Habitable Zones and the Frequency of Potential Habitable Planets in Extrasolar Planetary Systems
May 03, 2013 12:00p - 2:30p
Identifying terrestrial planets in the habitable zones (HZs) of other stars is one of the primary goals of ongoing exoplanet surveys and proposed space-based flagship missions. In this talk, I will discuss about our recent results on new estimates of HZs around Main-sequence stars. According to our new model, the inner and outer HZ limits for our
Solar System are at 0.99 AU and 1.67 AU, respectively, suggesting that the present Earth lies near the inner edge. Our model does not include the radiative effects of clouds; thus, the actual HZ boundaries may be broader than our estimates. Applying the new HZ limits to cool, low mass stars (M-dwarfs) in NASA's "Kepler" data, we find that potentially habitable planets around M-dwarfs are more common than previously reported. The mean distance to the nearest habitable planet may be as close as 7 light years from us.
On Earth, microorganisms appear to inhabit all physical space that provides the minimum requirements for life. These include the availability of water, carbon, nutrients, and light or chemical energy. While these are generally abundant in surface or near-surface environments, their mode and distribution in the subsurface are poorly constrained. Nevertheless, it has now been shown unequivocally that archaea and bacteria inhabit deeply buried rocks and sediments where they contribute to biogeochemical cycles. All evidence suggests that these subsurface ecosystems are spatially enormous and diverse. On other planets, at least in our solar system, putative extant or extinct life would most likely reside underground or in massive ice shells. With a focus on near-by planets where landed missions are more than just a possibility, access to the subsurface will be highly desirable. Robust strategies for subsurface life detection on Mars, Europa, or other potential targets are poorly developed. The search for extant life or its biosignatures is scientifically and technologically extremely difficult; on Earth, it is a formidable but tractable challenge. Our cross-disciplinary team from the University of Southern California, the California Institute of Technology, the Jet Propulsion Laboratory, the Rensselaer Polytechnic Institute, and the Desert Research Institute (DRI) will develop field, laboratory, and modeling approaches aimed at detecting and characterizing microbial life in the subsurface—the intraterrestrials.
In this talk, Glavin will describe the concept of a “habitable environment” and the requirements for life as we know it. Understanding the basic requirements for life and the prebiotic chemistry that led to the emergence of life on Earth will help guide our search for life on Mars. Glavin will also give an overview of NASA's Mars Science Laboratory mission with an update on the progress of the Curiosity rover and a summary of the analytical capabilities and measurement objectives of the SAM experiment. Curiosity is currently getting ready for the first SAM analysis of a drilled rock on Mars and will take a step closer to answering the question of whether Mars could have ever supported life.
The final stages of the growth of a planet consist of violently energetic impacts, but new observations of the Moon and Mercury indicate that the energy of accretion does not remove all the water and carbon from the growing planet. Models demonstrate that rocky planets that accrete with as little as 0.01 wt% water produce a massive steam atmosphere that collapses into a water ocean upon cooling. The low water contents required indicate that rocky planets may be generally expected to produce water oceans through this process, and that an Earth-sized planet would cool to clement conditions in just a few to tens of millions of years. These results indicate that most rocky planets in our solar system and rocky exoplanets are likely to have been habitable early in their evolution, increasing the likelihood of life on the estimated 17 billion Earth-sized planets in the Milky Way.
A promising path to the discovery and study of individual rocky planets in the Habitable Zone around a star is to search for planets around nearby M dwarfs, as their mass and smaller HZ planet orbits leads to a significantly larger Doppler radial velocity signal than that of the Earth on our Sun. Since the flux distribution mid-late M dwarfs peaks sharply in the NIR a stable high-resolution NIR spectrograph capable of delivering high RV precision in the bands is a promising route to detecting rocky planets. I will discuss significant advances in precision NIR spectroscopy that may help achieve this goal, on-sky tests with laser frequency combs, and the ongoing design and build of the fiber-fed Habitable Zone Planet Finder (HPF) high resolution spectrograph.
If planets could re-form or migrate inwards to just outside the Roche limit of white dwarfs stars, they would be warmed to Earth-like temperature for billions of years. These planets would be easy to detect in edge-on orbit via a large depth transit lasting a couple of minutes and repeating every 12 hours; thus, a ground-based transit survey of cool white dwarfs would be sensitive to Earth-sized planets in their habitable zones, should they exist in sufficient abundance. I will discuss the prospects for detection and characterization of Earth-like planets in the habitable zones of white dwarfs, as well as scenarios for planet formation and potential constraints on habitability.
Water is abundant on Earth, covering 75% of Earth's surface, and accounting for up to 0.1% of Earth's mass,
yet Earth may in fact be a very dry world. Liquid water is not only key for life, but understanding the origin of
Earth's water has important implications for habitability in both our solar system and in the numerous
extrasolar planetary systems that have been discovered with planets within their star's habitable zones.
This research question requires an interdisciplinary approach and is one of the themes for the UH NASA
Astrobiology Institute (NAI). In the 1980's observations and models first suggested that comets might play
a role in delivering water to Earth, and that the chemical fingerprint of this process was the D/H of ocean
water. A more modern look at the problem now seeks to uncover all the issues related to the origin of
Earth's water to identify the key unanswered questions and assess where these questions can
best be answered by interdisciplinary approaches. The UH NAI team is investigating the chemical
composition of primitive bodies and evidence of aqueous processes in the early solar system, dynamical
delivery of bodies to earth and the chemical signature of this delivery to earth. I will discuss
the interdisciplinary perspective of the origin of Earth's water, in part as a consensus view from
a recent workshop held in Iceland during Sep. 2011 to highlight where we are in our current understanding.
Astronomy is entering an exciting new era where ground- and space-based observatories open windows on planetary systems increasingly similar to our own. In this talk I will give a brief overview of the search for exoplanets and discuss techniques that allow us to characterize the atmospheres of extrasolar planets. I will illustrate these from our work on detecting and characterizing exoplanets. I will also introduce a new observing technique that provides an exciting look into the atmospheres, clouds, and surface features of exoplanets. We are now applying this technique, rotational phase mapping, to ultracool brown dwarfs using both the Hubble and the Spitzer Space Telescopes. I will discuss the exciting first results from these studies and the future applications of the technique to super-earths and earth-like planets, as a step toward understanding planets in or near to habitable zones.
In the next decade Mars will be visited by the most capable Rovers and instruments built to date. How will these missions find evidence of life on Mars and what does mean for us if there is no life detected. The robotic exploration of Mars and subsequent return of samples to earth will reveal a lot of information about our own origins whether or not life is discovered.
Comets and carbonaceous chondrites are the remnants of molecular cloud material from which our Solar System formed. These bodies are considered to be primitive in the sense that they have been subjected to the least modification following accretion. Both comets and carbonaceous chondrites contain relatively large quantities of refractory organic macromolecular material. For many decades, a lack of consensus has existed as to the ultimate origin of these extraterrestrial organic solids, where confusion largely stems from the fact that throughout the Galaxy there exist many regions were extensive organo-synthesis occurs. Origins theories for primitive Solar System organic solids span from the lowest temperatures in the Interstellar Medium up to 1000 K in the inner Solar System. The best constraint on the origin of refractory organic solids is obtained by detailed studies of the organic solids directly. Using advanced spectroscopic techniques we have identified a plausible source for these organic solids and show that the organic solids in both comets and carbonaceous chondrites share a common origin. The broader implications of these results, both in terms of our understanding of the early history of the inner Solar System objects and the origin of life on Earth, lie in linking the unique properties of these organic solids to events that shaped the origin and evolution of Earth.
As told by Maynard Smith and Szathmáry (The Major Transitions in Evolution, 1998) life's major transitions involve information and individuality. With equal justification, however, we can mark evolutionary milestones in terms of physiological innovation. A physiological complement to Maynard Smith and Szathmáry's list might include three major innovations associated with primary production (photosynthesis, oxygenic photosynthesis, and nitrogen fixation) and five that changed the face of heterotrophy (respiration, aerobic respiration, phagocytosis, bulk oxygen transport, and technology). Such a physiological perspective highlights interrelationships between evolving life and a physically dynamic planet. Geochemical data suggest that for much of the Proterozoic Eon, oxygen minimum zones of Earth's oceans tended toward euxinia. Under these conditions, nitrogen limitation would have favored primary producers capable of nitrogen fixation, as the geobiological record suggests. Despite the presence of oxygenic photoautotrophs, continuing anoxygenic photosynthesis likely played an important role in sustaining the redox structure of Proterozoic oceans. Late in the Neoproterozoic Eon, however, tectonic events appear to have nudged the biosphere toward a new state. Widespread rifting correlates with a switch from predominantly sulfidic to ferruginous waters in the OMZ; broadly coeval expansion of eukaryotes is consistent with the low sulfide tolerance exhibited by most eukaryotic clades. Four independent geochemical proxies suggest further redox transition 580-550 Ma, a time when rates of sediment accumulation increased markedly. Higher oxygen tensions and a receding challenge of anoxia likely facilitated animal diversification, but it was the evolution of anatomical mechanisms for bulk transfer that freed animals from the constraints of diffusion -- ushering in the age of bilaterians.
In the coming decades, the search for life outside our Solar System will be undertaken using astronomical observations of extrasolar
terrestrial planets. To better inform our search, the NASA Astrobiology Institute\'s Virtual Planetary Laboratory team uses a suite of
computer models to explore the interaction between a terrestrial planet and its parent star. The resulting models allow us to simulate
extrasolar terrestrial planetary environments and spectra, and to define and quantify likely signs of planetary habitability and life. This
talk will discuss the VPL models and results to date, including the detectability of planetary habitability and potential signs of life from
alternative biospheres.
Over 400 planets have been found around nearby stars, but none of them is thought to be at all like Earth. The goal now is to identify rocky planets within the
habitable zones of their stars and to search their atmospheres spectroscopically for signs of life. To do this, we need new space-based telescopes such as
NASA's proposed Terrestrial Planet Finders or ESA's Darwin mission (all of which are indefinitely postponed at the moment). If spectra of extrasolar planet
atmospheres can be obtained, the presence of O2, which is produced from photosynthesis, or O3, which is produced photochemically from O2, would under most
circumstances provide strong evidence for life beyond Earth. But "false positives" for life may also exist, and these need to be clearly delineated in advance
of such missions, if at all possible. I will also contrast my optimism about the search for complex life with the more pessimistic view expressed by Ward and
Brownlee in their book, Rare Earth.
A low-latitude distribution of continents may be a prerequisite for the global glaciations that appear to have affected Earth at least twice
during the emergence of animals. However, a preponderance of low-latitude continents may also make Earth more susceptible to true polar
wander, the process by which the mantle and crust spin relative to the fluid outer core to maintain rotational equilibrium. I will make a
case for a pair of true polar wander events circa 800 million years ago that started a cascade of changes in the geochemical cycling between
continents and oceans, and led to global glaciation. Did these changes in climate and ocean geochemistry finally allow the radiation of
animals, or was it the first animals that modified geochemical cycling and climate?
The oxygenation of the Earth's surface, which occurred ca. 2.3 billion years ago, is the earliest unambiguous imprint of biology
on a planetary scale. Therefore, the search for molecular oxygen features prominently in the prospective search for life on extrasolar planets. However, the connections between
the evolution of life and the evolution of the oxygen content of the Earth's atmosphere remain unclear. This talk will present emerging new perspectives on the Earth's transition
from an anaerobic to an aerobic world.
A fundamental challenge for astrobiology is to establish the relative contributions of chance versus predictability in the origin and evolution of life on our own planet. Thus, for example,
all Earth-life creates metabolism from an interacting network of protein molecules that catalyze various biochemical reactions. Furthermore, early during evolution it had arrived at a standard
set of 20 amino acid building-blocks with which to build each of these proteins. We now have good reason to think that many of these amino acids are formed in significant quantities throughout
the galaxy - but so are many others - so would alien life be like us, and how could we possibly know?
What is the best strategy for finding signs of life beyond the Solar System? Until recent years this was a purely philosophical question, but today we
have the technical ability to search for signs of life on exoplanets around nearby stars, so the question is now a practical one. To start, we ask what kind of
signs of life should we be looking for, and where should we be looking? Next we might ask about the methods we could use for such a search, and the kinds of evidence
that we expect to obtain. Finally we can ask about the prospects for starting this search in the coming decade.
The advent of cryogenic space-borne infrared observatories such as the Spitzer Space
Telescope has lead to a revolution in the study of extrasolar planets and planetary systems. Already Spitzer has characterized the
emergent infrared spectra of close-in giant exoplanets that orbit sun-like stars, using transit and eclipse techniques. Transits offer
enormous advantages in characterizing the bulk properties (mass, radius) as well as the atmospheric composition of extrasolar planets.
However, the nearest transiting and habitable extrasolar planet almost certainly does not orbit a Sun-like star. It orbits an M-dwarf
star, and it could be a scant 10 parsecs distant from us, or even closer. After we find this unusual habitable world, we will
characterize it using transit techniques. Already the ground-based MEarth survey has found a hot superEarth (T = 500 Kelvins) orbiting
the M-dwarf star Gliese 1214, 10 parsecs from our own Sun. A space-based all-sky survey could extend the MEarth results to
habitable-zone planets. When we have found such a world, the James Webb Space Telescope will be able to measure its atmospheric
composition, and possibly even search for biosignatures.
For the past decade missions to Mars have "followed the water". In this talk I will argue that future missions should
begin directly searching for signs of life. The most important result from the recent Mars missions in this regard was
the discovery of perchlorate by the Phoenix lander. Perchlorate could form the basis of a biological redox system on
Mars. Furthermore, reanalysis of the Viking GCMS results suggests that perchlorate and organics may have been present at
the Viking sites. Ice-cemented ground beneath dry permafrost in the high elevations of the Antarctic Dry Valleys
provides a model for considering the search for signs of life at the Phoenix site.
Formation scenarios of the solar nebula invoke two main reservoirs of water ice that may have taken part concurrently
into the production of solids. In the first reservoir, which is located within the heliocentric distance of 30 AU,
water ice infalling from the Interstellar Medium (ISM) initially vaporized into the hot inner part of the disk and
condensed in its crystalline form during the cooling of the solar nebula (Kouchi et al. 1994; Chick & Cassen 1997).
The second reservoir, located at larger heliocentric distances, is composed of water ice originating from ISM that did
not suffer from vaporization when entering into the disk. In this reservoir, water ice remained mainly in its
amorphous form. From these considerations, we discuss here the trapping conditions of volatiles in planetesimals
produced within the outer solar nebula and their implications for the origin and composition of gas giant planets,
their surrounding satellite systems and comets. In particular, we show that the formation of icy planetesimals
agglomerated from clathrate hydrates in the solar nebula can explain in a consistent manner the volatiles enrichments
measured at Jupiter and Saturn, as well as the composition of Titan's atmosphere.
Iron and sulfur redox chemistry support chemoautotrophic subsurface microbial communities on Earth, and could
potentially sustain a biosphere on Mars. In this talk, I will describe a highly productive ecosystem in an
extreme natural environment that is supported by air, water, and iron sulfide minerals. Through integrating
cultivation-independent molecular ('omic', imaging, and other) methods with geochemical approaches, it has
been possible to begin to determine how these communities are structured and to unravel complex interdependencies,
spatial organization, and evolutionary pathways.
The terrestrial geologic record from actual rocks extends back to about 4.02 billion years ago (4.02 Ga). Before that
time, what we know of the environment of the earliest Earth's surface from the time of formation to the start of the
continuous rock record is constrained by inferences derived from chemical and isotopic studies of the oldest zircon
grains (Zr-orthosilicate minerals) as old as 4.38 Ga found in younger rocks, physics of stars and how planets form, and
molecular phylogeny. Silicate planets form hot, but cool on timescales shorter than the tectonic cycle. Bolide impacts
subsequently become important modifiers of surface environments, but after the planetary "re-set button" was hit by the
Moon-forming event, were more beneficial than deleterious to early forms of life. Surface temperatures were likely warm
enough to stabilize liquid water even with a fainter young Sun since ca. 4.4 Ga. The oldest meta-igneous rocks are
interesting in that aside from their antiquity, they are rather mundane mid-crustal lithotypes. Evidence from the
Hadean zircons points to extensive recycling of crust at underthrust zones (plate tectonics?), generation of granitoid
melts and of (widespread?) continental crust and liquid water. The oldest know meta-sediments (ca. 3.81-3.83 Ga)
preserve chemical and isotopic signatures consistent with (but not proof of) elemental Sulfur metabolism, N-fixation,
CO2-fixation, and photoferrotrophy. In sum, by the time the continuous rock record starts at ~3.7 Ga, all of the key
features of the habitable Earth were already in place. To place firmer constraints on the establishment of the
habitable Hadean, we need to find yet older rocks. I will provide an update on this quest.
NASA's Stardust spacecraft returned samples from comet 81P/Wild 2 to Earth in January 2006. Examinations of the
organic compounds in cometary samples can reveal information about the prebiotic organic inventory present on the
early Earth and within the early Solar System, which may have contributed to the origin of life. Preliminary
studies of Stardust material revealed the presence of a suite of organic compounds including several
amines and amino acids, but the origin of these compounds (cometary vs. terrestrial contamination) could not
be identified. We have recently measured the carbon isotopic ratios of these amino acids to determine
their origin, leading to the first detection of a cometary amino acid.
Phosphorus is a key element in biological systems, acting in cell replication as RNA and DNA, in cell structure as phospholipids, and in
metabolism as ATP. Given its ubiquity in biochemistry, phosphorus was likely present in the origin or early evolution of life. I will
discuss sources of phosphorus on the early earth, concentrating primarily on extraterrestrial sources of reduced oxidation state phosphorus
compounds, and evidence that these sources were used by early biochemical systems. Additionally, I will show how these reduced oxidation
state phosphorus compounds could act in prebiotic or early biochemical systems to generate both key biologic compounds and metabolic energy.