27. Life in the Universe

Transcript: Carl Sagan was the first astronomer to attain widespread acclaim in the era of TV and the popular culture. He spent most of his career at the University of Cornell where he was a respected planetary scientist and editor of the main journal of planetary science, Icarus. He was involved in a number of NASA planetary missions including the Voyager and Pioneer probes. In the 1970s he developed, wrote, and starred in the TV series and associated books, Cosmos. Cosmos was perhaps the most influential TV show on science before or since. After that, he wrote the book Contact and was involved in the making of the movie by the same name which was also influential in bringing ideas about life in the universe to a wider public. In a shameful incident late in his career, Sagan was denied membership of the National Academy of Science, partly because of jealousy of the scientists and the idea that he demeaned himself by popularizing science. In fact, Sagan is a hero to most scientists for bringing the ideas of astronomy to so many millions of people.

Transcript: SETI, the Search for Extraterrestrial Intelligence, is a small subfield of astronomy. It doesn’t get most of the astronomy funding, and some astronomers even disapprove of it. SETI seeks to set aside speculation about intelligent life in the universe and conduct the experiment, to actually look. As the physicist Philip Morrison said, “If we do look, the odds of success are difficult to evaluate, but if we don’t look, the odds are zero.” SETI must make strong assumptions about the nature of intelligence, technology, and communication, but it's a very exciting prospect, to do an experiment that could lead to the answer of whether we have companionship in the universe, of whether the extraordinary experiment of biology that took place on this planet and lead to humans is unique or not.

Transcript: The very limited evidence that we have is consistent with the supposition that microbial life is quite common in the universe whereas intelligent life is quite rare. Evidence in favor of microbial life being potentially common is the fact that carbon, nitrogen, and oxygen, the essential life elements, are created readily in stars throughout the Milky Way and beyond, the fact that planets are found to exist around solar stars, even if Earth-like planets have not yet been found, the fact that life on Earth formed early in the history of this planet, that it is robust, and that microbial life has diversified into many evolutionary niches. Arguing against intelligent life being common is the fact that it happened relatively late in the evolution of life on Earth, after four billion years of evolution, the fact that only a handful of species under the most generous definition have evolved intelligence out of hundreds of millions in the history of Earth, and the fact that intelligent life forms like us are fragile, and that the idea of intelligence and communication may be a cultural tendency rather than a biological directive.

Transcript: We live in the Milky Way galaxy, a spiral. The universe has many galaxies, and there are different types: spirals, ellipticals, and irregulars. Is life equally likely in each of these vast types of stellar system? Possibly not. Spiral galaxies have an active history of star formation and have been creating heavy elements steadily for billions and billions of years, since about a billion years after the big bang. So there is plenty of carbon for chemistry and biology to develop. Elliptical galaxies form early and many of them have a small abundance of heavy elements which may mean they are less likely habitats for planets and life. These are of course statistical statements. For example, if life could develop around one in a million stars in the Milky Way, but only one in a billion stars in an elliptical galaxy, there would still be a hundred life forms in the elliptical galaxy as compared to ten thousand in the Milky Way. A much smaller number but not at all insignificant.

Transcript: Most searches for planets and life in the universe are being conducted in the solar neighborhood, but it’s worth asking the question of how likely life is in other environments within the Milky Way. It’s probably no accident that the Sun, the Earth, and life exist in the spiral arm of the Milky Way. It's a region of active star formation where there’s been a history of heavy element creation. There may be other high density environments where life is actually difficult to find. In the galactic center, star densities are probably high enough that planets are ejected from orbits around their stars. Also, in regions of intense star formation, the supernova rate may be high enough to eradicate life on planets around nearby stars. By contrast, in the halo of the Milky Way galaxy, the metal abundance is so low that there may not be enough carbon, nitrogen, and oxygen for planets and life to readily form.

Transcript: Since many stars in the Milky Way are in multiple star systems, that is they contain more than two stars in mutual orbits, it’s worth asking the question what’s the prospect of life around such stellar systems? The stability of planet orbits depends very much on the rate of close encounters of the stars. In situations of high stellar density such as globular clusters or the dense cores of open star clusters, the interaction rate is such that planets could probably not exist in stable orbits. They would either be perturbed into highly elliptical orbits, which would imply no stable environment in which life could develop, or they would be ejected completely. In general, it looks like the dense stellar environments are not conducive to stable planets and orbits. In fact, there are active searches underway in globular clusters for the free floating planets that astronomers expect to exist because they have been ejected by their parent stars.

Transcript: The Sun is not like most stars in one important way. The Sun is a single isolated star with its own planetary system. The majority of stars in the Milky Way galaxy are in binary or multiple systems. Astronomers have done dynamical experiments with computers to decide whether planet orbits could be stable in a binary star system. In general, there are two regimes where the orbits may be stable. One is the case of a tight binary star system where the planet orbit is at a distance that is ten or more times the distance between the two stars. This orbit is stable, but a tight binary stellar system usually has mass transfer which may lead to rapid evolution in one or both of the stars, leading to an unlikelihood that life could evolve. Another stable situation is a wide binary star system where the planet orbits close to a star such that the planet-star distance is about ten or more times less than the distance between the stars. In this case the planet orbit does not feel the second star and is stable. Situations in between, where the distance between the planet and its parent star is similar to the distance between the two stars, are almost always unstable and lead to the ejection of the planet in a very short time.

Transcript: It’s difficult to evaluate the possibility of life existing in a star after the main sequence stage. For the most massive stars, the death of the star is violent as a supernova which would almost certainly obliterate life on any planet that held it at the time of the star’s death. The stellar remnants are dark, neutron stars or black holes, with little prospect that they could lead to life. Lower mass stars like the Sun go through a giant phase. This enhancement of the envelope and heating up will probably also obliterate life in the terrestrial planet zone, but then the star becomes a white dwarf, not totally dark but with a habitable zone very small and very close to the star. All very low mass stars end this way as well, as white dwarfs. The universe is filled with white dwarfs, and these cooling embers of stars could potentially harbor life, perhaps by tidal heating of planets in elliptical orbits. But we have no way of evaluating the prospect.

Transcript: For Sun-like stars in the main sequence that are either more or less massive than the Sun, the prospect of life on planets around those stars is a trade off between the size of the habitable zone, and the number of planets it might contain, and the lifetime of the star. The highest mass main sequence stars, O and B stars, respectively a million and a thousand times the luminosity of the Sun, have lifetimes that are about one million and fifty million years. Far too little, we think, for complex life to develop before the stars go supernova. A stars and F stars, forming one and two percent of all main sequence stars respectively with twenty and seven times the luminosity of the Sun, live for a billion years in the case of A stars and two billion years in the case of F stars. Even for F stars, two billion years would only correspond to the time that it took to get multicelled organisms on Earth; then the star would die. So it seems that lower mass stars are the best possibilities. K stars, fifteen percent of all main sequence stars and a third the luminosity of the Sun, have main sequence lifetimes of twenty billion years, and the ubiquitous M stars, seventy-five percent of all main sequence stars with only 0.3 percent of the luminosity of the Sun, have main sequence lifetimes of hundreds of billions of years. However, their habitable zones are incredibly small, so the possibility of a planet existing at the right distance from such low luminosity stars is also small.

Transcript: Astronomers think that a long term stable environment is necessary for complex life to develop. A Sun-like star seems a good place. The Sun has a main sequence lifetime of ten billion years, so even if it takes life a good fraction of a billion years to develop, the star has plenty of time on the main sequence for life to evolve and become complex. We are now only halfway through the Sun’s main sequence lifetime, and life has already evolved intelligence and technology. Just imagine what another few billion years of evolution could do, if we can survive. So the Sun-like stars are the first place to look. But they form part of a sequence of stars with a range of masses and a range of properties, and astronomers are beginning to consider the possibilities of life on other stars in the main sequence.

Transcript: What is the long term role of life in the universe? In a sense, the universe seems like it was built for life. Carbon is produced readily in stars, and stars, with their energy and planets around them, appear to be ubiquitous not only in the Milky Way galaxy but probably in all the hundreds of billions of galaxies beyond the Milky Way. The longer the universe lives, the more carbon, nitrogen, and oxygen, essential life elements, are produced in the centers of stars and ejected into interstellar space where they can become part of the next generation of planets and stars. In this sense, it becomes more likely as the universe evolves for life to exist. Yet the long term future of the universe is cold death as the universe expands and things begin to cool and separate. Stars will eventually go out within all the galaxies and turn into dense stellar remnants. With a lack of energy in general available in a galaxy many billions of years from now, it’s hard to imagine how life might exist. Individual civilizations may circumvent the death of their own star, but can life in the universe circumvent the eventual death of stars in all galaxies? Nobody knows the answer to this question.

Transcript: The conventional assumption about life in the universe is that it exists on a terrestrial planet around a Sun-like star in the habitable zone, the region of distance where liquid water can exist on a planet’s surface. but we know within our own solar system that the habitable zone must extend to include the moons of the giant planets. In the larger scales of the universe, there may be many more suitable habitats for life if all it requires is an energy source and thermal disequilibrium. Low mass stars and brown dwarfs are far more common than Sun-like stars. Free floating planets might even exist in space, and this forms a huge set of potential sites for life. We have no idea whether planets form readily around brown dwarfs. The first planets ever detected were actually around a pulsar, a dead husk of a star that had gone supernova and left a compact core. We don't know if planets could hold life around a pulsar because there is no energy source. In the wilder speculations of theorists, it’s even surmised that life could potentially exist on the surface of a pulsar itself, working under the extreme gravity and perhaps with a vastly accelerated evolutionary clock. This is pure speculation, but with the subject of life in the universe we reach the edge of the scientific method. Induction cannot be implied when we do not understand how strange life might be elsewhere in the universe.

Transcript: We can use the idea of remote sensing of terrestrial planets in our own solar system to get an idea of what features we might look for in other planets around other stars. If we looked at the atmosphere of Venus with an infrared spectrum, we would see the strong absorption from carbon dioxide at fifteen microns and a more subtle absorption feature at eleven or twelve microns from sulfuric acid in the atmosphere. If we looked at Mars, we’d see the strong signature of its primary ingredient, carbon dioxide, in absorption at fifteen microns. If we looked at the Earth, we would see three interesting things. Carbon dioxide tracer would be there and also a strong edge due to water at about five or six microns. There would also be a deep absorption trough at about nine microns due to ozone. Ozone, a byproduct of oxygen, is a non-equilibrium gas and in the view of the Earth’s atmosphere from afar would be the strongest indication of life on this planet.

Transcript: If we could do spectroscopy of the atmospheres of extrasolar, Earth-like planets, we would look for the special tracers of non-equilibrium gases or materials that are associated with life and its processes. Primary among these is oxygen. Oxygen is highly reactive, and so when it exists out of equilibrium it almost always indicates a life process. The spectral features of oxygen are peaks at 0.7, 1.3 microns, and its associated ozone with a strong absorption at eight microns. Oxygen is a byproduct of photosynthesis in both plants and bacteria. Carbon dioxide has spectral features at 0.2, 4.3, and fifteen microns. Carbon dioxide is a byproduct of a number of biological processes including respiration. We would also look for methane, the spectral features at 3.3 and 7.7 microns, methane also a byproduct of metabolic processes. Water, an essential ingredient for life, has spectral features at 0.8, 1.0, 1.4, and 1.9 microns. Water we believe is the essential solvent for life and is a byproduct of respiration. Finally, we can look for the chlorophyll edge. Chlorophyll, ubiquitous ingredient of plants, its spectral edge gives plants their green coloration at 0.4 microns. Any or all of these spectral signatures might be seen in the dim light reflected by a planet around another star.

Transcript: Astronomers have successfully detected large extrasolar planets, and within a short period of time they will be able to actually make images of such planets. The next step is to detect lower mass planets extending down to terrestrial planets, places that we believe are hospitable habitats for life. Looking ten or twenty years ahead, there is the prospect for remote sensing on Earth-like extrasolar planets. This would involve taking the light of an extrasolar Earth, which is of course only reflected light from its parent star, dispersing it into a spectrum, and looking for spectral features that might indicate the atmospheric chemistry or the presence of life. This is an extraordinarily ambitious technique. Remember that the Sun outshines Jupiter by a billion and the Earth by a factor of ten billion, so we would be taking that fraction of the light and trying to disperse it into a spectrum. The experiment improves if conducted in the infrared, and there are many interesting molecular and chemical tracers in the infrared. Almost certainly these experiments will have to be done from space which is the only place where the suitably sharp images can be obtained.

Transcript: Each technique that is currently used to successfully detect extrasolar planets with a mass of Jupiter or larger could eventually and potentially be used to detect terrestrial planets or Earth-like objects. The direct detection technique is very difficult for Earths. The Sun outshines Jupiter by a factor of a billion, but the Earth by a factor of ten billion. The way to improve this experiment is to move into the infrared where the contrast improves by a factor of a thousand. A transit experiment can also be used. In an edge-on orbit, Jupiter would dim the Sun by one percent for one day every twelve years, and Earth, being ten times smaller, would dim the Sun by a hundred times less or only 0.01 percent, a tiny effect. The Doppler effect that has been used successfully to detect most extrasolar planets discovered so far requires extraordinary sensitivity if it’s used to detect Earths. The Sun pivots about its edge caused mostly by Jupiter, and so the detection of Jupiter requires a velocity precision of thirteen meters per second. Detecting an Earth with this technique requires a precision of 0.09 meters per second. Finally, the gravitational lensing technique, where brief magnification of a background star is caused by an intervening planet, can be used quite well to detect Earth-mass objects as well as Jupiter-mass objects. All of these techniques have an interesting prospect in the next ten or twenty years to succeed in detecting Earths. Probably they will have to be experiments done from space.

Transcript: Virtually every extrasolar planet found so far, and there are over a hundred, is an object like Jupiter or Saturn. These gas rich planets with giant atmospheres probably have conditions in their interiors that are utterly inhospitable for life. This fact is significant because the techniques used to find the extrasolar planets could have found objects ten times less massive than Jupiter and orbits considerably larger than the Jupiter orbit, and yet they have not found such systems. This raises the possibility that terrestrial planets or Earth-like planets might be unusual or rare. There is no way for us to know for sure, but it’s lead to a hypothesis called the Rare Earth Hypothesis. For example, in the known extrasolar planet systems, the presence of a giant planet so close to a star would act to eject smaller planets by the gravitational sling shot mechanism. So terrestrial planets or Earth-like planets could not exist in these systems except as moons. Until we find larger samples of extrasolar planets, we will not know how rare Earths might be.

Transcript: If we believe that life needs a planet as a site to form, then the discovery of extrasolar planets is very exciting because it shows that planets form naturally as a byproduct of star formation. Over a hundred extrasolar planets have been found. Most of them, however, are nothing like terrestrial planets in our solar system. They are almost all like Jupiter and Saturn. However, they are much closer to their stars than Jupiter and Saturn. Half of the extrasolar planets known are less than a half of an astronomical unit from their stars, similar to the distance of Mercury and much smaller than the distances to Jupiter and Saturn in our solar system. They also travel on highly elliptical orbits which mean that the temperatures will vary substantially over the orbit. Such hot and giant planets must have migrated inward from larger distances. This evolution, plus the extreme physical conditions, plus the heavy, dense atmospheres and difficult conditions on rocky cores if they exist mean that astronomers are very pessimistic that life could exist on the extrasolar planets that have been found so far.

Transcript: We are uncertain enough about the range of possible life processes elsewhere in the universe that we should be liberal-minded about the possibility of life far beyond the traditional habitable zone even in the solar system. Could life exist on interstellar space beyond the orbit of the most distant planets? These regions are cold and have extremely low density, yet radio telescopes have shown us that in interstellar space and even more in dense molecular clouds there are many types of molecules, over a hundred and twenty species, some involving as many as fourteen or fifteen atoms. We also know that comets, the denizens of the outer solar system spending most of their time at tens of thousands of astronomical units from the Sun, are like dirty snowballs which contain substantial amounts not only of ices but of organic materials. Meteorites, visitors from the outer solar system that reach the Earth, have been found with a total of seventy-four amino acids, a number of fatty acids, and all five of the DNA linking bases. All of this information from cold bodies in the outer parts of the solar system leads us to believe that organic material can survive in cold spaces, but that is a far step from replicating molecules and life.

Transcript: We are uncertain enough about the range of possible life processes elsewhere in the universe that we should be liberal-minded about the possibility of life far beyond the traditional habitable zone even in the solar system. Could life exist on interstellar space beyond the orbit of the most distant planets? These regions are cold and have extremely low density, yet radio telescopes have shown us that in interstellar space and even more in dense molecular clouds there are many types of molecules, over a hundred and twenty species, some involving as many as fourteen or fifteen atoms. We also know that comets, the denizens of the outer solar system spending most of their time at tens of thousands of astronomical units from the Sun, are like dirty snowballs which contain substantial amounts not only of ices but of organic materials. Meteorites, visitors from the outer solar system that reach the Earth, have been found with a total of seventy-four amino acids, a number of fatty acids, and all five of the DNA linking bases. All of this information from cold bodies in the outer parts of the solar system leads us to believe that organic material can survive in cold spaces, but that is a far step from replicating molecules and life.

Transcript: The damage we are causing to our planet, plus the knowledge that Venus and Mars may have been hospitable for life in the distant past, has lead to the idea of terraforming. Terraforming is the idea of transforming a planet so that life or even humans could survive. It’s an enormously ambitious undertaking, and we’ve only begun to decide the issues in principle, not in practice. In the case of Mars, the idea would be to add enormous numbers of microbes that generate carbon dioxide or water and steadily but slowly raise the temperature and pressure such that it went above zero degrees. At that point Mars would enter the habitable zone, and the gradual build up of the atmosphere could eventually make it habitable for humans. This would take a very long time and a huge amount of money. On Venus the issue is different because Venus is intolerably hot with its dense atmosphere of carbon dioxide, and inhospitable. So in this case we are seeking to lower the temperature to below a hundred degrees C. The way to do this would be with an enormous Sun shade or microbes that consume carbon dioxide. These technologies are fantastically speculative, but people have begun to see that they could be possible. The time that it would take to terraform a major terrestrial planet would be millions of years. This is no quick fix to our problem, and in any case there are moral and ethical implications of transforming a planet.

Transcript: Life itself, the atmosphere, the oceans, and the land form a complex interdependent system on the Earth. Although the Earth is chemically and biologically complex, it is not itself alive. There is a hypothesis called the Gaia hypothesis, named after an ancient goddess, that says that the entire ecosystem of the Earth acts like a living organism, but there’s no good scientific evidence for this. However, the interdependence of life on Earth is substantial and will affect our ability to survive on this planet. We can think of the metaphor of Spaceship Earth. All of our nutrients and our survivable conditions depend on maintaining the ecosystem of this planet which we have already altered with toxins, carbon dioxide release, and global warming. It's a sobering prospect, but the durability of life depends on the size and complexity of the organisms. Microbial life forms can form and survive in extreme conditions and are the most durable forms of life we know. The larger organisms on this planet, including ourselves, are much more fragile. It is clearly cheaper for us to survive on this planet than to move off Earth. Space travel is extraordinarily expensive. A simple manned mission to Mars will probably cost several hundred billion dollars, and the cost of a spaceship that could travel to the stars is beyond the resources of any country or even all countries on Earth.

Transcript: The extremely long term future of life is a sobering prospect. It’s nothing to lose sleep over. All of these effects take place over billions of years. As the Sun evolves and converts its hydrogen into helium, it will warm and warm and increase in its radiation until the Earth’s surface is too hot for liquid water to exist. At the end of its life, the Sun will become a red giant, and its compression and high core temperature will be accompanied by a vast, expanding envelope of gas that will sweep past and blast the Earth, extending to the orbit of Mars. Normal life as we know it could possibly not survive this event. At sometime even further in the future, the core of the Sun will become a white dwarf, and at that point the radiation produced is far too little to support life on any planet in the solar system. But, given the way things are going, we may be altering our planet to make life difficult for us and for other species long before any of these astrophysical events happen. Already there is evidence that our planet is becoming hotter and more toxic then we can handle. The eventual prospects for life on Earth probably do not involve biological adaptation to these new conditions but probably involve moving underground or creating entirely sealed ecosystems on the surface of the Earth. The other possibility is of course to move off Earth, either to form space colonies or to travel to nearby stars and find a better place to live.

Transcript: The traditionally defined habitable zone, the distance from the Sun within which liquid water can exist, extends from 0.8 to 1.7 AU. This range encompasses the Earth and Mars only within the solar system. However, our knowledge of the extreme possibilities of life surviving on Earth, extremophiles, and our detailed knowledge of environments elsewhere in the solar system leads us to believe that the true habitable zone could be much larger. For example, the conditions on Titan and Europa are possible to allow them to have life, and yet they are much further from the Sun. Titan is a moon of Saturn at ten astronomical units, and Europa is a moon of Jupiter at about five astronomical units. So the habitable zone could extend much further to the regime of giant planets and their moons. The key here is the energy source provided by tidal heating. Moons in orbit around large planets at close distances can have an extra heat source due to tidal heating which makes up for the shortfall in solar radiation. Thus the most generous bound on a habitable zone could include a significant number of objects in planetary systems around other stars.

Transcript: The traditional habitable zone for a star is defined in the terms of water remaining as a liquid, under the strong assumption that liquid water is required for life. Remember that the habitable zone depends enormously on the luminosity of a star, and the inverse square law determines what the radiation at any distance from a star is. The inner bound of the habitable zone in our solar system is 0.8 AU. Inside that distance from the Sun, the surface temperature on a planet would be too high for liquid water to exist. The water would boil. This distance is midway between the orbits of Venus and the Earth. The outer bound of the habitable zone is 1.7 astronomical units. This is slightly outside the orbit of Mars at 1.5 AU. Beyond this distance, water would be frozen. But these ranges can be modified because in inner regions atmospheres of certain compositions can act to shelter the water, and at distances beyond the edge of the formal habitable zone, greenhouse gases could possibly raise the temperature beyond that of energy incident from the Sun, allowing liquid water to exist.

Transcript: An important place to study prebiotic chemistry and perhaps to detect evidence of life itself is Titan. Titan is a major moon of Saturn, larger in size than Earth’s Moon or Pluto. Titan has a thick atmosphere of pressure one and a half bars, composed primarily of nitrogen, ninety percent, and small amounts of methane, ethane, and argon. At this distance from the Sun the temperature is low, minus a hundred and eighty degrees centigrade or minus two hundred and ninety degrees Fahrenheit. The surface is made of a mixture of rock and ice, where the ice is composed of water, methane, and ammonia. There is good evidence that Titan has liquid oceans composed primarily of ethane and methane, and it may have deep underground oceans made of ammonia and water.

Transcript: An important place to study prebiotic chemistry and perhaps to detect evidence of life itself is Titan. Titan is a major moon of Saturn, larger in size than Earth’s Moon or Pluto. Titan has a thick atmosphere of pressure one and a half bars, composed primarily of nitrogen, ninety percent, and small amounts of methane, ethane, and argon. At this distance from the Sun the temperature is low, minus a hundred and eighty degrees centigrade or minus two hundred and ninety degrees Fahrenheit. The surface is made of a mixture of rock and ice, where the ice is composed of water, methane, and ammonia. There is good evidence that Titan has liquid oceans composed primarily of ethane and methane, and it may have deep underground oceans made of ammonia and water.

Transcript: Scientists are very uncertain what the probability is of life on or in Europa. The Galileo probe first mapped out the fissure network of surface ice that covers a liquid water layer. We only have rough estimates or models of what the thickness of the ice and water layers are. But it’s likely that the ice layer is ten kilometers thick, and the liquid layer could be as much as a hundred kilometers thick which would mean that Europa has as much liquid water on it as the sum of all the oceans of water on Earth. The surface of Europa is geologically young and is perhaps been kept active by geological activity within the small moon. The water is kept liquid, and the geological activity may be spurred by tidal heating by the large planet Jupiter which is nearby, the same mechanism that produces volcanoes on Io. The key requirement for life this distance from the Sun would be an energy source. Photosynthesis may be possible on the surface layers of the ice, but deep within the ice there’s very little energy. The best prospect is if geological activity activates deep sea vents which can foster life forms the way they do in the oceans of the Earth.

Transcript: One of the most interesting places to search for life in the solar system will be Europa, one of the Galilean moons of Jupiter. Europa is about the size of Earth’s Moon, and it has a thin atmosphere where oxygen is a major constituent. Europa has a fractured water-ice surface with good evidence for liquid water oceans below the ice. The ice crust has been partially melted due to tidal heating from the planet Jupiter. The surface temperature of Europa is very cold given its distance from the Sun, minus one hundred and fifty degrees centigrade or minus two hundred and forty degrees Fahrenheit.