of these requirements is frequently undertaken in considering habitability. Cockell et al. (2016) argue that the term habitability is inherently binary: An environment either can or cannot sustain life. This is an important starting point in evaluating the life-hosting potential of…
Are we alone in the universe? How did life arise on our planet? How do we search for life beyond Earth? These profound questions excite and intrigue broad cross sections of science and society. Answering these questions is the province of the emerging, strongly interdisciplinary field of astrobiology. Life is inextricably tied to the formation, chemistry, and evolution of its host world, and multidisciplinary studies of solar system worlds can provide key insights into processes that govern planetary habitability, informing the search for life in our solar system and beyond. Planetary Astrobiology brings together current knowledge across astronomy, biology, geology, physics, chemistry, and related fields, and considers the synergies between studies of solar systems and exoplanets to identify the path needed to advance the exploration of these profound questions.
Planetary Astrobiology represents the combined efforts of more than seventy-five international experts consolidated into twenty chapters and provides an accessible, interdisciplinary gateway for new students and seasoned researchers who wish to learn more about this expanding field. Readers are brought to the frontiers of knowledge in astrobiology via results from the exploration of our own solar system and exoplanetary systems. The overarching goal of Planetary Astrobiology is to enhance and broaden the development of an interdisciplinary approach across the astrobiology, planetary science, and exoplanet communities, enabling a new era of comparative planetology that encompasses conditions and processes for the emergence, evolution, and detection of life.
Enceladus, a small moon in orbit around Saturn, has been full of surprises since it was first identified in 1789. The discovery of Enceladus is credited in part to William Herschel, who was fortuitously observing Saturn at equinox. During equinox, the Sun and Earth are aligned edge-on with the rings of Saturn, reducing the observed reflection of sunlight off the rings (ringshine), which is usually bright enough to mask Enceladus from most telescopes. Enceladus is named after one of the Titans, giants in Greek mythology, and contrary to its small size (504 km or 313 miles in diameter), it has…
The quest for both habitable and inhabited worlds beyond Earth is key to understanding the potential distribution of life in the universe. This ongoing search seeks to answer profound questions: Are we alone? How unique is Earth? Should the hunt for life beyond Earth uncover a multitude of habitable worlds and few (if any) inhabited ones, humanity would begin to understand just how lonely and fragile our situation is. On the other hand, if our hunt yields a true diversity of inhabited worlds, then we would learn something fundamental about the tenacity of life in the cosmos.
Astrobiology is intended to be the science of life in the cosmos. When fully developed it will encompass the origins of life and all of lifes manifestations in the full variety of astrophysical environments in which it will be found. It is a big picture, but with one exception it is all future conditional. This is why one begins a book on planetary astrobiology with Earth: Earth is our only known role model for how to succeed in the cosmos. This chapter explores the processes involved in planet formation and which of them may play important roles in establishing the…
Full Citation: Meadows V. S., Arney G. N., Des Marais D. J., and Schmidt B. E. (2020). Preface. In Planetary…
Our capacity to search for life on other worlds is rapidly advancing, both within our solar system and beyond. In the last few decades robotic missions have explored every major target of astrobiological interest in our solar system, collecting data relevant to assessing habitability across diverse environments, from rocky planets to icy satellites. Outside our solar system, we have now identified thousands of worlds orbiting other stars, many of which may be habitable, suggesting there are many places beyond our solar system where life could, at least in principle, emerge and persist. The next challenge in advancing the search for…
The discovery of extrasolar planets demonstrated that the current solar system-inspired paradigm of planet formation was on the wrong track. Most extrasolar systems bear little resemblance to our well-ordered solar system. While the solar system is radially segregated by distance from the star, with small inner rocky worlds and more distant giant planets, few known exo-systems follow the same blueprint. Models designed with the goal of reproducing the solar system failed spectacularly to understand why other planetary systems looked different than our own.
Yet exoplanets represent a huge sample of outcomes of planet formation, and new ideas for solar system…
The development of astrobiology, the study of life in the universe, is intimately tied to the development of spaceflight in the mid to late twentieth century, which enabled the discovery and exploration of non-Earth environments that might harbor life. In 1976, the Viking missions to Mars supported a core goal to search for signs of life beyond Earth (Klein et al., 1972). However, the Viking life detection experiments provided inconclusive evidence of organics and life (e.g., Klein, 1979; Biemann, 1979; Quinn et al., 2013), a result that put a damper on both Mars exploration and exobiology (as astrobiology was known…
A major focus of astrobiology is the search for habitable conditions and life beyond Earth. We have scrutinized Earth intensely to understand how we might search for habitable conditions elsewhere. In many ways, Venus is the most Earth-like planet in the solar system with its similar mass, radius, and bulk density. Indeed, it is likely that Venus and Earth had very similar starting conditions in terms of their relative compositions of both volatiles and refractory compounds. Yet Earth has been habitable since at least the start of the Archean geological eon (about 3.8 b.y. ago) and possibly during the Hadean…
We have only one example of an inhabited world, namely Earth, with its thin veneer of water, the solvent essential to known life. Is a terrestrial planet like Earth that lies in the habitable zone and has the ingredients of habitability a common outcome of planet formation or an oddity that relied on a unique set of stochastic processes during the growth and subsequent evolution of our solar system?
Ultimately, water originates in space, likely formed on dust grain surfaces via reactions with atoms inside cold molecular clouds (Tielens and Hagen, 1982; Jing et al., 2011). Grain surface water ice…
In the near future, extremely large ground-based telescopes may conduct some of the first searches for life beyond the solar system. High spectral resolution observations of reflected light from nearby exoplanetary atmospheres could be used to search for the biosignature oxygen. However, while Earth’s abundant O2 is photosynthetic, early ocean loss may also produce high atmospheric O2 via water vapor photolysis and subsequent hydrogen escape. To explore how to use spectra to discriminate between these two oxygen sources, we generate high-resolution line-by-line synthetic spectra of both a habitable Earth-like and post-ocean-loss Proxima Centauri b.
Context. GJ 1148 is an M-dwarf star hosting a planetary system composed of two Saturn-mass planets in eccentric orbits with periods of 41.38 and 532.02 days.
Aims. We reanalyze the orbital configuration and dynamics of the GJ 1148 multi-planetary system based on new precise radial velocity measurements taken with CARMENES.
Methods. We combined new and archival precise Doppler measurements from CARMENES with those available from HIRES for GJ 1148 and modeled these data with a self-consistent dynamical model. We studied the orbital dynamics of the system using the secular theory and direct N-body integrations. The prospects of potentially habitable moons around GJ 1148 b were examined.
Results. The refined dynamical analyses show that the GJ 1148 system is long-term stable in a large phase-space of orbital parameters with an orbital configuration suggesting apsidal alignment, but not in any particular high-order mean-motion resonant commensurability. GJ 1148 b orbits inside the optimistic habitable zone (HZ). We find only a narrow stability region around the planet where exomoons can exist. However, in this stable region exomoons exhibit quick orbital decay due to tidal interaction with the planet.
Conclusions. The GJ 1148 planetary system is a very rare M-dwarf planetary system consisting of a pair of gas giants, the inner of which resides in the HZ. We conclude that habitable exomoons around GJ 1148 b are very unlikely to exist.
High-contrast direct imaging of exoplanets can provide many important observables, including measurements of the orbit, spectra that probe the lower layers of the atmosphere, and phase variations of the planet, but cannot directly measure planet radius or mass. Our future understanding of directly imaged exoplanets will therefore rely on extrapolated models of planetary atmospheres and bulk composition, which need robust calibration. We estimate the population of extrasolar planets that could serve as calibrators for these models. Critically, this population of “standard planets” must be accessible to both direct imaging and the transit method, allowing for radius measurement.
Liquid water oceans are at the center of our search for life on exoplanets because water is a strict requirement for life as we know it. However, oceans are dynamic habitats—and some oceans may be better hosts for life than others. In Earth’s ocean, circulation transports essential nutrients such as phosphate and is a first-order control on the distribution and productivity of life. Of particular importance is upward flow from the dark depths of the ocean in response to wind-driven divergence in surface layers.
Strong seismic waves from the July 2019 Ridgecrest, California, earthquakes displaced rocks in proximity to the M 7.1 mainshock fault trace at several locations. In this report, we document large boulders that were displaced at the Wagon Wheel Staging Area (WWSA), approximately 4.5 km southeast of the southern terminus of the large M 6.4 foreshock rupture (hereafter “the large foreshock”) and 9 km southwest of the nearest approach of the M 7.1 mainshock surface rupture