Carl Sagan

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.

SETI

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.

Intelligent Life in the Universe

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.

Life and Galaxies

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.

Life and the Galactic Environment

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.

Life and Multiple 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.

Life and Binary Stars

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.

Life and Stars Beyond Main Sequence

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.

Life and Main Sequence Stars

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.

Life and Sun-Like Stars

27. Life in the Universe

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.

Pan Spermia

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.

Future of Life in the Universe

Remote

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.

Remote Sensing Earths

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.

Atmosphere of Terrestrial Planets

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.

Detecting Terrestrial Planets

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.

Terraforming

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.

Ecosystems

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.

Habitable Zone

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.

Evidence of Life on Titan

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