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...

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...

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,...

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...

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....

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...

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...

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...

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...

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...

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...

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...

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...

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. ...

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...

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. ...

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...

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...

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...

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...

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...

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...

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...

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...

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...