The Origin of Life As We Know It
Prof. Jeffrey Bada (Scripps Institution of Oceanography, UCSD)
There are two fundamental requirements for life as we know it, liquid water and organic polymers, such as nucleic acids and proteins. Water provides the medium for chemical reactions and the polymers carry out the central biological functions of replication and catalysis. During the accretionary phase of the Earth, high surface temperatures would have made the presence of liquid water and an extensive organic carbon reservoir unlikely. As the Earth's surface cooled water and simple organic compounds, derived from a variety of sources, would have begun to accumulate. This set the stage for the process of chemical evolution to begin in which one of the central facets was the synthesis of biologically important polymers, some of which had a variety of simple catalytic functions. Increasingly complex macromolecules were produced and eventually molecules with the ability to catalyze their own imperfect replication appeared. Thus began the processes of multiplication, heredity and variation, and this marked the point of both the origin of life and evolution. Once simple self-replicating entities originated they evolved first into the RNA World and eventually to the DNA/Protein World, which had all the attributes of modern biochemistry. If the basic components water and organic polymers were, or are, present on other bodies in our solar system and beyond, it is reasonable to assume that a similar series of steps that gave rise of life on Earth could occur elsewhere.
Submarine Hydrothermal Vents: Limits of Life, Biofilms and Early Evolution
Dr. John Baross (University of Washington)
The two types of hydrothermal vent environments, magma-driven and peridotite-hosted, offer many contrasting habitat conditions for microbial communities. These environments span a wide range of chemical and physical conditions that include almost all of the extremes in temperature, Eh, salinity and heavy metal concentrations that limit where life can exist. Moreover, vent microorganisms have adapted to habitat conditions that include flowing fluids, porous spaces within basalt, sulfides and sediments, the surfaces of rocks and animals and the subseafloor potentially to depths in the crust exceeding 6 km. Hydrothermal systems produce volatiles, such as H2, H2S, CH4, CO, CO, and trace metals that are important sources of carbon and energy, and nutrients for organisms. The sources of volatiles include magma degassing, water/rock reactions, and abiotic reduction of CO2 to methane and possibly other organic compounds. All of these reactions take place in the subseafloor and are not always dramatically expressed on the seafloor. Recently, a peridotite-hosted hydrothermal vent environment was discovered on the Mid-Atlantic Ridge. This environment, named the "Lost City Hydrothermal Field" is a source of high concentrations of hydrogen and methane, hydrocarbons and possibly organic acids produced abiotically from serpentinization reactions that take place in the crust. Hallmark characteristics of both types of high temperature hydrothermal vent microbial communities are that they utilize hydrogen as a primary energy source and they exist as biofilms. This is interesting in that there are parallels between the energy metabolic reactions of these microbial biofilms and the chemistry of the H2-CO2 redox couple that are present in hydrothermal systems, thus indicating the possibility that vent autotrophs might provide clues about the kinds of reactions that initiated the chemistry of life. Hydrothermal vents thus unite microbiology and geology to address the important question - what is the origin of life? - And by inference, a model for understanding the environmental conditions that could support a second genesis on other planets and moons.
Mars: What We Could Know in 20 Years (and How)
Dr. Luann Becker (John Hopkins University)
The search for extinct organic matter (i.e., organic matter generated by now-extinct organisms) in rocks, sediments, and ices from Mars or ‘extant’ (living organisms) organic matter on Mars is critical to the determination of where life exists present or past. Future missions to Mars also have the potential to address whether life arose there in a separate origin and may further provide information about our own prebiotic evolution, a record that has all but been erased from Earth’s crust. Several joint missions between NASA and ESA are planned in the next decade that may enable us to search for clues of life in a liquid water environment. It seems clear that the potential for learning about life beyond our own planet is one of considerable interest to scientists and the general public alike. Yet, as we learned from the Viking and Phoenix missions, the search for life signs is problematic and requires an appropriate strategy that will maximize our opportunities to properly examine these compelling questions. Another potential hurdle to the search for past or present life on Mars is the forward contamination of the planet with either terrestrial organisms or biomolecules. This problem makes it essential that organic analyses be carried out as ‘cleanly’ as possible in order to provide a useful baseline data set for comparison with other landed missions and possibly a sample return mission in the future. This talk discusses, briefly, what is known about the organic matter on Mars and further addresses techniques and strategies for future missions to Mars and beyond.
Life As We Don't Know It
Dr. Steven Benner (Foundation for Applied Molecular Evolution, Inc.)
Exobiology, the field that hunts for alien life, is a science without a subject matter. This makes difficult the use of "the" scientific method in the hunt. Tools used routinely to detect life on Earth are not likely to detect alien life. Nothing illustrates this better than recent exploration of Mars. Metabolism-like reactions, carbon fixation, oxygen release, perchlorate and (just last week) methane have all been observed on Mars. All might be considered to be signs of life, but might also arise without biology. Needed, but missing in exobiology, is a "theory" of life, an overarching framework that connects chemistry, information, and physiology to Darwinian processes, which are believed to be the only way that matter can spontaneously organize itself to give the attributes of life. A theory of life as a universal "natural kind" must come indirectly, as "universal life" cannot be observed directly. This talk will feature recent efforts to build a general theory, efforts that include the resurrection of ancient forms of life for study in the laboratory, the combination of geology and chemistry to understand life's origins, and the construction of artificial chemical systems capable of Darwinian evolution in a "synthetic biology."
Mass and Orbit Estimation Near the Detection Limit
Robert Brown (STScI)
An extrasolar planet’s orbit and mass have both scientific and practical significance: the values of these parameters help us better to understand the physical conditions on the planet and efficiently plan future analytic observations. However, estimating the mass and predicting the future planetary position are challenging for terrestrial planets in the habitable zone—especially for Earth-twins—due to rapid motion, orbital obscuration, and small signals by any technique. This presentation discusses a variety of related issues, particularly in the low signal-to-noise regime, approaching the detection limit. These issues include search completeness, the accuracy of estimates of mass and orbit, and the limitations due to the actual inventory of nearby stars.
Potentially Habitable Worlds III: Europa
Chris Chyba (Princeton University)
There is very strong evidence from measurements of Europa’s gravity field that the outer ~100 km of Europa’s interior is water. There is strong evidence (theoretical and observational, but especially from magnetometer results) that most of this layer is in the liquid phase. Combining magnetometer results with simple spherical interior models implies that the ice is thin (less than 10 km in thickness) and that the underlying ocean is very saline. Europa’s likely bulk composition suggests that so-called biogenic elements should be abundant. Important remaining questions include the availability and cycling of these elements, the availability of energy to power life, and the presence of conditions that might be conducive to the origin of life. Models of radiation-driven chemistry in the surface ice suggest that biologically useful energy should be available. As is the case for Mars, preventing the forward contamination of Europa by terrestrial microorganisms carried on spacecraft should be an important priority in future exploration.
Cultural Perspective: How Detection of Extraterrestrial Life Will Change Our World Views
Paul Davies (ASU)
It is often remarked that the discovery of life beyond Earth would transform our view of ourselves and our place in the universe. However, the implications for science and society depend crucially on the precise nature of the discovery. In my talk I shall review a spectrum of possibilities, from finding a microbe on Mars to picking up a message from an advanced alien community. I shall discuss how we might obtain, right now, critical information about our place in the universe by identifying a shadow biosphere on Earth.
The Seeds of Life - Exogenous Delivery of Organics to Earth
Prof. Pascale Ehrenfreund (Space Policy Institute)
The variety of interstellar environments offers many chemical pathways that lead to the formation of carbon compounds. Observations throughout the electromagnetic spectrum show a large variety of organic molecules in interstellar clouds. Simple molecules such as CO, CH, CN, OH, C2, C3 as well as more complex organics including nitriles, aldehydes and alcohols are identified. The most abundant interstellar carbon fraction, macromolecular carbon and polycyclic aromatic hydrocarbons, is produced in interstellar and circumstellar regions. Our solar system was formed about 4.6 Gyr ago through the gravitational collapse of an interstellar cloud. Recent data from the Stardust mission confirmed large-scale mixing in the solar nebula. Thus, the carbonaceous inventory of our solar system represents a mixture of materials including: (i) highly processed material that was exposed to high temperature and radiation (ii) newly formed compounds and (iii) relative pristine material with strong interstellar heritage. Small bodies, such as comets, asteroids and their fragments, meteorites and interplanetary dust particles (IDPs) bear witness of processes occurring at the time of solar system formation. Carbonaceous meteorites exhibit evidence of thermal and aqueous alteration on their parent bodies. Their insoluble carbon fraction is composed of macromolecular aromatic carbon; their soluble carbon fraction contains carboxylic acids, hydrocarbons, and several of the key prebiotic compounds such as amino acids, nucleobases and polyols. Small bodies delivered large quantities of extraterrestrial material to young terrestrial planetary surfaces in the early history of our solar system that may have provided the material necessary for the emergence of life.
Terrestrial Planet Bombardment and Habitability
Dr. Jane Greaves (University of St Andrews)
I will review the importance of bombardment by comets and asteroids in the context of the development of life. Although we have only our own life-bearing planet from which to speculate, some aspects such as a catastrophic impact at the level of destroying a planet's crust can not be good for biosystems! The Earth is now very unlikely to experience such a massive hit, as the Kuiper Belt has been largely cleared out, although ~10 km impactors every ~100 Myr may change the environment enough that new species arise. However in exo-systems, very large cometary populations are detected via the study of dusty debris, and these can persist over the entire main-sequence stellar lifetimes. I will present results on the comet populations of nearby Sun-like stars and place the Sun within this ensemble, ending with an estimate of the distance to the nearest analogue system to our own that could harbour a habitable Earth.
Deciphering Spectral Fingerprints of Biomarkers on Exo-Earths
Dr. Lisa Kaltenegger (Harvard/CfA)
In this talk we discuss how we can read a planet’s spectrum to assess its habitability. What can we look for in a spectral fingerprint of Earth and super-Earths that can indicate life? In this talk we explore biomarkers on rocky planets, at different wavelengths, geological epochs, different biota, and their detectability. To search for signs of life we need to set the planet’s atmosphere in context with the observable features. The detection and of Earth-like planets is approaching rapidly and ground as well as space based telescopes to characterization them, are already in development phase (ELT, TNT, GMT, James Webb Space Telescope, Darwin, TPF, NWO). We will assess their effectiveness and the best observation strategy to search for the signatures of a biosphere.
The Evolution of Earth’s Atmosphere and Surface
Prof. James Kasting (Penn State University)
Earth is the only planet known to harbor life and thus serves as our model both for a place where life may have originated and as a planet on which life could be remotely detected. Conditions on the early Earth, however, are not well understood, partly because of the lack of a rock record before about 4.0 Ga (billions of years ago). We deduce that the early atmosphere was a weakly reduced mixture of N2 and CO2, with smaller amounts of H2, CO, and some CH4. This composition could conceivably have been altered by large impact events, and so the question of whether the heavy bombardment occurred over an extended period, or as a pulse near 3.9 Ga, is critical. Climate on the early Earth remains an enigma, as well. Despite the faintness of the young Sun, the early Earth appears to have been warm, or perhaps even hot. Taken at face value, oxygen and silicon isotopes in ancient cherts imply a mean surface temperature of 70(+/-15)oC at 3.3 Ga. A recently published analysis of the thermal stability of ancient proteins supports this conclusion. This evidence for hot early surface temperatures must be weighed against theoretical considerations, as well as geomorphic evidence for glaciation at 2.9 Ga, 2.4 Ga, and 0.6-0.7 Ga. Such models must also account for the well documented correlation between the rise of O2 at 2.4 Ga and the Paleoproterozoic glaciations which occurred at that same time.
Extrapolating the Rules for Life: Determining Alternative Photosynthetic Biosignatures Evolved With Other Stars and Atmospheres
Dr. Nancy Kiang (NASA Goddard Institute for Space Studies)
Photosynthesis – broadly, the utilization of light by life to drive biochemical processes – is so successful a process that it provides the foundation for virtually all life on Earth. Its presence is visible at the global scale, in our abundant atmospheric oxygen and in the wide distribution of chlorophyll over the Earth’s surface. Since stellar radiation is such a ubiquitous energy source for a planet, we expect photosynthesis to be successful also on other habitable planets. However, to detect alien photosynthetic life, should we look for the same gaseous and pigment biosignatures as on Earth? We can obtain some clues as to what sort of extrasolar photosynthesis might dominate on another planet from the diversity of photosynthetic organisms on Earth and their path of evolution with our changing atmosphere: the constraints are both environmental resources (stellar radiation spectrum and intensity, carbon source, nutrients, and electron donors) and molecular mechanisms (light harvesting, redox requirements), and how the one is bounded by the other. This talk aims to summarize the biology for the astronomers to help inform strategies for detection of photosynthetic life. What are the likely rules for adaptation of photosynthesis to other stars and atmospheres? What has been the range of biosphere productivity from the Archaen to modern times on Earth? How much can the spectral characteristics of pigments vary? What do we need to know to determine is the long wavelength limit of photosynthesis, and of oxygenic photosynthesis in particular?
Dr. Ralph Lorenz (JHU Applied Physics Laboratory)
Titan is a target of outstanding interest from three broad perspectives. First, it is a large icy satellite, with a geophysical structure and evolution that may be compared with other satellites such as Ganymede. Second, it shows geomorphological and meteorological processes characteristic of the terrestrial planets - most obviously our own Earth. Mountains, lakes, dunefields and fluvial channels are evidently evolving via familiar processes under unfamiliar conditions to yield a varied landscape much like our own. Similarly, methane rainstorms resemble tropospheric moist convection on Earth, and the Titan stratosphere exhibits seasonal change in gas and aerosol composition, with a polar structure that has parallels with the Earth's ozone hole. Finally, Titan has a rich atmospheric photochemistry which produces an array of organic molecules and macromolecules. Titan's surface is the ultimate sink for this material, forming lakes at the poles and giant seas of sand at low latitude. It has even been shown that terrestrial microbes can metabolize laboratory analogs of this photochemical material ('tholin'). Titan is one of the richest chemical environments in the solar system, and Titan-like moons doubtless circle many extrasolar planets. As our sun evolves into a red giant, Titan may become a habitable or even inhabited world, and many extrasolar Titans may already have done so. Titan's organic-rich surface environment lends itself to in-situ exploration : there is no radiation hazard and the atmosphere makes it easy to deliver large payloads softly to the surface, and permits global-scale mobility by vehicles such as hot-air balloons.
Scientific Perspective: The Search for Life in the Universe
Chris McKay (NASA Ames)
The search for a second genesis of life in the universe address deep philosophical and scientific questions: Is life common in universe? Are there alternative biochemistries for life? Several observations motivate the search. These include the presence of organic material in the outer solar system, in meteorites, and the interstellar medium; the evidence for Earth-like planets around other starts; the early occurrence of life on Earth; the presence of past and present liquid water environments in other worlds of our solar system. The search for a second genesis of life includes work in laboratories aimed at creating life, missions to other worlds in our solar system, the search for signs of life on extrasolar planets, and of course SETI. Probably, the best strategy for a second genesis of life in the worlds of our solar system is the detection and detailed characterization of organic material. This make use of the fact that life selects certain organic molecules, abiotic chemistry does not. On extrasolar planets the detection of atmospheric oxygen is the most promising approach. High levels of oxygen on an Earth-like planet orbiting a Sun-like star would be convincing evidence for life and suggestive evidence for complex life. Neither the vast number of stars and planets, nor the principle of mediocrity give us any guidance on the likelihood of life beyond the Earth. The question for a second genesis of life in the universe is an empirical one and all methods of search should be engaged.
Exploring the Habitability of Icy Worlds: The Europa Jupiter System Mission
Dr. Robert Pappalardo (Jet Propulsion Laboratory, California Institute of Technology)
The search for life in the outer solar system focuses on active, ocean-bearing moons. While several icy satellites may contain oceans within, Europa’s ocean seems astrobiologically most promising, with a relatively thin ice shell above, and direct contact with a rocky mantle below. This differs from the more common “ocean sandwich” within large moons as exemplified by Ganymede, with ice I above, and higher density ice polymorphs below. The Europa Jupiter System Mission (EJSM) is a planned combined NASA-ESA endeavor to explore Europa, Ganymede, and the Jupiter system. The mission concept has as its overall theme: “The emergence of habitable worlds around gas giants.” It consists of a NASA-led Jupiter Europa Orbiter (JEO) and an ESA-led Jupiter Ganymede Orbiter (JGO), which would execute a choreographed exploration of the Jupiter System for ~2.5 years before settling into orbit around Europa and Ganymede, respectively, for 9-12 months to investigate the potential habitability of these worlds. JEO would characterize Europa’s ocean and deeper interior, ice shell and any subsurface water, surface composition and chemistry, geology including potential future landing sites, and interactions with the external environment including the tenuous atmosphere, and JEO will understand Europa in the context of the Jovian system. JGO’s analogous detailed exploration of Ganymede would provide unprecedented comparative planetology of these active, ocean-bearing worlds. EJSM would shed new light on the potential habitability of icy worlds in our solar system and beyond.
Stellar Evolution and The Habitable Zone: Overall Properties and the Impact of Activity and Rotation
Prof. Marc Pinsonneault (Ohio State University)
The impact of stellar properties on the habitable zones around them are discussed. The predicted luminosity as a function of mass, composition, and age is well-constrained. However there is recent data indicating that the radii of lower-mass stars are inconsistent with theoretical predictions. I will present evidence in favor of a significant activity-radius effect as the likely explanation. I will also review effects typically neglected in stellar models, especially rotation and activity, and argue that they can have a significant impact on habitability.
Theoretical Models of Exoplanet Atmospheres and Interiors: Implications for Life Detection
Sara Seager (MIT)
Over 300 exoplanets are known to orbit nearby stars. Now that their existence is firmly established, a new era of “exoplanet characterization” has begun. A subset of exoplanets—called transiting planets—pass in front of their stars as seen from Earth. Transiting planets have immeasurably changed the field of exoplanets because their physical properties, including average density and atmospheric properties, can be now be routinely measured. Highlights include identification of molecules in exoplanetary atmospheres and constraints on interior composition. The race to find habitable exoplanets has accelerated with the realization that “big Earths” orbiting close to small stars can be both discovered and characterized with current technology. What will it take to identify such habitable worlds with the observations and theoretical tools available to us?
Metabolism in the Origin of Life
Prof. Robert Shapiro (New York University, Dept. of Chemistry)
In one widely accepted theory of the development of life on Earth, evolution began with the appearance of organic polymers with the ability to catalyze their own replication. By natural selection, these polymers evolved into an RNA world and then into a DNA/ protein world. This theory does not explain, however, the processes by which the unorganized, diverse mixtures of small organic chemicals produced by abiotic processes were converted into highly organized, functional polymers. Some experimenters have attempted to mimic the process by carrying out multi-step syntheses using pure reagents, controlled conditions and sophisticated equipment. But the odds that these detailed procedures would be duplicated in the absence of scientists and laboratories are insignificant. A remedy for this conceptual gap lies in a hypothesis called “metabolism-first”: The first steps of self-organization were driven by the flow of available free energy through an appropriate chemical mixture confined within a natural compartment or on a surface. One key requirement is that the energy be made available, or coupled to the system in a way that promotes self-organization. This can be visualized by considering a cycle of chemical reactions that contains a “driver” reaction – one that interacts with the external energy source in a way that is favored by thermodynamics. Material would be drawn into this cycle in order to maximize the discharge of free energy. The cycle would adapt to environmental changes to maintain this process, and in doing so evolve into a network of increasing complexity which eventually produces a replicator.
LUCA (Last Universal Cellular Ancestor) and the 3 Domains of Life
Janet Siefert (Rice University)
This talk will trace research efforts to understand the progression of life from cellular entities, to the last universal common ancestor of life on earth, to our efforts to understand life's pedigree in a three domain regime. This talk will place these efforts in a historical context and consider what future directions are possible to better refine our understanding.
Remote Detection of Biological Activity via Circular Polarization of Light
Bill Sparks (STScI)
The identification of a universal biosignature that could be sensed remotely is critical to the prospects for success in the search for life elsewhere in the universe. A candidate universal biosignature is homochirality, which is likely to be a generic property of all biochemical life. Due to the optical activity of chiral molecules, it has been hypothesized that this unique characteristic may provide a suitable remote sensing probe using circular polarization spectroscopy. Here we describe a study of the circular polarization spectra of photosynthetic microbial organisms. Given their major importance to astrobiology, the advantages accrued by the adoption of photosynthesis and the natural exposure of photosynthesis to observation, such microbes are plausibly commonplace and amenable to remote sensing. We show that the circular polarization spectra exhibit distinctive features apparently related to the biophysics of photosynthesis. We conclude that circular polarization spectroscopy could provide a powerful remote sensing technique for generic life searches.
Past And Current Searches For Life On Mars
Steve Squyres (Cornell University)
For more than a century, Mars has been considered as a possible abode for life. Early telescopic studies investigated "canals" presumed to have been built by intelligent life, and a "wave of darkening" thought to perhaps result from changing vegetation cover. Subsequent flyby and orbital spacecraft disproved these ideas, but also provided evidence for warmer, wetter conditions on ancient Mars. The Viking landers tested the hypothesis that microbial life exists today in near-surface martian soil, and did not produce a convincing confirmation. Controversial evidence for ancient martian microbial life has been argued for in an ancient martian meteorite, and more recent spacecraft studies have focused primarily on better understanding the habitability of ancient Mars. These studies have produced strong evidence for ancient water on Mars, including subsurface aquifers, surface flow, hydrothermal systems, and a wide range of aqueous alteration and precipitation processes. All of these could have provided habitable niches on Mars, although some conditions, particularly salinity and acidity, would have posed challenges to life. Widespread sulfate deposits suggest largely acidic conditions over much of martian history, while phyllosilicate deposits indicate that some martian waters had a more neutral pH. Very recent telescopic data have revealed methane in the martian atmosphere, suggesting either recent geologic or biological activity on the planet. Planned missions to Mars will search for ancient biomarkers, including organic carbon, in the planet's rocks and soils.
Detecting Traces of Primitive Life on Earth (in Context of Solar System Exploration)
Dawn Sumner (University of California, Davis)
The vast majority of ancient organisms leave no discernible evidence of their existence beyond a minute contribution to the average chemistry of their environment. Preservation of distinctive chemical or morphological signatures is unfortunately (for paleobiologists) a very rare occurrence. Thus, detecting the traces of early life on earth requires three approaches: 1) a deep understanding of how the chemistry of environments is influenced by both abiotic and biological processes; 2) a targeted search for distinctive chemical signatures of specific types of life where they can be well preserved; and 3) wise use of morphological signatures to identify and distinguish abiotic and biological structures. We use all three approaches to identify and understand the early evolution of life on earth, and these same three approaches are appropriate to use in searching for evidence of life elsewhere in the solar system. Identifying traces primitive life is difficult on earth, and the challenges are somewhat different elsewhere. Earth is currently saturated with life, unlike any other body in the solar system, making distinguishing evidence of ancient versus younger ecosystems extremely important. Such a distinction is unnecessary on any other body where the detection of any trace of any life would be a momentous discovery. However, studies of the primitive terran biosphere demonstrate that traces of life are difficult to unambiguously identify even when life was abundant. These studies raise the caution that sparse life elsewhere may be exceedingly difficult to identify, especially in fossil form.
SETI Turns 50 - Five Decades of Progress in the Search for Extraterrestrial Intelligence
Dr. Jill Tarter (SETI Institute)
The 1959 Nature article by Giuseppe Cocconi and Phil Morrison provided the theoretical underpinnings for SETI, accompanied in 1960 by Project Ozma, the first radio search for signals by Frank Drake at the National Radio Astronomy Observatory (NRAO). Well over 100 search programs have been conducted since that time, primarily at radio and optical wavelengths, (see www.seti.org/searcharchives) without any successful signal detection. Some have suggested that this means humans are alone in the cosmos. But that is far too strong a conclusion to draw from far too small an observational sampling. An appropriate analogy would be to retrieve one glass of water from the ocean, and having found no fish in that sample, to conclude that there were no fish in the ocean. The experiment might have worked – the smallest fish is ~ 1 mm in size and many other species of fish could have fit within the glass and been visible to naked eye inspection – but the sample was far too small to have any significant probability of success. Instead of concluding that intelligent life on Earth is unique, it is more appropriate to note that in 50 years our ability to search for electromagnetic signals has improved by at least 14 orders of magnitude and that these improvements are still occurring at an exponential rate. In addition, in the past 50 years, the detection of exoplanets, and a growing appreciation of the robustness of extremophiles have given the cosmos at least an appearance of being more biofriendly. If we are looking in the right way, our observational tools will evolve to sample sufficiently large volumes of phase space before SETI turns 100, that evidence for cosmic company can reasonably be expected. If we are not yet looking in the right way, this same exponential growth in our astronomical tools and in new technologies may well uncover evidence of a type we are not wise enough today to predict. The last sentence of the Cocconi and Morrison paper is as wise today as it was in 1959: "The probability of success is difficult to estimate, but if we never search, the chance of success is zero."
Advantages and Strategies for Direct Imaging and Characterization of Exoplanets
Dr. Wesley Traub (JPL)
Knowing that exoplanets exist, we naturally ask if there are any counterparts to the Earth, and if they show signs of life. To answer these questions, we must first find Earth- or super-Earth size planets, then characterize them with spectroscopy. It is unlikely that we will be able to take advantage of transiting terrestrial planets, when they are found, to characterize them using spectroscopy. Instead, direct imaging of planets around nearby stars offers great advantages for characterization, in the visible and infrared. In the US community, our current strategy for accomplishing this enterprise engages a suite of future space missions: astrometric detection, coronagraphic characterization in the visible, and interferometric characterization in the infrared. This paper will discuss the options available with these tools, and the science that we hope to gain under these options.
Complex Organic Molecules in Star- and Planet-Forming Regions
Ewine van Dishoeck (Leiden)
Organic compounds are ubiquitous in space: they are found in diffuse clouds, in the envelopes of evolved stars, in dense star-forming regions, in protoplanetary disks, in comets, on the surfaces of minor planets, and in meteorites and interplanetary dust particles. This brief overview summarizes the observational evidence for the types of organics found in these regions, with emphasis on the complex organic molecules found in low- and high-mass star-forming regions with existing millimeter telescopes. In addition, mid-infrared spectroscopy of disks has revealed surprisingly high abundances of simple organic molecules like C_2H_2 and HCN in the planet-forming zones of disks (inner few AU), which are the building blocks of larger organics. The results will be placed in the context of evolutionary models following the "trail" of organics from collapsing cloud to disks. Prospects for future facilities, in particular ALMA and Herschel at submillimeter wavelengths, as well as JWST-MIRI and ELTs at mid-infrared wavelengths will be discussed.