Astronomy 141 - Life in the Universe - Autumn Quarter 2009

Course finale and summary. We look back over where we've been the last eleven weeks, and bring together all of the main themes of this course on Life in the Universe. Recorded live on 2009 Dec 4 in Room 1005 Smith Laboratory on the Columbus campus of The Ohio State University.

How will life, the Universe, and everything end? This lecture looks at the evolution of our expanding Universe to project the prospects for life into the distant cosmological future. Recent observations show that we live in an infinite, accelerating universe. I will trace the evolution of the universe from the current age of stars into the future. The final state of the Universe will be cold, dark, and disordered, and ultimately inhospitable to life as we understand it or perhaps can imagine it. Recorded live on 2009 Dec 3 in Room 1005 Smith Laboratory on the Columbus campus of The Ohio State University.

What is the future of life on Earth and in our Solar System? The Sun is the source of energy for life on the Earth, but it will not shine forever. This lecture looks at the impact of the various stages of the evolution of the Sun on the habitability of the Solar System, with particular emphasis on the continued habitability of the Earth. I will refer to state-of-the-art computer models of the Sun to get is properties at various stages in its past and future life. NOTE: Due to a recorder malfunction this lecture was re-recorded later in the day on 2009 Dec 2, rather than being live from the class room in Smith Laboratory. As such, it is about 10 minutes longer than usual (my pacing is off when not in front of class).

What does extraterrestrial life look like? This lecture explores current thinking about what extraterrestrial life might be like not by guessing their appearances, but instead applying lessons learned from our growing understanding of how evolution and biochemistry work on Earth. I will discuss Universal versus Parochial characteristics, Convergent Evolution, Radical Diversity, and other ideas from evolutionary biology that might inform how life might emerge on other worlds. We will then look at alternatives to carbon biochemistry, specifically the possibility of silicon-based life, and alternatives to liquid water as a solvent medium for biochemistry, specifically the possible role of Ammonia. Finally I will give one example of a highly speculative idea about life without chemistry. In the end, the outcome of such studies may not be to tell us much about extraterrestrials as to help focus questions on how we ourselves arose. Recorded live on 2009 Dec 1 in Room 1005 Smith Laboratory on the Columbus campus of The Ohio State University.

So, Where is Everybody? Interstellar colonization, in principle, is an exponential growth process that would fill the galaxy in a few million years even with a very modest star flight capability. This is a small fraction of the lifetime of the Milky Way Galaxy, so the Galaxy should be teaming with life. But, we so far have no compelling evidence of extraterrestrial visitations, alien artifacts, or any other evidences that the Galaxy is populated. Physicist and Nobel Laureate Enrico Fermi's apparent paradox and some of the proposed resolutions are the topic of this lecture. I will review the Fermi Paradox and describe the most common possible resolutions. The Fermi Paradox is useful in helping to frame the question of extraterrestrial life, even if we so far have no answers. At the end I only touch on the Rare Earth Hypothesis, but this is a very nuanced question which requires a whole other lecture to explore that I have not had time to fully prepare for during this busy quarter. Recorded live on 2009 Nov 30 in Room 1005 Smith Laboratory on the Columbus campus of The Ohio State University.

If we ever detect life elsewhere, how will we go visit? This lecture considers the challenges of interstellar travel and colonization. The problem is one of basic physics (the enormous energy requirements of star flight) coupled with the vast, irreducible distances between the stars. I will describe various starship concepts that use reasonable extrapolations of current technologies (nuclear propulsion and solar sails), ignoring for our discussions science-fiction exotica like faster-than-light drives and wormholes. My interest is in the scientific aspects of the problem, not an exploration of speculative fiction. I then turn to interstellar colonization, and how even a relatively modest star-flight capability might allow a determined civilization to colonize the entire galaxy very rapidly. This has implications for how we might interpret the results of Drake Equation type arguments about the frequency of intelligent life in the Galaxy, and leads to the Fermi Paradox that will be the topic of the next lecture. Recorded live on 2009 Nov 25 in Room 1005 Smith Laboratory on the Columbus campus of The Ohio State University.

Is anybody out there? This lecture reviews the ideas behind SETI, the Search for Extra-Terrestrial Intelligence, an effort to find other intelligent communicating civilizations by tuning in on their radio or other electromagnetic communications. I will discuss the basic approaches being taken by various SETI efforts, and what we expect to find. In addition to listening, we have also been broadcasting, intentionally or otherwise, messages into space, and we have sent physical artifacts with descriptions of our home on robotic spacecraft headed out of our solar system into interstellar space. Recorded live on 2009 Nov 24 in Room 1005 Smith Laboratory on the Columbus campus of The Ohio State University.

How many intelligent, communicating civilizations live in our Galaxy? We have no idea. One way to approach the question and come up with quasi-quantitative estimates is the Drake Equation, first introduced by radio astronomy Frank Drake in the 1960s. I will use the Drake equation as an illustration of the issues related to the question of extraterrestrial intelligence, and to set the stage for future lectures on the likelihood of finding other intelligences in our Universe. Recorded live on 2009 Nov 23 in Room 1005 Smith Laboratory on the Columbus campus of The Ohio State University.

Are there other Earths out there? Do they have life on them? This lecture looks at the search for ExoEarths - Earth-sized planets in the habitable zones of their parent stars, and what we might learn from measuring them. The ultimate goal of all planet searches is to find other Earth's, what the late Carl Sagan so poetically called the "pale blue dot" as seen from the depths of space. This lecture discusses what we might learn about such planets from studies of our own Earth, spectroscopic biomarkers that might reveal life, and variability studies that might give us insight into surface features (continents and oceans) and weather (clouds and even climate). Recorded live on 2009 Nov 19 in Room 1005 Smith Laboratory on the Columbus campus of The Ohio State University.

What are the properties of the 400+ exoplanets we have discovered so far? This lecture reviews the properties of exoplanets, and finds a couple of surprises: Jupiter-mass planets orbiting close to their parent stars, and Jupiter-mass planets in very elliptical orbits. Both seem to require some mechanism for migration: strong gravitational interactions with either the protoplanetary disk or other giant planets to cause the planets to move inward from their birth places beyond the "Ice Line". We will then briefly discuss why we are seeing systems very different from our own, mostly we think a selection effect due to our search methods to date. Microlensing, however, is more sensitive to systems like ours, and is starting to find them. Earths, however, remain elusive so far, but the hunt is on. Recorded live on 2009 Nov 18 in Room 1005 Smith Laboratory on the Columbus campus of The Ohio State University.

Are there planets around other stars? This lecture reviews the methods used to hunt for exoplanets and the results thus far. I will describe direct imaging methods, indirect methods relying on the gravitational influence of the planet on its parent star, planetary transits in which a planet blocks part of its parent star's light, and gravitational microlensing. There has been an explosion in our knowledge of planets around other stars, from little or nothing in the early 1990s to more than 400 planets around some 340-odd stars as of today. Recorded live on 2009 Nov 17 in Room 1005 Smith Laboratory on the Columbus campus of The Ohio State University.

What stars are near the Sun? Now that we have some idea of what we are looking for - rocky planets in the habitable zones of low-mass main-sequence stars - what are the prospects near the Sun? This lecture examines the hunting ground for planets, the nearby stars that make up the Solar Neighborhood. I will describe our nearest neighbor, the Proxima Centauri/Alpha Centauri triple system, and then look at the properties of our nearest stellar neighbors. What we will find is that G-type stars like the Sun are uncommon, only about 7% of all nearby main-sequence stars. Red dwarfs, on the other hand, are very common, about 75%. To find Sun-like main sequence stars, we will have to extend our search to larger distances into our Milky Way galaxy proper. Recorded live on 2009 Nov 16 in Room 1005 Smith Laboratory on the Columbus campus of The Ohio State University.

Which stars are the most hospitable for life? This lecture examines the factors affecting the habitability of stars, with a goal of understanding where we should search for life-bearing planets. We will do this by generalizing the idea of a Habitable Zone developed for the Sun back in Lecture 30. In this context, we find that the best places to search for life would be rocky planets in the habitable zones of low-mass main-sequence stars. There are a number of caveats we will discuss - tidal locking, stellar flares, and UV radiation - and limitations to the approach, but it seems to be a good place to start our search. Recorded live on 2009 Nov 13 in Room 1005 Smith Laboratory on the Columbus campus of The Ohio State University.

What happens to a star when it runs out of hydrogen in its core? This lecture describes the post main-sequence evolution of stars. What happens depends on the star's mass. Low mass stars swell up into Red Giants, and eventually shed their envelopes and end their lives as white dwarf stars. High mass stars become Red Supergiants, and if large enough, end their lives in a spectacular supernova explosion that leaves behind a neutron star or black hole. The explosion itself creates massive quantities of heavy elements, which then seed interstellar space with metals to be incorporated into subsequent generations of stars. Recorded live on 2009 Nov 12 in Room 1005 Smith Laboratory on the Columbus campus of The Ohio State University.

Why do stars shine? How long do they shine? This lecture describes the physics of stars on the main sequence, describes the mass-luminosity relation of main sequence stars, introduces nuclear fusion power and the nuclear fusion lifetimes of stars. From this we gain an important insight into one of the criteria we might apply to the search for life around other stars: we want planets around low-mass main sequence stars that can shine more or less steadily for more that 500 Myr to 1 billion years - maybe longer if our goal is to find intelligent life. Recorded live on 2009 Nov 10 in Room 1005 Smith Laboratory on the Columbus campus of The Ohio State University.