HABITABLE ZONES AROUND LOW MASS STARS AND THE SEARCH FOR EXTRATERRESTRIAL LIFE

Habitable planets are likely to exist around stars nottoo different from the Sun if current theories about terrestrialclimate evolution are correct. Some of these planets may have evolved life, and some of the inhabited planets may have evolved O₂-rich atmospheres. Such atmospheres could be detectedspectroscopically on planets around nearby stars using a space-based interferometer to search for the 9.6m band of O₃.Planets with O₂-rich atmospheres that lie within the habitable zone around their parent star are, in all probability, inhabited.     

Which Type of Planets do We Expect to Observe in the Habitable Zone?

We used a sample of super-Earth-like planets detected by the Doppler spectroscopy and transit techniques to explore the dependence of orbital parameters of the planets on the metallicity of their host stars. We confirm the previous results (although still based on small samples of planets) that super-Earths orbiting around metal-rich stars are not observed to be as distant from their host stars as we observe their metal-poor counterparts to be. The orbits of these super-Earths with metal-rich hosts usually do not reach into the Habitable Zone (HZ), keeping them very hot and inhabitable. We found that most of the known planets in the HZ are orbiting their GK-type hosts which are metal-poor. The metal-poor nature of planets in the HZ suggests a high Mg abundance relative to Si and high Si abundance relative to Fe. These results lead us to speculate that HZ planets might be more frequent in the ancient Galaxy and had compositions different from that of our Earth.     

Importance of Biologically Active Aurora-like Ultraviolet Emission: Stochastic Irradiation of Earth and Mars by Flares and Explosions

Habitable planets will be subject to intense sources of ionizing radiation and fast particles from a variety of sources--from the host star to distant explosions--on a variety of timescales. Monte Carlo calculations of high-energy irradiation suggest that the surfaces of terrestrial-like planets with thick atmospheres (column densities greater than about 100 g cm(-2)) are well protected from directly incident X-rays and gamma-rays, but we find that sizeable fractions of incident ionizing radiation from astrophysical sources can be redistributed to biologically and chemically important ultraviolet wavelengths, a significant fraction of which can reach the surface. This redistribution is mediated by secondary electrons, resulting from Compton scattering and X-ray photoabsorption, the energies of which are low enough to excite and ionize atmospheric molecules and atoms, resulting in a rich aurora-like spectrum. We calculate the fraction of energy redistributed into biologically and chemically important wavelength regions for spectra characteristic of stellar flares and supernovae using a Monte-Carlo transport code and then estimate the fraction of this energy that is transmitted from the atmospheric altitudes of redistribution to the surface for a few illustrative cases. For atmospheric models corresponding to the Archean Earth, we assume no significant ultraviolet absorbers, only Rayleigh scattering, and find that the fraction of incident ionizing radiation that is received at the surface in the form of redistributed ultraviolet in the biologically relevant 200-320 nm region (UV-C and UV-B bands) can be up to 4%. On the present-day Earth with its ultraviolet ozone shield, this fraction is found to be 0.2%. Both values are many orders of magnitude higher than the fraction of direct ionizing radiation reaching the surface. This result implies that planetary organisms will be subject to mutationally significant, if intermittent, fluences of UV-B and harder radiation even in the presence of a narrow-band ultraviolet shield like ozone. We also calculate the surficial transmitted fraction of ionizing radiation and redistributed ultraviolet radiation for two illustrative evolving Mars atmospheres whose initial surface pressures were 1 bar. We discuss the frequency with which redistributed ultraviolet flux from parent star flares exceeds the parent star ultraviolet flux at the planetary surface. We find that the redistributed ultraviolet from parent star flares is probably a fairly rare intermittent event for habitable zone planets orbiting solar-type stars except when they are young, but should completely dominate the direct steady ultraviolet radiation from the parent star for planets orbiting all stars less massive than about 0.5 solar masses. Our results suggest that coding organisms on such planets (and on the early Earth) may evolve very differently than on contemporary Earth, with diversity and evolutionary rate controlled by a stochastically varying mutation rate and frequent hypermutation episodes.     

Pathways to Earth-Like Atmospheres

We discuss the evolution of the atmosphere of early Earth and of terrestrial exoplanets which may be capable of sustaining liquid water oceans and continents where life may originate. The formation age of a terrestrial planet, its mass and size, as well as the lifetime in the EUV-saturated early phase of its host star play a significant role in its atmosphere evolution. We show that planets even in orbits within the habitable zone of their host stars might not lose nebular- or catastrophically outgassed initial protoatmospheres completely and could end up as water worlds with CO₂ and hydrogen- or oxygen-rich upper atmospheres. If an atmosphere of a terrestrial planet evolves to an N₂-rich atmosphere too early in its lifetime, the atmosphere may be lost. We show that the initial conditions set up by the formation of a terrestrial planet and by the evolution of the host star’s EUV and plasma environment are very important factors owing to which a planet may evolve to a habitable world. Finally we present a method for studying the discussed atmosphere evolution hypotheses by future UV transit observations of terrestrial exoplanets.     

Vacant habitats in the Universe

The search for life on other planets usually makes the assumption that where there is a habitat, it will contain life. On the present-day Earth, uninhabited habitats (or vacant habitats) are rare, but might occur, for example, in subsurface oils or impact craters that have been thermally sterilized in the past. Beyond Earth, vacant habitats might similarly exist on inhabited planets or on uninhabited planets, for example on a habitable planet where life never originated. The hypothesis that vacant habitats are abundant in the Universe is testable by studying other planets. In this review, I discuss how the study of vacant habitats might ultimately inform an understanding of how life has influenced geochemical conditions on Earth.     

ORIGIN OF THE BIOLOGICALLY IMPORTANT ELEMENTS

The chemical elements most widely distributed in terrestrial living creatures are the ones (apart from inert helium and neon) that are commonest in the Universe — hydrogen, oxygen, carbon, and nitrogen. A chemically different Universe would clearly have different biology, if any. We explore here the nuclear processes in stars, the early Universe, and elsewhere that have produced these common elements, and, while we are at it, also encounter the production of lithium, gold, uranium, and other elements of sociological, if not biological, importance. The relevant processes are, for the most part, well understood. Much less well understood is the overall history of chemical evolution of the Galaxy, from pure hydrogen and helium to the mix of elements we see today. One implication is that we cannot do a very good job of estimating how many stars and which ones might be orbited by habitable planets.     

A Scientometric Prediction of the Discovery of the First Potentially Habitable Planet with a Mass Similar to Earth

Background

The search for a habitable extrasolar planet has long interested scientists, but only recently have the tools become available to search for such planets. In the past decades, the number of known extrasolar planets has ballooned into the hundreds, and with it, the expectation that the discovery of the first Earth-like extrasolar planet is not far off.

Methodology/Principal Findings

Here, we develop a novel metric of habitability for discovered planets and use this to arrive at a prediction for when the first habitable planet will be discovered. Using a bootstrap analysis of currently discovered exoplanets, we predict the discovery of the first Earth-like planet to be announced in the first half of 2011, with the likeliest date being early May 2011.

Conclusions/Significance

Our predictions, using only the properties of previously discovered exoplanets, accord well with external estimates for the discovery of the first potentially habitable extrasolar planet and highlight the the usefulness of predictive scientometric techniques to understand the pace of scientific discovery in many fields.     

Planetary systems and extraterrestrial life

The paper revies the present status of the problem of the existence of other planetary systems in the Galaxy. Observational data and theoretical results are presented to show that the occurrence of planetary systems is, most probably, not a universal phenomenon. Study of the stability of planetary orbits in the vicinity of double stars indicates that, in general, planetary systems can not survive around them over long periods. Therefore, we should rule out the possibility of the existence of planetary systems similar to our own in the neighborhood of double stars. In the solar neighborhood, at least 60% of the stars are known to be members of double systems. The nature of the dark companions is discussed and it is concluded that they are stellar objects and not planets. Recent work on the absence of a perturbation in the motion of Barnard's star is discussed. Comments are made on the existence of extraterrestrial life in the solar system and around other stars in the Galaxy.     

The search for life on Earth and other planets

As the NASA rover Curiosity approaches Mars on its quest to look for signs of past or present life there and sophisticated instruments like the space telescopes Kepler and CoRoT keep discovering additional, more Earth-like planets orbiting distant stars, science faces the question of how to spot life on other planets. Even here on Earth biotopes remain to be discovered and explored.     

Maximum Number of Habitable Planets at the Time of Earth's Origin: New Hints for Panspermia?

New discoveries have fuelled the ongoing discussion of panspermia, i.e. the transport of life from one planet to another within the solar system (interplanetary panspermia) or even between different planetary systems (interstellar panspermia). The main factor for the probability of interstellar panspermia is the average density of stellar systems containing habitable planets. The combination of recent results for the formation rate of Earth-like planets with our estimations of extrasolar habitable zones allows us to determine the number of habitable planets in the Milky Way over cosmological time scales. We find that there was a maximum number of habitable planets around the time of Earth's origin. If at all, interstellar panspermia was most probable at that time and may have kick-started life on our planet.     

ANALOGS OF THE EARLY SOLAR SYSTEM

Within the last few decades, the existence of protoplanetary disks has been inferred on the basis of emission from T Tauri stars that does not arise from a stellar photosphere. More recently, high-resolution interferometric techniques have resolved the dust continuum emission, and millimeter arrays have imaged circumstellar molecular gas. These measurements corroborate the disk interpretation; many T Tauri stars are surrounded by centrifugally supported circumstellar disks with radial sizes of order 100 AU. Further proof issues from Hubble Space Telescope images of disks that are illuminated externally. The morphology of circumstellar dust is revealed in striking detail and affirms the prevalence and dimensions of disks imaged at longer wavelengths. The fate of circumstellar material around young stars must be understood in order to discern the degree to which these disks are proto-planetary. Observational studies of circumstellar disks which are in the beginning of a dispersal phase are challenging and place great demands on astronomical techniques. Nevertheless, the connection between disks and the formation of extra-solar planets is supported by increasing circumstantial evidence. Optically thin dust continuum emission persists in T Tauri stars and is detected around some young main sequence stars. Since the dust is subject to rapid dispersal by radiation pressure and Poynting-Robertson drag, some mechanism of replenishment is required. Disks around nearby young main sequence stars show evidence for inner voids and disk asymmetries that should also disappear on short timescales. The presence of large orbiting bodies which collide and interact with the resulting debris can explain both the persistence of optically thin dust and the maintenance of otherwise-ephemeral dynamical features. Together with recent detections of extra-solar planets, these observations lend some support to the hypothesis that circumstellar disks commonly give birth to planetary systems.     

Lichens, new and promising material from experiments in astrobiology

Many lichen species are regarded as extremophiles in terms of temperature, radiation and desiccation survival. Therefore, lichens have been previously proposed, together with unicellular algae and bacteria, as the living system most likely to resist the extreme conditions of outer space. This enables, following the “Panspermia” theory, speculation about the possibility of life transfer between Earth and other planets. Different experiments have been designed to establish the survival capability of these organisms exposed to space conditions. In particular, the damaging effect of solar UV was studied under various protecting conditions. Different lichen species were exposed to space in the BIOPAN-5 and BIOPAN-6 facilities of the European Space Agency located at the outer shell of the Russian Earth orbiting FOTON M2 satellite. Chlorophyll fluorescence and gas exchange systems were used for the measurement of photosynthetic parameters. All exposed lichens, independently of the filters used, showed after the flight nearly the same photosynthetic activity as measured before the flight. These findings suggest that lichens could stay alive in space even completely exposed to massive UV and cosmic radiation, which have been proved being lethal for bacteria and other microorganisms. Improvements and possible upgrading of the existing experiment designs are also explored in view of a future and more intensive use of lichens in Astrobiology.     

What Possible Life Forms Could Exist on Other Planets: A Historical Overview

Speculations on living beings existing on other planets are found in many written works since the Frenchman Bernard de Fontenelle spoke to the Marquise about the inhabitants of the solar system in his Entretiens sur la pluralité des mondes (1686). It was an entertainment used to teach astronomy more than real considerations about the habitability of our solar system, but it opened the way to some reflections about the possible life forms on other planets. The nineteenth century took up this idea again in a context of planetary studies showing the similarities as well as the differences of the celestial bodies orbiting our Sun. Astronomers attempted to look deeper into the problem of habitability such as Richard Proctor or Camille Flammarion, also well-known for their fine talent in popular writings. While the Martian canals controversy was reaching its height, they imagined how the living forms dwelling in other planets could be. Nowadays, no complex exo-life is expected to have evolved in our solar system. However, the famous exobiologist Carl Sagan and later other scientists, formulated audacious ideas about other forms of life in the light of recent discoveries in planetology. Through these few examples, this paper underlines the originality of each author’s suggestions and the evolution and contrast of ideas about the possible life forms in the universe.     

Thermodynamics of thermal radiation from stars photoautotrophs and biospheres

Photoautotrophs are almost the exclusive providers of chemical free energy to the Earth biosphere. Their importance in coadjuvating the evolutionary development of higher forms of life in other planets is briefly discussed from this point of view. A simple analysis based on the nonequilibrium thermodynamics of thermal radiation fields is performed. The analysis relates well known standard parameters of stars of the main sequence to the thermodynamic bounds on the free energy acquisition of planetary photosynthetic processes activated by the star radiation. Upper bounds to permissible wavelengths, active in photosynthesis easily follow. Simple inferences can then be made about the possible types of main sequence stars with planetary systems where exobiological photoautotrophs might have evolved. As red dwarves constitute both the great majority of companions to the Sun and the majority of main sequence stars in our Galaxy, emphasis is placed on discussing the case of planetary systems of stars with low photospheric temperatures. Bounds to the free energy intake per unit area by the biospheres of planets in planetary systems of late type main sequence stars are estimated and compared. Some simple conclusions are drawn.Special Symposium on Photochemistry and the Origins of Life, Sixth International Congress on Photobiology, Bochum, Germany.     

Space biomagnetics

Astronauts who venture from their spacecraft onto the lunar surface and the surfaces of our neighboring planets will be exposed for a few hours in duration to magnetic-field intensities which are markedly less than that of the earth's field. The intensities of magnetic fields to which they will be exposed while inside their spacecraft can be stated only after completing a detailed survey of the contribution made to these fields by the functioning electronic components of spacecraft. Assessment of individuals regularly working in and exposed continuously for 10 days to magnetic fields less than 100 gammas in intensity indicate that extremely low-intensity magnetic fields encountered during a nominal Apollo moon mission should not affect astronaut health or performance. Careful physiological and psychological observations first on higher primates, then on man exposed to such fields for more prolonged periods of time must be carried out before this conclusion can be drawn for longer exposures.Recent technological advances in propulsion and radiation protection have made it possible that astronauts might also be exposed intermittently to high-intensity, relatively low-gradient magnetic fields during space missions. The duration of such exposures could range from less than an hour if an activated magnetohydrodynamic engine must be serviced, to several days if pure magnetic or plasma-radiation shielding is used for astronaut protection from solar flare radiation. From past experience with personnel who enter high-intensity magnetic fields for brief periods of time in their work, magnetic-field exposures while servicing magnetohydrodynamic engines should not be hazardous to astronauts. On the other hand, past exposures of man and sub-human systems to high-intensity magnetic fields do not indicate whether or not astronauts who are exposed for up to several days to currently estimated magnetic-field intensities associated with pure magnetic or plasma-radiation shielding could suffer impairment of their health or performance. This answer can be obtained only by carefully conducted experiments which closely simulate such exposures, and look closely for physiological, psychological and pathological changes, especially in exposed higher primates, before assessing the response of man to such exposures.Magnetic force is animate or imitates life; and in many things surpasses human life, while this is bound up in the organick body.Prepared under Contract NASr-115 at The Lovelace Foundation for Medical Education and Research, Albuquerque, N.M., U.S.A.     

Carbonaceous Chondrite Meteorites: the Chronicle of a Potential Evolutionary Path between Stars and Life

The biogenic elements, H, C, N, O, P and S, have a long cosmic history, whose evolution can still be observed in diverse locales of the known universe, from interstellar clouds of gas and dust, to pre-stellar cores, nebulas, protoplanetary discs, planets and planetesimals. The best analytical window into this cosmochemical evolution as it neared Earth has been provided so far by the small bodies of the Solar System, some of which were not significantly altered by the high gravitational pressures and temperatures that accompanied the formation of larger planets and may carry a pristine record of early nebular chemistry. Asteroids have delivered such records, as their fragments reach the Earth frequently and become available for laboratory analyses. The Carbonaceous Chondrite meteorites (CC) are a group of such fragments with the further distinction of containing abundant organic materials with structures as diverse as kerogen-like macromolecules and simpler compounds with identical counterparts in Earth’s biosphere. All have revealed a lineage to cosmochemical synthetic regimes. Several CC show that asteroids underwent aqueous alteration of their minerals or rock metamorphism but may yet yield clues to the reactivity of organic compounds during parent-body processes, on asteroids as well as larger ocean worlds and planets. Whether the exogenous delivery by meteorites held an advantage in Earth’s molecular evolution remains an open question as many others regarding the origins of life are. Nonetheless, the natural samples of meteorites allow exploring the physical and chemical processes that might have led to a selected chemical pool amenable to the onset of life.

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Graphical Abstract ?

     

Aliens at home?

If we ponder how alien life might look like on other planets, we don''t have to go far, Simon Conway Morris argues, since life forms on Earth have already pushed life to the limits.When in 1609 Galileo first saw the moons of Jupiter, he must have been spellbound. I was certainly so enrapt when I saw Europa and her three companions strung like a line of jewels. Galileo may have appreciated the irony that my guide was a Jesuit priest, and the somewhat antiquated telescope we used was but a few yards from the Papal summer residence in Castel Gandolfo. Galileo prized open the door and before long, scientific imagination was fired by the prospect of innumerable inhabited worlds. As the centuries progressed, imagination raced ahead of facts, with the Moon optimistically colonized by Selenites, and Mars transformed by immense canals to supply the parched regions of a planet plunging into desertification. From this dying planet H.G. Wells propelled his aliens to terrorize southern England with immense tripods housing sinister octopoids.Now we might be closer to knowing if Wells was in any sense on the right track. The spectacular success in detecting extrasolar planets has produced a roster in excess of 450, and this technology potentially allows us to detect Earth-like planets. Even if many of the known planets are too large to be habitable and lie, for the most part, beyond the inferred ‘habitable zones'', before long we will get some clues as to how densely our galaxy is inhabited. The consensus points in two directions. First, life is a universal. Second, our biosphere will be of almost no use when it comes to comparisons. Let me draw your attention to a remarkably unappreciated fact: if you want to understand aliens, stay at home.Am I serious? After all it is already clear that extrasolar planetary systems are vastly different to our Solar System. Immense planets orbit their suns every few days, their surfaces far more torrid than that of Venus. Other planets most likely possess giant oceans, hundreds of kilometres deep. The diversity of moons and planets in our Solar System is a reminder of what may await us light years from Earth. Even among our neighbours, a case can be made for possible life in the clouds of Venus and Jupiter, the oceans of Europa and hydrocarbon lakes of Titan, and—with perennial optimism—in the permafrost of Mars. We might assume, therefore, that the range of environments available to life, its ‘habitation box'', is gigantic, and that Earth''s biosphere just nestles in one tiny corner. Oddly enough the evidence is exactly the opposite. Life on Earth has reached the limits of what is possible—anywhere.Temperature? The current limit on Earth is 122 °C. Plunging in the opposite direction the evidence is just as remarkable. At temperatures well below freezing, life carries on cheerfully. Even far beyond the eutectic, in which free water cannot form, organisms remain in a state of suspended animation with rates of damage and repair almost precisely matched. What of extreme desiccation? Evidently life has reached the limits of water activity. Entertainingly some of the hardiest forms are fungi that inhabit the weird alien world of Blue Stilton cheese. So, too, the bright colouration of salt pans is a familiar sight, and these osmotic extremes not only host rich microbial faunas but life that can flourish in the most bitter of brines. What of the extremes of pH—bleach versus battery acid? Once again, alkaliphiles and acidophiles disport themselves in ponds and streams that would have the Health and Safety officers in a state of panic. Pressure, either crushingly high or extremely attenuated? Life, of course, exists in the deepest oceanic trenches, but how much deeper might be viable? The weakest link seems to be the pressure sensitivity of the phospholipid membranes, suggesting that even on planets with titanic oceans life won''t survive much deeper than in the Mariana Trench. The same argument applies to the deep crust: at about 5 km the crushingly high pressures also coincide with the thermal limits imposed by the geothermal gradient. Shall we look to the skies? Clouds carry bacteria, but even at quite modest heights it seems to be accidental freight rather than a nebulous ecosystem.Terrestrial life has conquered nearly all of the ‘habitation box'' and its evolution begs so many questions. Are some forms, such as the hyperthermophiles, survivors from the Earth''s apocalyptic beginnings? Maybe, but most have clearly been reinvented several times. Getting to the limits of life isn''t that difficult, but how do extremophiles not only survive but flourish in these environments? Often the adaptations seem minor, which merely means they are more subtle than we might realize. What of the future? So far as the Earth is concerned it must cope with ever increasing solar luminosity: the last men will long predecease the last microbe. Possibly long before, we will engage in the first great galactic diaspora; but wherever our biologists journey they may find that life ‘out there'' got no further than the blue jewel that is Earth.     

Ontogeny of Spectral Transmission in the Eye of the Tropical Damselfish, Dascyllus albisella (Pomacentridae), and Possible Effects on UV Vision

The visual ecology of fishes places changing demands on their visual system during development. Study of changes in the eye can suggest possible changes in behavioral ecology. The spectral transmission of the pre-retinal ocular media controls the wavelength of light that reaches the retina and is a simply measured indication of their potential visual capabilities. Dascyllus albisella is a coral reef planktivore known to have UV-sensitive retinal cone cells. UV vision probably aids in detection of zooplankton. As a juvenile it is very closely associated with branching coral heads or, more rarely, sea anemones. As it matures, it ventures farther from its coral, above the reef, and eventually assumes a more vagile life style, moving farther and more frequently afield. Their eyes contain short-wavelength blocking compounds in the lens, cornea and humors. As they age, both the lens and the cornea accumulate blocking compounds that increase the 50% transmission cutoff of the whole eye from ca. 330nm in 2–3cm juveniles to ca. 360nm in the largest adults. The cornea increases its cutoff wavelength faster than the lens and becomes the primary filter in large adults. The cutoff of the aqueous and vitreous humors combined does not change with size. The slope of the transmission cutoff curve increases with the size of the fish. The increased blocking of UV radiation is likely not an adaptation to protect the eye from short-wavelength induced damage. Instead it probably reduces the image degradation effects of short-wavelength light in the largest eyes and still allows sufficient penetration of UV radiation to permit functional UV vision.     

Fitness in the universe: Choices and necessities

We live in a universe of chance, but not of accident. Repeatedly in the course of its development choices have been made for which one can ask the reasons. One such choice is fundamental: if the proton had not so much greater mass than the electron, all matter would be fluid; and if the proton did not have exactly the same numerical charge as the electron — or some simple multiple of that charge — virtually all matter would be charged. If a universe were started with charged hydrogen, it could expand, but probably nothing more. Hydrogen, carbon, nitrogen and oxygen play as fundamental — and irreplaceable — roles in the metabolism of stars as of living organisms. Both metabolisms are coupled, through radiation from the stars providing the energy on which life must come ultimately to run on the planets. In the course of their evolution on the Earth, living organisms have found their way repeatedly and exclusively to certain types of organic molecule to perform specific functions; so, for example, the chlorophylls for photosynthesis, and carotenoids for plant phototropism and for vision. It is argued that some measure of necessity has governed these choices; and that an extended principle of natural selection has operated at all levels of material organization to produce such elements of order and compatibility in the universe.     

Bit by bit: the Darwinian basis of life

All known examples of life belong to the same biology, but there is increasing enthusiasm among astronomers, astrobiologists, and synthetic biologists that other forms of life may soon be discovered or synthesized. This enthusiasm should be tempered by the fact that the probability for life to originate is not known. As a guiding principle in parsing potential examples of alternative life, one should ask: How many heritable “bits” of information are involved, and where did they come from? A genetic system that contains more bits than the number that were required to initiate its operation might reasonably be considered a new form of life.Thanks to a combination of ground- and space-based astronomical observations, the number of confirmed extrasolar planets will soon exceed 1,000. An increasing number of these will be said to lie within the “habitable zone” and even be pronounced as “Earth-like.” Within a decade there will be observational data regarding the atmospheric composition of some of those planets, and just maybe those data will indicate something funny going on—something well outside the state of chemical equilibrium—on a potentially hospitable planet. Perhaps our astronomy colleagues should be forgiven for their enthusiasm in declaring that humanity is on the brink of discovering alien life.But haven''t we heard this before? Didn''t President Clinton announce in 1996 that a Martian meteorite recovered in Antarctica [1] “speaks of the possibility of life” on Mars? (No, it turned out to be mineralic artifacts.) Wasn''t some “alien” arsenic-based life discovered recently in Mono Lake, California [2]? (No, it''s a familiar proteobacterium struggling to survive in a toxic environment.) Didn''t Craig Venter and his colleagues recently create a synthetic bacterial cell [3], “the first self-replicating species we''ve had on the planet whose parent is a computer”? (No, its parent is Mycoplasma mycoides and its genome was dutifully reconstructed through DNA synthesis and PCR amplification.)Why are we so confused (or so lonely) that we have such trouble distinguishing life from non-life and distinguishing our biology from another? A key limitation is that we know of only one life form, causing us to regard life from that singular perspective (Figure 1). We see life as cellular, with a nucleic acid genome that is translated to a protein machinery. Life self-reproduces, transmits heritable information to its progeny, and undergoes Darwinian evolution based on natural selection. Life captures high-energy starting materials and converts them to lower-energy products to drive metabolic processes. Life exists on at least one temperate, rocky planet, where it has persisted for about four billion years. There are likely to be tens of thousands of “habitable” planets within a thousand light years of Earth, and more than a billion such planets in our galaxy, so surely (say the astronomers) we are not alone.Open in a separate windowFigure 1Phylogenetic tree of life based on small-subunit ribosomal RNA sequences, showing representative species from each of the three kingdoms (compiled by Pace [11]).The root of the tree is indicated by a horizontal line. The locations on the tree of Halomonas sp. (GFAJ-1) [2] and Mycoplasma mycoides (JCVI-syn1.0) [3] are indicated by black circles adjacent to Escherichia and Bacillus, respectively.