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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.
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©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