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27. Life in the Universe

27. Life in the Universe

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Pan Spermia

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.

1min

28 Jul 2011

Rank #1

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Life and Main Sequence Stars

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.

1min

28 Jul 2011

Rank #2

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Future of Life in the Universe

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.

1min

28 Jul 2011

Rank #3

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Remote Sensing Earths

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.

1min

28 Jul 2011

Rank #4

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Atmosphere of Terrestrial Planets

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.

1min

28 Jul 2011

Rank #5

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Detecting 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.

1min

28 Jul 2011

Rank #6

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Terraforming

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.

1min

28 Jul 2011

Rank #7

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Ecosystems

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.

1min

28 Jul 2011

Rank #8

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Habitable Zone

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.

1min

28 Jul 2011

Rank #9

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Evidence of Life on Titan

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.

1min

28 Jul 2011

Rank #10