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NOVA scienceNOW: The Search for ET

Viewing Ideas

Before Watching

  1. Survey students' opinions about whether we are alone in the universe. Is there life elsewhere in the universe? Is there intelligent life? Have students discuss these questions with a partner or in small groups and record their answers and reasoning. Next, present the class with the two hypothetical situations below. Ask them to discuss and record their reactions.

    Hypothetical Situation 1: We are completely alone in the universe. What would this mean for you? How would it make you feel? How would it change your feeling about your place in your community, your country, the world, or the universe?

    Hypothetical Situation 2: We have received a signal from outer space and know that we are not the only intelligent life in the universe. How would this knowledge change things for you? How would you feel? What would you do? What, if anything, do you think would change as to how you interact with the world and with other human beings? How do you think the U.S. and the world at large would deal with and respond to this knowledge?

  2. Practice working with large numbers. Considering the scale of the universe, the Milky Way galaxy, and the search for extraterrestrial life involves very large numbers. Before class, prepare one class set of 100 pennies. In class, give each student one penny. Do they consider a penny to be worth anything? How about ten pennies? 100? In teams or as a class, have the students lay out 100 pennies on a table in various groupings (e.g., as a ten-by-ten grid, in ten groups of ten, or in four groups of 25). How much is 100 pennies worth? (One dollar) What if you were to give them 1,000 pennies? (That's 10 dollars, or 10 x 10 x 10 pennies. It's 103 pennies, expressed in scientific notation.) 10,000 pennies? 100,000 pennies? Ask students how many pennies there are in one million dollars. (The difference between one cent and one million dollars is, quite simply, the difference between 1 penny and 100 million pennies.)

    Alternatively, make and print out a page of periods. At an eight-point font size on a page with one-inch margins, one can display 10,600 periods. It takes 94 such pages to get a million periods and 9400 pages for 100 million periods. That is nearly two cases of copy paper! Pass around the sheet of dots. Then stack up, one by one, 20 reams of copy paper to show how truly staggering 100 million really is. Scientists in the segment report that this is the number of radio channels the Allen Radio Telescope Array is scanning.

    Have students do some large-number calculations. Tell them to keep these values in mind as they watch the segment and try to spot where this comparison is mentioned.

    1. How many times does 1000 go into 300 billion? (300,000,000,000/1,000 = 300,000,000 or 300 million times)
    2. What is 300 million times 40? (300,000,000 x 40 = 1,200,000,000 or 1.2 billion)
    3. What is 300 billion times 100 billion? (300,000,000,000 x 100,000,000,000 = 30,000,000,000,000,000,000 or 30 quintillion or 3 x 1019)

    After you show the segment, briefly revisit the calculations, referencing the specifics of what each calculation represents.

    1. There are around 300 billion stars in the Milky Way Galaxy. During the first 40 years of SETI, they searched 1000 star systems, which means that they searched only 1/300 millionth of the Milky Way!
    2. At a rate of 1000 star systems every 40 years, it would take 1.2 billion years to search all the stars in the Milky Way!
    3. With around 100 billion galaxies in the universe, and if the Milky Way Galaxy is considered typical, there are approximately 30 quintillion stars in the universe around which planets with intelligent life might orbit.

  3. Use a small sample to make a point about confidence and prediction. Before class, fill a nontransparent container with rice. Color one grain and push it deep into the center. (You can use any collection of small, easily available objects as long as one is visually distinct.) Show the container to your students, explain what is inside, and ask them what they predict their chances would be of randomly pulling out the colored grain if they are only permitted to remove a single grain. What if they were allowed to take out ten grains? 100? Have one student actually pull out a single grain. Ask another student to pull out ten. Have the class discuss whether, based on these two samples, they think there actually is a colored grain in the container. How many grains would they want to take out of the jar to be completely, totally convinced that there is no colored grain inside? (almost all)

  4. Review the electromagnetic spectrum. The search for intelligent extraterrestrial life is actually a search for the electromagnetic signals that technologically advanced life might broadcast into space. In order to understand the search, it is important to appreciate some basics about the electromagnetic spectrum. Remind your class of these key facts:

    • The electromagnetic spectrum is the range of all possible electromagnetic radiation.
    • Electromagnetic radiation along the spectrum is classified into types: radio waves (at the long-wavelength end), microwaves, infrared radiation, visible light (what we can see), ultraviolet radiation, X-rays, and gamma rays (at the short-wavelength end).
    • The electromagnetic spectrum covers wavelengths from thousands of meters down to nanometers.

    Mention that astronomers learn about the universe by "catching" electromagnetic waves from space. In addition, tell students that Search for Extraterrestrial Intelligence (SETI) is seeking signals from intelligent extraterrestrials via the radio and microwave portions of the spectrum (everything longer than about 1 millimeter). These longer wavelengths are used for several reasons. First, longer-wavelength radiation traverses great distances without being scattered by our galaxy's interstellar dust. Second, a signal from an extraterrestrial civilization would likely be very weak by the time it reached Earth. This very weak signal would be all but impossible to detect in the shorter wavelengths, a region of the spectrum where Earth is "noisy." Finally, hydrogen, which is the most abundant element in the universe, emits a signal in the radio portion of the spectrum, at a wavelength of about 21 centimeters (1,420 megahertz). As early as 1959, this scientifically significant wavelength was identified as one that might be chosen for communications by extraterrestrials. These longer wavelengths, and thus the SETI Search, require larger telescopes, such as the Allen Telescope Array presented in the segment.

After Watching

  1. Conduct a search in your classroom. In the segment, Jill Tarter compares the SETI search thus far to dipping a glass in the ocean and deciding that, because there are no fish in the glass, there must be no fish in the ocean. Walk your students through a sampling and analysis of your classroom. First, have them imagine that a cat has gotten loose in the school and that a search is being conducted. However, only a few people are willing to help with the search, and the search method available to them is very unusual—they must select only one room in the school to search (yours, as it turns out), and they only explore three one-cubic-foot spaces in the room. What do your students think of the searchers' chances of finding the cat? Challenge them to be specific. What is the volume of the classroom (length by width by height)? With the scrutiny of just three one-cubic-foot sample areas, what percentage of the room is being searched? Of the school?

    Have students brainstorm a list of all the things:

    • that make the search of the school difficult (They can't search all the rooms; they can't search each room completely; the cat could be hiding; the cat could simply be asleep and therefore less noticeable; the cat could be on the move.)
    • searchers could do to improve their chances of finding the cat. (Search rooms in the vicinity where the cat has been seen; search only on surfaces and not in mid-air; search more rooms; increase the number of sample areas per room.)
    • that make the search of the universe difficult (SETI scientists can't search every star in the galaxy; they can't search all wavelengths; even if there is life on a planet around a star being searched it might not be broadcasting signals at all, or it might not be broadcasting signals at the wavelengths being searched)
    • scientists could do to improve their chances of detecting a signal from extraterrestrial life (Search stars that they think are more likely to be appropriate; search more stars; search more wavelengths coming from each star; use better search tools—the Allen Telescope Array introduced in the segment is such a tool.)

  2. Use the Drake Equation to calculate the possibility of extraterrestrial life. The Drake Equation is a tool for estimating the number of technological civilizations within our galaxy that we might be able to detect. First presented by Dr. Frank Drake in 1961, the equation doesn't have a "right" answer—it's an approximate calculation based on a series of reasoned estimates for various factors. Unlike a fixed mathematical equation, it's a way to define the relevant elements, discuss each one, develop reasonable estimates, and arrive at a prediction. The equation is usually written as follows:

    N = R* • fp • ne • fl • fi • fc • L

    N = An estimate of the number of civilizations in the Milky Way Galaxy whose electromagnetic emissions might be detectable

    R* = The formation rate of stars that are suitable for the development of intelligent life, OR the number of suitable stars that form in our galaxy per year [1 is a reasonable estimate.]

    fp = The fraction of the suitable stars that have planetary systems [Somewhere between 1 (all) and 0.5 (half) is a reasonable estimate; more extrasolar planets are being discovered every year.]

    ne = The number of planets per planetary system that have an environment suitable for life, with liquid water being a primary need of life [Within our solar system, there is at least one (Earth) and there may be more (e.g., Mars, Europa, Titan)]

    fl = The fraction of suitable planets on which life actually appears [If you think life appears anywhere that conditions are appropriate, 1 is a reasonable estimate; if you think only 1 in 5 planets suitable for life actually develop life, 0.2 is a reasonable estimate; etc.

    fi = The fraction of planets harboring life on which intelligent life emerges [If you think every planet with life eventually produces intelligent life, 1 is a reasonable estimate; if you think only 1 in 10 planets with life actually produce intelligent life, 0.1 is a reasonable estimate; etc.)

    fc = The fraction of intelligent-life civilizations that develop a technology that broadcasts detectable signals into space [Human beings didn't start broadcasting signals until the 20th century, and not all intelligent beings might develop or choose to use such technologies. If you think all civilizations have and use broadcasting technologies, 1 is a reasonable estimate; if you think only 1 in 5 civilizations do so, 0.2 is a reasonable estimate; etc.]

    L = The length of time such technological civilizations broadcast detectable signals into space OR the number of years a civilization actually broadcasts signals [Human beings have only been broadcasting signals for around 100 years. If you think that civilizations don't remain able to or interested in broadcasting signals for very long, 100 years is a reasonable estimate; if you are more optimistic, 1,000 years, 500,000 years, 1,000,000 years, or even more is a reasonable estimate.]

    Present the "equation" to your students, have them discuss the values for each factor, and then calculate a value for N. You can do this using the online calculator available on the PBS Life Beyond Earth Web site, or by writing the Drake equation on the board. (Unless you use the Web version, you will want a calculator for this activity.)

  3. Research extrasolar planets. Extrasolar planets are planets that orbit stars other than the sun. Hundreds of extrasolar planets have been discovered since 1994, when radio astronomer Dr. Alexander Wolszczan discovered the first one. Have your students search the Web for information on extrasolar planets, the ongoing search, and the latest discoveries. Have them focus on Earth-like planets, which are the ones most likely to support detectable intelligent life. A good source of information is the page about extrasolar planets on the PBS Seeing in the Dark Web site.

Links and Books

Web Sites

NOVA scienceNOW
Offers resources related to the SETI, including additional activities, streamed video, and reports by experts.

Life Beyond Earth: Drake Equation
Provides an online calculator for the Drake Equation as part of the Web site for the PBS film, Life Beyond Earth.

National Astronomy and Ionosphere Center: Arecibo Observatory
Provides information about the Arecibo Observatory telescope, research, and more.

SETI Institute
Presents the mission, research, and resources of the SETI Institute, including the Allen Telescope Array.

Astrobiology: Life in the Universe
Provides information on the study of the origin, evolution, distribution, and future of life in the universe.


by Carl Sagan.
Pocket, 1985.
A classic science fiction novel about scientists receiving a message from outer space and the world's reactions to the event.

by Carl Sagan.
Ballantine Books, 1985.
Presents a transformative, though now somewhat outdated, look at the science of the origin of life, the search for extraterrestrial intelligence, and the universe. (The 13 episodes of the television series are also available.)

The Living Cosmos: Our Search for Life in the Universe
by Chris Impey.
Random House, 2007.
Describes the foundations of astrobiology and the science of life in the universe.

SETI 2020: A Roadmap for the Search for Extraterrestrial Intelligence
by R. D. Ekers, D. Kent Cullers, and John Billingham.
SETI Press, 2002.
Lays out how SETI scientists should direct their efforts from the time this book was published in 2002 through the year 2020.

Activity Author

Teon Edwards is a curriculum developer with a background in astrophysics, mathematics, and the use of technology and multimedia in teaching and learning. Since 1996, she has developed numerous science and mathematics education materials for school, home, and informal learning environments.

Teacher's Guide
NOVA scienceNOW: The Search for ET

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