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AS Science In Society 1.11 Teacher Notes Introduction The activities here are about evidence for the existence of life and some of the difficulties in detection. Students can attempt to make an estimation of the number of life-supporting planets that might exist. They also consider two different examples of research into the possibility of life elsewhere – The SETI Institute and the ESA Darwin Project. Resources Part 1 Access to http://photojournal.jpl.nasa.gov/Help/ImageGallery.html and to the internet in general for obtaining images and information about Venus, Earth and Mars. Part 3 Power point slides the Drake equation Part 4 Internet access for research into the SETI Institute and the Darwin Project, at addresses: http://www.seti.org/Page.aspx?pid=570 and http://www.esa.int/science/darwin The activity Part 1 Examination of evidence begins with consideration of the Earth and sister planets in the Solar System, making comparisons in order to explore how life might be recognised from a very long way away. The following is one POSSIBLE scoring, but there are no absolutely correct answers: Feature of planet Seasonal changes Presence of solid water Presence of liquid water Temperature range Presence of CO2 in the atmosphere Presence of O2 in the atmosphere Emission of modulated radio waves TOTAL Venus 0 0 0 Earth 2 2 5 Mars 1 2 1 0 1 5 1 2 1 0 5 1 0 5 0 1 25 8 Note that emission of modulated radio waves is significant because it suggests the inhabitants have the technology for radio wave transmission (as has happened on Earth for the past 100 years). Page 1 ©The Nuffield Foundation, 2008 Copies may be made for UK in schools and colleges AS Science In Society 1.11 Teacher Notes Discuss the fact that the presence of oxygen does not lead to a certain conclusion about the existence of life on the planet. We know, with reasonable confidence, that on Earth the oxygen is ‘caused’ by the presence of life. However, while oxygen on other planets might be consistent with the presence of life based on photosynthesis there may be other unknown causes. This means that the level of confidence in supposing that life is the ‘cause’ of the oxygen is reduced. Part 2 There is then some work on evidence for the existence of exoplanets – on how exoplanets can be discovered. An important point to make is that in looking at light curves of stars for signs of planetary transits, astronomers are looking for an effect that is difficult to distinguish from ‘noise’. They must therefore develop techniques for identifying signals hidden by noise. This is a ‘work in progress’ – such techniques continue to be developed and thus smaller and smaller planets are being discovered. Part 3 Show the powerpoint slides the Drake Equation to illustrate this section. The presentation includes more factors from the equation than are included in this activity. The simplification can be discussed. This leads to the use of a simplified version of the Drake equation in order to make an estimate of the number of life-supporting planets that might exist. The important point here is that there are no correct answers. S is a large number, f L is probably a small number and is unknown. A large proportion of stars are hosts to planets, but the number of these that have the right ‘goldilocks zone’ conditions (such as temperature), and have the right size and so on for the presence of liquid water, is unknown. That life has evolved on Earth suggests that the same processes, being part of nature, can happen elsewhere, IF the conditions are right and there is an abundance of time for natural selection processes to take place. These are quite big ‘ifs’, and so f E is also likely to be very small, and is unknown Part 4 Students compare the nature and funding of two examples of exploration of the possible existence of life elsewhere. These raise issues of the motivations of scientists, their funding, and their communications to each other and to the wider world. October 2008 Page 2 ©The Nuffield Foundation, 2008 Copies may be made for UK in schools and colleges AS Science In Society 1.11 Student sheets Introduction If there were intelligent living things far away from us they might be looking for signs of life elsewhere in their Universe. If they could detect remote planets then they might have discovered that the Sun has a family of planets. If they can then explore the radiation reflected or emitted by these planets they might be able to begin to work out some details, such as temperature, nature of atmosphere, seasonal changes, and so on. They could speculate whether any of them could be homes for living things like themselves. Venus, Earth and Mars are three rocky planets that could get them interested. These are all very roughly the same size and all between 105 and 230 million kilometres from their star (the Sun). The activity Part 1 Planetary features Obtain colour images of full globes of Venus, Earth and Mars. Also obtain images, if possible, of closeups of surface features. A good source of images can be found at: http://photojournal.jpl.nasa.gov/Help/ImageGallery.html Look for signs of dynamic processes such as: winds active volcanoes water in one or more states flowing water Discuss whether these features are likely to encourage or discourage the idea that a planet might support life. In the table below, give each planet a score of between, say, 0 and 5 in each cell of the table. A score of 0 means that the planet is not likely to be hospitable to life due to the relevant feature. A score of 5 means that the planet is likely to be highly hospitable to life. Feature of planet Seasonal changes Presence of solid water Presence of liquid water Temperature range Presence of CO2 in the atmosphere Presence of O2 in the atmosphere Emission of modulated radio waves TOTAL Page 1 Venus Earth Mars ©The Nuffield Foundation, 2008 Copies may be made for UK in schools and colleges AS Science In Society 1.11 Student sheets 1. Discuss your scoring with the rest of the class. Justify your scores to others. 2. What assumptions are you making in using each of these features as an indicator of life? 3. Do you think that the total score is a good guide for judging whether an exoplanet might support life of any kind? Part 2 Looking for more We are not aliens looking at the Solar System from a long way away, but we are natives of the Solar System looking out. We have begun to discover exoplanets in orbit around many stars, and we are developing the technology for looking more closely at them. Discovering objects that are much smaller and darker than the stars that they orbit is not easy, but it is possible. One way is to detect the very small changes in the detected brightness of star that can be explained by a planet passing in front of the star. The path of the planet across the star might look like this: The resulting pattern of brightness of the star might look like this: 1. Write a brief explanation of the link between the path of the planet and the pattern of the star’s brightness. 2. Why, despite the long term interest in exoplanets is it only now that they are being ‘found’. 3. Why do we only know about the larger exoplanets so far? Part 3 Estimating So, exoplanets exist. But what are the chances that any of them support life? Watch the power point slides You can find an interesting article on possible habitable planets, how to identify them and what kinds of life they might support, at: http://www.astrobio.net/news/article2462.html In 1960, some scientists met to discuss the search for extra-terrestrial intelligence (SETI). One of the scientists, Frank Drake, suggested an equation that could be useful for making a very rough estimate of the number of planets in our Galaxy that could be homes for intelligent living things. Page 2 ©The Nuffield Foundation, 2008 Copies may be made for UK in schools and colleges AS Science In Society 1.11 Student sheets This is an amended and simplified version that can be used for estimating the number of planets (N) with any kind of life: N = S x f L x fE S is the number of stars in the Galaxy. fL is the fraction of stars that have been capable of supporting life for the last billion years. fE is the fraction of those stars on which evolution of life has actually taken place. This is a useful table of conversions between different ways to write numbers: Longhand number Number in powers of ten Longhand number Number in powers of ten Number as a percentage of 1 100 000 000 000 1011 10 000 000 000 1010 1000 000 000 109 100 000 000 108 10 000 000 107 1000 000 106 100 000 10 000 105 104 1000 100 10 1 1/10 1/00 1/1000 1/10 000 1/100 000 103 102 101 100 10-1 10-2 10-3 10-4 10-5 100 000 10 000 1000 100 10 1 0.1 0.01 0.001 There are a lot of stars in the galaxy – about 100 billion, which we can write as 100 000 000 000 or as 1011. That is the value of S in the equation. Since the 1990s, very many ‘exoplanets’ have been found. It is estimated that much more than 10% of Sun-like stars have planets in orbit around them. Not all of these planets are suitable for life. Make an estimate of fL. Complex chemicals exist in much of the Universe. The development of simple living things from these chemicals may (or may not) be quite common, provided that the conditions are right. Make an estimate of fE. Now use your estimates to work out a value for N. Note: The full Drake equation provides estimates of the number of ‘civilisations’ of intelligent living things that exist in the Galaxy. A complication is that civilisations, such as human civilisation, might not last very long. They might destroy themselves by using weapons that are just too efficient, or by spreading disease due to easy travel, or by climate change. Human settlements have only existed for about 10 000 years (while the Earth has existed for 4 500 000 000 years). The full Drake equation takes account of this. Part 4 The SETI Institute is a private non-profit organisation in the USA. It carries out research and education projects relating to life elsewhere in the Universe. You can find out more at: http://www.seti.org/Page.aspx?pid=570 Page 3 ©The Nuffield Foundation, 2008 Copies may be made for UK in schools and colleges AS Science In Society 1.11 Student sheets The Darwin Project of the European Space Agency (ESA) will use a flotilla of satellites in orbit just above the Earth to detect faint signs of Earth-like planets. It will do so by detecting signs of particular chemicals, including water and carbon dioxide, in the light that planets reflect. You can find out more about the Darwin project at: http://www.esa.int/science/darwin 1. What are the sources of funding for these two activities? 2. What benefits do they offer to the people who run them? 3. What benefits do they offer to the world in general. In other words, what are the benefits of research into extraterrestrial life? Page 4 ©The Nuffield Foundation, 2008 Copies may be made for UK in schools and colleges