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Home , Extraterrestrial life, Tholin

Copyright 1994 , Inc. The Search for Extraterrestrial The remains the only inhabited world known so far, but scientists are finding that the abounds with the chemistry of life

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n the past few decades the human the compounds it can form; liquid wa- RNA), which constitute the hereditary species has begun, seriously and ter provides a superb, stable medium in instructions, and the proteins, which, as I systematically, to look for evidence which organic can dissolve enzymes, catalytically control the chem- of life elsewhere. While no one has yet and interact. What is more, organic mol- istry of cell and organism. The code- found living organisms beyond the ecules are surprisingly common in the book for translating nucleic acid infor- earth, there are some reasons to be en- universe. Þnd evidence for mation into protein structure is essen- couraged. Robotic space probes have them everywhere, from interstellar gas tially identical for all life on the earth. identiÞed worlds where life may once and dust grains to to many This profound uniformity in the hered- have gained a toehold, even if it does worlds in the outer . itary chemistry suggests that every or- not ßourish there today. The Galileo Some other moleculesÑhydrogen ganism on our has evolved from spacecraft found clear signs of life dur- ßuoride, for exampleÑmight approach a common instance of the origin of life. ing its recent ßight past the earthÑa in their ability to dissolve other If so, we have no way of knowing which reassurance that we really do know molecules, but the cosmic abundance of aspects of terrestrial life are necessary how to sniÝ out at least certain kinds ßuorine is extremely low. Certain atoms, (required of all living things anywhere) of life. And rapidly accumulating evi- such as silicon, might be able to take and which are merely contingent (the dence strongly suggests that the uni- on some of the roles of in an al- results of a particular sequence of hap- verse abounds with planetary systems ternative life chemistry, but the variety penstances that, had they gone other- something like our own. of information-bearing molecules they wise, might have led to organisms hav- In practice, the community of scien- provide seems comparatively sparse. ing very diÝerent properties). We may tists concerned with Þnding life else- Furthermore, the silicon equivalent of speculate, but only by examining life where in the solar system has contented (silicon dioxide, the ma- elsewhere can biologists truly deter- itself with a chemical approach. Human jor component of ordinary glass) is, on mine what else is possible. beings, as well as every other organism all planetary surfaces, a solid, not a gas. The obvious place to start the search on the earth, are based on liquid water That distinction would certainly com- for life is in our own solar system. Ro- and organic molecules. (Organic mole- plicate the development of a silicon- bot spacecraft have explored more than cules are carbon-containing compounds based metabolism. 70 , satellites, and aster- other than carbon dioxide and carbon On extremely cold worlds, where wa- monoxide.) A modest search strategyÑ ter is frozen solid, some other solventÑ looking for necessary if not suÛcient liquid , for instanceÑmight be CARL SAGAN is noted as a researcher, criteriaÑmight then begin by looking a key to a diÝerent form of biochemis- lecturer and author. He received his Ph.D. for liquid water and organic molecules. try. At low temperatures, certain class- in and from the Of course, such a protocol might miss es of molecules require very little acti- in 1960. In 1968 forms of life about which we are whol- vation energy to undergo chemical re- he joined Cornell University, where he is ly ignorant, but that does not mean we actions, but because our laboratories David Duncan Professor of Astronomy could not detect them by other meth- are at room temperature and not, say, and Space and director of the ods. If a silicon-based giraÝe had walked at the temperature of NeptuneÕs satel- Laboratory for Planetary Studies. He has participated in numerous National Aero- by the Viking landers, its portrait lite , our knowledge of those mol- nautics and Space Administration plane- would have been taken. ecules may well be inadequate. For the tary missions, among them Viking, Voy- Actually, focusing on moment, though, carbon- and water- ager and Galileo. His research interests and liquid water is not nearly so paro- based life-forms are the only kinds we embrace the origin of life, the physics chial and chauvinistic as it might seem. know or can even imagine. and chemistry of planetary atmospheres No other comes close On the earth the signature molecules and surfaces, and the search for extra- to carbon in the variety and intricacy of of life are the nucleic acids (DNA and terrestrial . His books include ; Contact, a Þctional account of a meeting between humans and an alien ; and the recently published SATURNÕS (at the upper right in this Voyager 2 composite image) is Pale Blue Dot: A Vision of the Human Fu- an intriguing natural laboratory for prebiotic chemistry. Titan has a thick atmo- ture in Space. sphere in which complex organic solids form and rain down onto the surface.

Copyright 1994 Scientific American, Inc. SCIENTIFIC AMERICAN October 1994 93 oids at distances varying from about it also permitted the spacecraft to make tons of visible light than of . 100 to about 100,000 kilometers. These close-up observations of our planet. But photons of visible light are too fee- ships have been equipped with magne- GalileoÕs instruments were not designed ble to sever the H-OH bond in water. If tometers, charged-particle detectors, for an earth-encounter mission, so cir- there were a way to combine two visi- imaging systems, and photometric and cumstance fortuitously arranged a con- ble light photons to break apart the spectrometric instruments that sense trol experiment: a search for life on the water , then everything would radiation ranging from ultraviolet to ki- earth using a typical modern planetary have a ready solution. Yet so far as we lometer-wavelength radio. For the moon, probe. The results of GalileoÕs December know, there is no way to accomplish and Mars, observations from or- 1990 encounter with the earth proved this featÑexcept through life, speciÞc- biters and landers have conÞrmed and quite enlightening. ally through photosynthesis in plants. expanded on Þndings transmitted back The prevalence of molecular in from ßyby spacecraft. n observer looking at the data from the earthÕs atmosphere is our Þrst clue None of these encounters has yielded Galileo would immediately notice that the planet bears life. compelling, or even strongly suggestive, A some unusual facts about the When Galileo photographed the earth, indications of . Still, earth. When my co-workers and I exam- it found unmistakable evidence of a such life, if it exists, might be quite un- ined spectra taken by Galileo at near-in- sharp absorption band painting the like the forms with which we are famil- frared wavelengths (just slightly longer continents: some substance was soak- iar, or it might be present only margin- than red light), we noted a strong dip ing up radiation at wavelengths around ally. Or the remote-sensing techniques in brightness at 0.76 micron, a wave- 0.7 micron (the far red end of the visible used for examining other worlds might length at which molecular oxygen ab- spectrum). No known show be insensitive to the conceivably subtle sorbs radiation. The prominence of the such a feature, and it is found nowhere signs of life on another world. The most absorption feature implies an enor- else in the solar system. The mystery elementary test of these techniquesÑ mous abundance of molecular oxygen substance is in fact just the kind of the detection of life on the earth by an in the earthÕs atmosphere, many orders light-absorbing pigment we would ex- instrumented ßyby spacecraftÑhad, un- of magnitude greater than is found on pect if visible photons were being add- til recently, never been attempted. The any other planet in the solar system. ed together to break down water and National Aeronautics and Space Ad- Oxygen slowly combines with the generate molecular oxygen. Galileo de- ministrationÕs Galileo has rectiÞed that rocks on the earthÕs surface, so the oxy- tected this pigmentÑwhich we know omission. gen-rich atmosphere requires a replen- as chlorophyllÑcovering most of the Galileo is a dual-purpose spacecraft ishing mechanism. Some oxygen is land area of the earth. (Plants appear that incorporates a orbiter and freed when ultraviolet light from the green precisely because chlorophyll re- entry probe; it is currently in interplan- splits apart molecules of water (H2O), ßects green light but traps the red and etary space and is scheduled to reach and the low-mass atoms pref- blue.) The prevalence of the chlorophyll the Jupiter system in December 1995. erentially escape into space. But the red band oÝers a second reason to think For technical reasons, NASA was unable great concentration of oxygen (20 per- that the earth is an inhabited planet. to send Galileo on a direct course to Ju- cent) in the earthÕs dense atmosphere GalileoÕs infrared spectrometer also piter; instead the mission incorporated is very hard to explain by this process. detected a trace amount, about one part three gravitational assistsÑtwo from If visible light, rather than ultraviolet, per million, of . Although that the earth and one from VenusÑto send could split water molecules, the abun- might seem insigniÞcant, it is in - it on its journey. This looping course dance of oxygen could be understood, tling disequilibrium with all that oxy- greatly lengthened the time, but because the sun emits many more pho- gen. In the earthÕs atmosphere, methane

What Is Life?

he search for extraterrestrial life must begin with the Biochemical definitions—for example, defining life in Tquestion of what we mean by life. “I’ll know it when I terms of nucleic acids, proteins and other molecules—are see it” is an insufficient answer. Some functional defini- clearly chauvinistic. Would we declare an organism that tions are inadequate: one might identify life as anything can do everything a bacterium can dead if it was made of that ingests, metabolizes and excretes, but this descrip- very different molecules? The definition that I like best— tion applies to my car or to a candle flame. Some more so- life is any system capable of reproduction, mutation and phisticated definitions—for example, life as recognizable reproduction of its mutations—is impractical to apply by its departure from thermodynamic equilibrium—fall when we set down a spacecraft on another world: repro- afoul of the circumstance that much of (such as duction may not be done in public, and mutations might lightning and the layer) is out of equilibrium. be comparatively infrequent.

94 SCIENTIFIC AMERICAN October 1994 Copyright 1994 Scientific American, Inc. rapidly oxidizes into water and carbon dioxide. At thermodynamic equilibrium, calculations indicate that not a single molecule of methane should remain. Some unusual processes (which we know to include bacterial metabolism in bogs, rumina and termites) must steadi- ly refresh the methane supply. The pro- found methane disequilibrium is a third sign of life on the earth. Finally, GalileoÕs plasma-wave instru- ment picked up narrow-band, pulsed, amplitude-modulated radio emissions coming from the earth. These signals begin at the frequency at which radio transmissions on the earthÕs surface are Þrst able to leak through the ionosphere; they look nothing like natural sources of radio waves, such as lightning and the earthÕs magnetosphere. Such unusu- al, orderly radio signals strongly suggest the presence of a technological civiliza- tion. This is a fourth sign of life and the only one that would not have been ap- parent to a similar spacecraft ßying by the earth anytime within the past two billion years. The Galileo mission served as a signif- icant control experiment of the ability of remote-sensing spacecraft to detect life at various stages of evolutionary de- velopment on other worlds in the solar system. These positive results encour- LIFE ON THE EARTH betrays its presence in images and measurements made by age us that we would be able to spot the Galileo spacecraft. This false-color infrared image reveals a mysterious red-ab- the telltale signature of life on other sorbing pigment (chlorophyll, which appears orange-brown here) painting the con- worlds. Given that we have found no tinents. No such pigment is seen anywhere else in the solar system. Spectra indi- such evidence, we tentatively conclude cate that the earthÕs atmosphere is unusually rich in molecular oxygen and that widespread biological activity now methane. Galileo has boosted scientistsÕ conÞdence that they may be able to spot exists, among all the bodies of the so- the telltale signs of life even if it is diÝerent from life on the earth. lar system, only on the earth.

ars is the nearest planet whose er, to see if there were life-forms in the ticism is that an additional experiment surface we can see. It has an at- soil that ate the food and oxi- to look for organic molecules in the M mosphere, polar ice caps, sea- dized it, giving oÝ radioactive carbon Martian soil gave uniformly negative sonal changes and a 24-hour day. To dioxide. A third experiment exposed the results, even though the instruments generations of scientists, writers and the Martian soil to radioactive carbon diox- could detect such molecules at a sensi- public at large, Mars seemed the world ide and to determine tivity of around one part per billion. most likely to sustain extraterrestrial if any of it was taken up by microbes. We expected that any life on MarsÑas life. But ßybys past and orbiters around To the initial astonishment of, I think, with life on the earthÑwould be an ex- Mars have found no excess of molecular all the scientists involved, each of the pression of the chemistry of carbon- oxygen, no substancesÑwhatever their three Viking experiments gave what at based molecules. To Þnd no such mol- natureÑenigmatically and profoundly Þrst seemed to be positive results. Gas- ecules at all was daunting for the opti- departing from thermodynamic equilib- es were exchanged; organic matter was mists among the exobiologists. rium, no unexpected surface pigments oxidized; carbon dioxide was incorpo- The apparent positive results of the and no modulated radio emissions. In rated into the soil. life-detection experiments on the two 1976 NASA set down two Viking landers But there are reasons that these Viking landers is now generally attribut- on Mars. I was an experimenter on that provocative results are not generally ed to chemicals that oxidize the soil. mission. The landers were equipped thought to provide a convincing argu- These chemicals form when solar ultra- with instruments sensitive enough to ment for . The putative violet light irradiates the Martian atmo- detect life even in unpromising deserts metabolic processes of Martian mi- sphere. A handful of Viking scientists and wastelands on the earth. crobes occurred under a wide range of still wonder whether extremely tough One experiment measured the gases conditions: wet and dry, light and dark, and resilient organisms might exist, so exchanged between Martian surface cold (only a little above freezing) and thinly spread over the Martian soil that samples and the local atmosphere in hot (almost the normal boiling point of their organic chemistry could not be the presence of organic nutrients car- water). Many microbiologists deem it detected but their metabolic processes ried from the earth. A second experi- unlikely that Martian microbes would could. Those scientists do not deny the ment brought a wide variety of organic be so capable under such varied condi- presence of ultraviolet-generated oxi- foodstuÝs marked by a radioactive trac- tions. Another strong reason for skep- dants, but they emphasize that nobody

Copyright 1994 Scientific American, Inc. SCIENTIFIC AMERICAN October 1994 95 LANDER scooped up bits of ized the earth, if life had already arisen. had melted the permafrost. Most plan- Martian soil and tested them for the Things did not calm down until about etary scientists agree that searching for presence of life and organic molecules. 4.0 billion years ago. And yet re- chemical or morphological fossils of Despite tantalizing initial results, the Vik- veal that by 3.6 billion years ago the ancient life should have high priority in ing experiments suggest that Mars is, at earth abounded with microbial life (in- future Martian exploration. Although it present, a dead world. Future missions cluding large, basketball-size stromato- is a long shot, searching for life in con- may search for fossils of organisms that lites, colonies of ). These temporary Martian oases might also be might have lived billions of years ago, early forms of life seem to have been a productive endeavor. when Mars was warmer and wetter. biochemically very adept. Many were It is now clear that organic chemistry photosynthetic, slowly contributing to has run rampant through the solar sys- the earthÕs bizarre oxygen-rich atmo- tem and beyond. Mars has two small has yet been able to explain fully the sphere. Manfred Schidlowski of the Max satellites, Phobos and Deimos, which, Viking life-detection results on the ba- Planck Institute for Chemistry in Mainz because of their dark color, seem to be sis of oxidants alone. A few researchers has studied carbon isotope ratios pre- made of (or at least covered by) organic have made tentative claims of Þnding served in ancient rocks; that work pro- matter. They are widely thought to be organic matter in a class of meteorites vided (disputed) evidence that life was captured from farther out in (the SNC meteorites) that are thought already ßourishing 3.8 billion years ago. the solar system. Indeed, there seems to be bits of the Martian surface blast- The inferred time available for the to be a vast population of small worlds ed into space during ancient impacts. origin of life on the earth is thus being covered with organic matter : the so- More likely, the organic material con- squeezed from two directions. Accord- called C- and D-type asteroids in the sists of contaminants that entered the ing to current knowledge, that amount main belt between Jupiter and after its arrival on our world. of time may be as brief as 100 million Mars; the nuclei of comets such as Hal- So far there are no claims of discover- years. When I Þrst drew attention to this leyÕs ; and the newly discovered ing Martian microbes in these rocks ÒsqueezeÓÑin 1973, after lunar samples class of asteroids near the outermost from the sky. returned by Apollo clariÞed the chron- planets. In 1986 the European Space For the moment, it is safe to say that ology of impacts on the moonÑI ar- AgencyÕs Giotto spacecraft ßew directly Viking found no compelling case for life gued that the rapidity with which life into the cloud of dust surrounding Hal- on Mars. No unambiguous signatures arose on the earth may imply that it is leyÕs Comet, revealing that its nucleus of life emerged from four very diÝer- a likely process. It is dangerous to ex- may be made of as much as 25 percent ent, extremely sensitive experiments trapolate from a single example, but it organic matter. conducted at two sites 5,000 kilome- would be a truly remarkable circum- A fairly abundant type of meteorite ters apart on a planet where fast winds stance if life arose quickly here while on the earth, known as carbonaceous transport Þne particles around the on many other, similar worlds, given chondrite, is thought to consist of frag- globe. The Viking Þndings suggest that comparable time, it did not. ments from C-type asteroids in the Mars is, today at least, a lifeless planet. Between 4.0 and 3.8 billion years ago, main belt. Carbonaceous meteorites conditions on Mars, too, may have fa- contain an organic residue rich in aro- ould Mars have supported life in vored the emergence of life. The surface matic and other . Scien- the distant past? The answer de- of Mars is covered with evidence of an- tists have also identiÞed a number of Cpends very much on how quick- cient rivers, lakes and perhaps even amino acids (the building blocks of the ly life can arise, a topic about which we more than 100 meters deep. The proteins) and nucleotide bases (the remain sadly ignorant. Astronomers are Mars of 4.0 billion years ago was much ÒrungsÓ of the DNA double helix, which quite certain that, initially, the earth was warmer and wetter than it is today. Tak- spell out the genetic code). inhospitable to life because of the col- en together, these pieces of information Asteroidal and cometary fragments lisions of planetesimals, the planetary suggest, although they hardly prove, plunging into the atmosphere of the building blocks that accreted together that life may have arisen on ancient carried with them vast stores to form the earth. Early on, the earth Mars as it did on the ancient earth. If of organic molecules. Some of these sur- was covered by a deep layer of molten so, as Mars evolved from congenial to vived the intense heating on entry and rock. After that magma froze, the occa- desolate, life would have held on in the therefore may have made a signiÞcant sional arrival of large planetesimals last remaining refugiaÑperhaps saline material contribution to the origin of would have boiled the oceans and steril- lakes or places where the interior heat life. Impacts would have delivered sim-

96 SCIENTIFIC AMERICAN October 1994 Copyright 1994 Scientific American, Inc. ilar supplies of organic matter, along we can judge from its density (much the optical constants of Titan , with water, to other worlds. Those lower than that of solid rock) and from we Þnd that it beautifully matches the worlds need not be as richly endowed the composition of nearby worlds, Titan optical constants derived from obser- with liquid water as is the earth for crit- should have a great deal of water ice on vations of the Titan haze. No other pro- ical steps in prebiological chemistry to and near its surface. A few simple or- posed material comes close. occur. The water could be found in ganic moleculesÑhydrocarbons and ni- Organic molecules continually form ponds, in subsurface reservoirs, as thin trilesÑare found to be minor constitu- in the upper and Þlms on grains or as ice melts ents of TitanÕs atmosphere. slowly fall out as new are gen- formed by impacts. Ultraviolet light from the sun, charged erated in the upper air. If this process particles trapped in SaturnÕs magneto- has continued over the past four billion ne of the most fascinating and sphere and cosmic rays all bombard Ti- years, TitanÕs surface must be covered instructive worlds illustrating tanÕs atmosphere and initiate chemical by tens, maybe even hundreds, of me- O prebiological organic chemistry reactions there. When W. Reid Thomp- ters of tholin and other organic prod- is SaturnÕs giant moon, Titan (which is son of Cornell University and I consid- ucts. Moreover, Thompson and I have as large as the planet Mercury). Here we ered the eÝects of ultraviolet irradiation calculated that over the history of the can see the synthesis of complex organ- and simulated those of auroral elec- solar system, a typical location on Ti- ic molecules happening before our eyes. tron bombardment, we found the re- tan has something like a 50Ð50 chance Titan has an atmosphere 10 times as sults agree well with the observed abun- of having experienced centuries of liq- massive as the earthÕs, composed main- dances of gaseous organic constituents. uid water from the heat released by im- ly of molecular , along with a My colleague Bishun N. Khare and I pacts. When we mix Titan tholin with few percent to 10 percent methane. at Cornell simulated the pressure and water in the laboratory, we make ami- When Voyager 2 approached Titan in composition of the appropriate levels no acids. There are also traces of nucle- 1981, it could not see the surface, be- in TitanÕs atmosphere and irradiated the otide bases, polycyclic aromatic hydro- cause this world is entirely socked in by gases with charged particles. The exper- and a wonderful brew of oth- an opaque, reddish orange haze. The iment produced a dark, organic solid er compounds. If 100 million years is surface temperature is very low, about that we call Titan tholin, from the Greek enough for the origin of life on the 94 kelvins, or Ð179 degrees Celsius. If word for Òmuddy.Ó When we measure earth, could 1,000 years be enough for

TITAN THOLIN 1 POLY-HCN

10Ð1

10Ð2 POLY-C H TITAN HAZE 2 2 (MODEL OF OBSERVATIONS) (IMAGINARY PART) 10Ð3 COMPLEX REFRACTIVE INDEX 10Ð4 0.01 0.11.0 10 100 1,000 WAVELENGTH (MICRONS)

LABORATORY SIMULATION of TitanÕs nitrogen-methane at- of Titan tholin closely match those of TitanÕs haze (bottom mosphere (left ) yields a tarlike accumulation of complex right ). When combined with liquid water, Titan tholin pro- organic molecules, which the author calls Titan tholin. Analo- duces amino acids, nucleotide bases and other molecules im- gous chemical reactions may give rise to the haze that ob- portant to terrestrial life. Such molecules might have formed scures TitanÕs surface (top right ). The optical characteristics in temporary lakes created by cometary impacts on Titan.

Copyright 1994 Scientific American, Inc. SCIENTIFIC AMERICAN October 1994 97 it on Titan? Could life have started on Does Intelligent Life Exist on Other Worlds? Titan during the centuries following an impact, when lakes of water or water- he search for extraterrestrial intelligence is an attempt to use large radio ice slurries brießy formed? The Þrst Ttelescopes, sophisticated receivers and modern data analysis to detect close-up examination of TitanÑby a hypothetical signals sent our way by advanced on planets around orbiter and Titan entry probeÑ other . Necessarily, there are great uncertainties in selecting the appro- is scheduled to occur when the ESA- priate wavelength, band pass, polarization, time constant and decoding al- NASA Cassini mission reaches the Sat- gorithm with which to search for those signals. Nevertheless, radio technolo- urn system in about 2004. gy is inexpensive, likely to be discovered early in the of a techno- When we look beyond our solar sys- logical civilization, readily detectable (not just over interplanetary distances, tem, into the gas and grains that popu- as Galileo has done, but over vast interstellar distances) and capable of trans- late interstellar space, again we Þnd mitting enormous amounts of information. The first large-scale, systematic striking signs of the prevalence of or- search program, covering a significant fraction of the wavelengths thought ganic chemistry. Astronomers examin- optimal for interstellar communication, was initiated by the National Aero- ing microwaves emitted and absorbed nautics and Space Administration on October 12, 1992. Congress canceled by molecules at distinctive frequencies the program a year later, but it will soon be resuscitated using private mon- have identiÞed more than four dozen ey. Meanwhile some smaller efforts have made provocative findings. simple organic compounds in interstel- One promising project is the Megachannel Extraterrestrial Array (META), lar spaceÑhydrocarbons, , alco- which is led by Paul Horowitz, a physics professor at Harvard University, and hols and , some of them having funded mainly by , the largest space interest group in long, straight carbon chains, such as the world. The antenna used for META appears below. After five years of HC11N. When a cloud of interstellar dust continuous sky survey and two years of follow-up, Horowitz and I found a grains lies between the earth and some handful of candidate radio signals that have extremely narrow bandwidths, more distant infrared source, it is pos- that do not seem to share the earth’s rotation and that cannot be attributed sible to determine which infrared wave- to specific sources of noise or interference. The only trouble is that none of lengths are absorbed by the grains and these sources repeats, and in science nonrepeating data are usually not hence to learn about their composition. worth much. The tantalizing aspect of the META findings is that the five Some of the missing infrared light is strongest signals all lie in the plane of the . The likelihood that this widely presumed to have been absorbed alignment happens by chance is something like 0.5 percent. We think more by polycyclic aromatics, complex hy- comprehensive searches are worth doing. drocarbons similar to the compounds found in coal tar. In the part of the in- frared spectrum near 3.4 microns, three distinct absorption features are seen. The same patterns appear in the spec- tra of comets, in tholins made from the irradiation of ices and in meteoritic organic matter. That infra- red Þngerprint is probably caused by linked (aliphatic) groups of carbon and hydrogen: ÐCH3 and ÐCH2. Yvonne Pen- dleton and her colleagues at the NASA Þnd that the best spectral Þt seems to be with meteoritic organic matter.

he infrared match among com- ets, asteroids and interstellar T clouds may represent the Þrst di- rect evidence that asteroids and comets contain organic matter that originated on interstellar grains before gathering together in the infant solar system. But the data are also amenable to an oppo- site interpretationÑthat some of the organic matter that formed in the early solar nebula accumulated into asteroids and comets, while some was ejected by the sun into interstellar space. If 100 bil- lion other stars did likewise, they could account for a signiÞcant fraction of the organic matter in all the interstellar grains in the . The prevalence of organic material in the outer solar sys- tem, in comets that come from far be- yond the outermost planets and in in- terstellar gas and grains strongly sug- gests that complex organic matterÑ

98 SCIENTIFIC AMERICAN October 1994 Copyright 1994 Scientific American, Inc. 0 0.10 0.20 0.30 0.40 0.50 0.60 0.70 0.80 0.90 1.0

A B C

PSR B1257 + 12

SUN MERCURY VENUS EARTH

0 0.10 0.20 0.30 0.40 0.50 0.60 0.70 0.80 0.90 1.0 DISTANCE FROM STAR (EARTH TO SUN = 1)

EXTRASOLAR PLANETS seem to orbit the star PSR B1257+12, sizes of the planets are not drawn to scale). It is possible that the tiny, dense remnant of an ancient supernova explosion. a more distant, habitable planet circles this stellar corpse. The spacing of the three surrounding planetsÑknown as A, B There is also increasing evidence that planetary systems or- and CÑresembles the circumstances of our solar system (the bit many sunlike stars, which oÝer better prospects for life.

relevant to the origin of lifeÑis widely at Pennsylvania State, unambiguously decades astronomers will begin accu- spread throughout the Milky Way. detected earthlike planets in a place mulating a sizable inventory of planets Organic molecules on bone-dry inter- where most astronomers least expect- around nearby stars. stellar grains fried by ultraviolet light ed to Þnd them: around a , the We have every reason to believe that and cosmic rays seem an unlikely habi- swiftly spinning neutron-star remnant there are many water-rich worlds some- tat for the origin of life, however. Life from a supernova explosion. Based on thing like our own, each provided with seems to need liquid water, which in variations in the timing of radio emis- a generous complement of complex or- turn seems to require planets. Astro- sions from the pulsar PSR B1257+12, ganic molecules. Those planets that cir- nomical observations increasingly indi- Wolszczan has deduced the presence cle sunlike stars could oÝer environ- cate that planetary systems are com- of three planets (so far called only A, B ments in which life would have billions mon. A surprisingly large number of and C) orbiting the pulsar. of years to arise and evolve. Should not nearby young stars of roughly solar These worlds are closer to their star there be an immense number and di- mass are surrounded by just the kind than the earth is to ours, and PSR versity of inhabited worlds in the Milky of disks of gas and dust that scientists B1257+12 emits in charged particles Way? Scientists diÝer about the strength going back to and Pi- several times as much energy as does of this argument, but even at its best it erre Simon, the Marquis de Laplace, say the sun in electromagnetic radiation. If is very diÝerent from actually detecting is needed to explain the origin of the all the charged particles intercepted by life elsewhere. That monumental dis- planets in our system. These disks pro- A, B and C are transformed into heat, covery remains to be made. vide a persuasive though still indirect these worlds must almost certainly be indication that there is a multitude of too hot for life. But Wolszczan Þnds planets, presumably including earthlike hints of at least one additional planet FURTHER READING worlds, around other stars. situated farther from the pulsar. For all MARS. Edited by H. H. KieÝer, B. M. Ja- George W. Wetherill of the Carnegie we know, this superÞcially unpromising kosky, C. Snyder and M. S. Matthews. Institution of Washington has devel- system, 1,400 light-years from the earth, Press, 1992. oped detailed models for predicting the may contain a dark but habitable plan- TITAN: A LABORATORY FOR PREBIOLOGI- distribution of the planets that should et. It is not clear whether these planets CAL ORGANIC CHEMISTRY. Carl Sagan, W. Reid Thompson and Bishun N. Khare be formed in such circumstellar disks. survived from before the supernova ex- in Accounts of Chemical Research, Vol. Meanwhile James F. Kasting of Pennsyl- plosion or, more likely, formed after- 25, No. 7, pages 286Ð292; July 1992. vania State University has calculated ward from surrounding debris. Either FIVE YEARS OF PROJECT META: AN ALL- the range of distances from their way, their presence suggests that plan- SKY NARROW-BAND RADIO SEARCH FOR at which planets can support liquid wa- etary formation is an unexpectedly com- EXTRATERRESTRIAL SIGNALS. Paul Horo- ter on their surfaces. Taken together, mon and widespread process. witz and Carl Sagan in Astrophysical these two lines of inquiry suggest that Numerous searches for planets in in- Journal, Vol. 415, No. 1, pages 218Ð233; a typical should con- fant and mature sunlike systems are September 20, 1993. A SEARCH FOR LIFE ON EARTH FROM THE tain one and maybe even two earthlike under way. The pace of exploration is GALILEO SPACECRAFT. Carl Sagan et al. planets circling at a distance where liq- becoming so quick, and so many new in Nature, Vol. 365, No. 6448, pages uid water is possible. techniques are about to be employed, 715Ð721; October 21, 1993. Recently Alexander Wolszczan, also that it seems likely that in the next few

Copyright 1994 Scientific American, Inc. SCIENTIFIC AMERICAN October 1994 99