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Origins: Where are the Aliens?

Classroom Activities

Exploring Spectra


To learn how planetary spectra can be used to search for life on other worlds and analyze a mystery planet's spectrum for potential signs of life.

Materials for teacher
  • copy of the "Exploring Spectra" student handout (PDF or HTML)
Materials for students
  • copy of the "Mission: The Search for Life" student handout (PDF or HTML)
  • copy of the "Research Journal" student handout (PDF or HTML)
  • copy of the "Planet Spectra" student handout (PDF or HTML)
  • copy of the "Mystery Planet's Spectrum" student handout (PDF or HTML)
Materials for each team
  • copy of the "Research Reading: In Search of ET's Breath" student handout (PDF or HTML)
  • copy of the "Research Reading: Terrestrial and Jovian Planets" student handout (PDF or HTML)
  • copy of the "Research Reading: Chemical Fingerprints" student handout (PDF or HTML)
  • access to print and internet resources

  1. Currently, the search for life elsewhere in the galaxy employs ground-based telescopes that seek signals from intelligent life. In the future, scientists hope to send telescopes into space to look at the atmospheres of Earth-like planets that may be near other, larger gas planets that have already been identified. Scientists want to use spectroscopy—a technique that allows chemicals to be identified by their unique light signatures—to decode the compositions of these atmospheres and learn whether they might be capable of supporting either primitive or complex life (as it is currently understood). In this activity, students will learn which chemicals scientists are searching for, why those chemicals were chosen, and the kind of spectral signature each chemical emits. Then they will apply their knowledge to a mystery planet's spectrum to determine whether the planet might be a candidate for life.

  2. In order to complete this activity successfully, students must understand concepts about the electromagnetic spectrum and absorption spectroscopy (see Prior Student Knowledge below for a complete list of concepts). Primer information and activities on these concepts can be found at
  3. To begin the activity, tell students they have been hired by NASA to determine whether a mystery planet has the potential for life. In order to do this, students will need to learn how scientists would like to use planetary spectra to determine whether other worlds may be suitable for life.

  4. Review the "Mission: The Search for Life," "Research Journal," and "Planet Spectra" student handouts to familiarize yourself with the activity. Then distribute the handouts to students and review the mission and activity procedure with them.

  5. Prior to having students conduct their research, explain absorption spectra by analyzing a graphic spectra example. Using the "Exploring Spectra" overhead, show students what stellar hydrogen absorption looks like in a continuous spectrum and represented as a graph. (The overhead shows hydrogen being absorbed in four specific bands of visible light. The two absorption lines just beyond 400 nanometers are caused by calcium in the Sun's atmosphere.) Note to students that this graphic represents stellar absorption spectra (in which specific wavelengths of starlight have been absorbed by gases in the sun's lower atmosphere or Earth's atmosphere). In this activity, students will be studying planetary spectra (in which specific wavelengths of starlight have been absorbed by a planet's atmosphere). Also note to students that the overhead represents spectra that are mostly in the visible part of the electromagnetic spectrum; students will be studying absorption spectra that exist in the infrared region.

  6. Once students have a basic understanding of spectra, they can begin their research. Organize students into teams of three or four. Distribute a set of the Research Reading handouts to each team.

  7. You may want to instruct students to begin their research by reading "In Search of ET's Breath," which contains an overview regarding the search for life on other worlds. Then they can read the other handouts and conduct their research using print and Internet resources. (Because the field of astrobiology is so new, there are few books on the topic. See the Links and Books section below for print resources.) Have students use their research findings to answer the Research Questions listed on their "Research Journal" handouts. Answering the Research Questions on their handouts will help students meet Project Requirement #1 (identify the characteristics of planets with the best chances of harboring life).

  8. Monitor students and provide assistance as needed (see Activity Answer on for more information). The Research Reading handouts that contain the information for each answer are referenced in the Activity Answer, so that you can direct students who need additional assistance to the appropriate reading.

  9. Once students have completed their research, have them make comparisons of the spectra in their "Planet Spectra" handouts and answer the Data Analysis Questions listed on their "Research Journal" handouts. Answering the Data Analysis Questions will help students meet Project Requirement #2 (make a comparison of the data provided). All team members will need to use the Research Reading handouts to conduct research to interpret the data.

  10. After students have analyzed the planet spectra, have them draw conclusions and compile individual final reports. Encourage students to choose their own report format, including slide shows, skits, stories, computer presentations, or written accounts. Direct students to address the material outlined in the Final Report Requirements section of their "Research Journal" handouts when compiling their reports.

  11. Have students present their reports to the class. (See Activity Answer for more information on what to look for in student reports.)

  12. As a final assessment, provide each student with the "Mystery Planet's Spectrum" handout. Ask students to determine the likelihood of finding life on this planet based on the signs of life scientists are currently looking for. Allow students to use their research journals and final report to aid them. Students should support their opinions with evidence.

Prior Student Knowledge

This activity investigates planetary spectral analysis. Prior to beginning the activity, make sure students understand the following key concepts and terms:

  • White light is composed of colors that can be seen when light is dispersed into a spectrum.

  • The electromagnetic (EM) spectrum consists of radio, microwave, infrared, visible, ultraviolet, X-rays and gamma rays. Humans can only see visible light.

  • All matter is composed of elements, compounds, and mixtures.

  • Chemical symbols are used to represent elements and compounds.

  • Key terms: absorption, Archean, atmosphere, extrasolar, extraterrestrial, intensity, nanometer, ozone, spectrum (spectra), wavelength (see Activity Answer for definitions). Students should also be able to read and interpret graphs.

Related Activities

Characteristics of Life
Investigate the nature of life on Earth.

Explore the question of life beyond Earth and discover how scientists find extrasolar planets in this American Museum of Natural History site that offers articles and student materials related to NOVA 's "Where Are the Aliens?" program.

Remote Communications
Perform a simulated mission to another planet to search for evidence of life or the conditions where life might form.

Activity Answer

You may want to review the following terms with students:

absorption: The process by which light transfers its energy to matter. For example, a gas cloud can absorb starlight that passes through it. After the starlight passes through the cloud, dark lines called absorption lines appear in the star's continuous spectrum at wavelengths corresponding to the light-absorbing elements.

Archean: A geologic period in Earth's history marked by the emergence of life, about 3.8 billion to 2.5 billion years ago.

atmosphere: The layer of gases surrounding the surface of a planet, moon, or star.

brightness temperature: The temperature an object must have to produce the observed intensity.

extrasolar: An adjective meaning "beyond the solar system." For example, an extrasolar planet orbits a star other than the sun.

extraterrestrial: An adjective that means "beyond the Earth." The phrase "extraterrestrial life" refers to possible life on other planets.

intensity: The amount, degree, or quantity of energy passing through a point per unit time. For example, the intensity of light that Earth receives from the sun is far greater than what it receives from any other star because the sun is the closest star to Earth.

nanometer: A nanometer is one billionth of a meter (10-9).

ozone (O3): A form of molecular oxygen containing three atoms instead of the normal two. It is created by the action of ultraviolet light on oxygen (O2). Earth's ozone layer protects the planet by absorbing the sun's high-energy ultraviolet radiation, which is harmful to life.

spectrum (pl spectra): The result of spreading a beam of electromagnetic radiation so that components with different wavelengths are separated.

wavelength: The distance between one peak or crest of a wave and the next.

Research Questions

  1. What gases does life (as we know it) require? What gases does life produce?

    Different types of organisms require different gases. Plants require carbon dioxide (CO2) for photosynthesis, while animals require oxygen (O2) for respiration. However, some primitive life forms (e. g., anaerobic bacteria) require neither. As a result of their metabolism, plants give off oxygen (O2), and animals give off carbon dioxide (CO2). Some bacteria produce methane (CH4). So, oxygen, carbon dioxide, and methane are all gases that can be produced by life. However, other natural processes can also produce these gases. In order to be more confident that they have found the potential for life, scientists would like to find more than one of these gases in the same atmosphere. Finding both oxygen and methane in a planet's atmosphere would be a very good indication that life could exist on that planet.

    Students can find the answer to this question in "Research Reading: Chemical Fingerprints."

  2. What is the difference between a terrestrial planet and a Jovian planet?

    Terrestrial planets include Mercury, Venus, Earth, and Mars. Jovian (meaning Jupiter-like) planets include Jupiter, Saturn, Uranus, and Neptune. Terrestrial and Jovian planets differ in size and structure. Terrestrial planets have smaller sizes and masses, while Jovian planets have much larger sizes and masses. In our solar system, terrestrial planets are closer to the sun than Jovian planets, and are warmer than Jovian planets. Terrestrial planets have rocky, solid surfaces and atmospheres made mostly of carbon dioxide or nitrogen (except for Mercury, which has almost no atmosphere). In contrast, Jovian planets do not have a solid surface and are made mostly of gases. (They are also known as gas giants.) Their atmospheres are mostly hydrogen and helium.

    Students can find the answer to this question in "Research Reading: Terrestrial and Jovian Planets."

  3. What does it mean for a planet to be in the "habitable zone"?

    The planets that seem most likely to harbor life are located in the "habitable zone;" that is, the region around a star where scientists can expect to find liquid water at the surface of a terrestrial planet. If a planet is too hot, the water becomes a gas. If a planet is too cold, the water freezes. Either of these conditions would make a planet extremely inhospitable for life. The habitable zone of our solar system starts just beyond Venus and ends just before Mars.

    But the habitable zone may be larger than originally conceived. A strong gravitational pull caused by large planets may produce enough energy to sufficiently heat the cores of orbiting moons (such as Jupiter's moon Europa). Life survives in a wide variety of environments on Earth. Perhaps it could thrive in more extreme environments.

    Students can find the answer to this question in "Research Reading: In Search of ET's Breath."

  4. Which planets in the solar system are in (or near) the habitable zone?

    Earth is right in the middle of the habitable zone, while Venus and Mars are close to, but just outside of, the habitable zone.

    Students can find the answer to this question in "Research Reading: In Search of ET's Breath."

  5. Why is it important to look at Archean Earth?

    Earth's atmosphere has changed over time, and early (photosynthetic) life had a significant impact on it. During the first billion years, single-celled ancestors of modern-day bacteria evolved into primitive photosynthetic organisms that released oxygen into the atmosphere. During this time, Earth's Archean atmosphere contained methane (CH4), but not oxygen (O2). Today, Earth's atmosphere contains about 21 percent oxygen and .0002 percent methane. So, the absence of oxygen doesn't necessarily mean that no life exists.

    Students can find the answer to this question in "Research Reading: In Search of ET's Breath" and "Research Reading: Chemical Fingerprints."

Data Analysis Questions

  1. Which gases, if any, are common to all four planet spectra?

    Carbon dioxide (CO2) appears in all four planet spectra.

  2. What does your answer to question 1.) mean in terms of the search for life on other planets?

    Since carbon dioxide appears on a planet even if it doesn't have any life, carbon dioxide is not a good indicator for finding life.

  3. If ozone (O3) is present, is normal oxygen (O2) also present? Does the presence of oxygen automatically mean life?

    Yes, if ozone is present, then normal oxygen is probably there also. But the presence of oxygen doesn't automatically mean life, because there are non-biological processes that can produce oxygen. For instance, ultraviolet sunlight (or starlight) can break apart water (H2O) molecules into hydrogen and oxygen. The hydrogen, having very low mass, can escape into space while the heavier oxygen is left behind.

  4. How does the spectrum of Archean Earth compare to that of present-day Earth? Why is it important to consider the atmosphere of Archean Earth when considering how to look for life on other worlds?

    An infrared spectrum from the atmosphere of modern-day Earth would show carbon dioxide, water), and ozone. However, the spectrum from Archean Earth would show carbon dioxide, water, and methane. These are both suggestive of life, because they are gases that living organisms give off.

    For roughly the first billion years of Earth's history, oxygen-producing, photosynthetic life had not yet evolved. Instead, the microorganisms that dominated the planet tapped energy from gases that leaked out of Earth's interior. Some microbes created methane as a byproduct.

    Because of the methane-producing organisms, methane was present in the Archean Earth's atmosphere. But organisms were not yet producing an abundance of oxygen, and therefore ozone is absent in the spectrum of Archean Earth's atmospheric gases.

    In more recent times, photosynthesis has resulted in abundant oxygen in the atmosphere. Therefore, ozone is present in the modern Earth's atmosphere, while methane is present only in trace amounts. On a planet with a similar geology to Earth, methane levels greater than about 100 parts per million would suggest the presence of life. But methane doesn't necessarily imply life. Planets of a different geological make-up might have high methane levels and no life.

  5. What gases are likely to be present in the atmosphere of a planet harboring life? Is the answer different depending on whether it's primitive life or advanced life?

    The atmosphere of a planet harboring life would likely show carbon dioxide, water, and oxygen, ozone and/or methane. A planet with only primitive life would likely have an atmosphere containing carbon dioxide, water, and methane. With complex life (i.e., plenty of oxygen-producing organisms), a planet would be more likely to have a substantial amount of oxygen in its atmosphere (in the form of oxygen or ozone).

  6. Can the infrared portion of a planetary spectrum be used to look for signs of life? What spectral features are of interest for this?

    Yes, because most of the gases produced by life create observable features in the infrared part of the spectrum, scientists could look for ozone, methane, and water. Ozone and methane are signs of life, and water is an indication that the planet is not so cold as to be completely frozen. Water, if liquid, potentially provides a resource for life.

    Normal oxygen does not show up in the infrared part of the spectrum. So, the way to detect oxygen is to look for one feature of ozone that appears at a wavelength of approximately 9,500 to 9,700 nanometers. Methane produces a dip in the spectrum at a wavelength of approximately 7,600 nanometers. One feature of water appears at 6,000 nanometers.

Student Final Reports

In addition to providing information about where and how to search for habitable planets, students' final reports should include the following conclusions:

  • finding oxygen is good, but it should not be the only target. Other gases should be included—such as water vapor and methane—since life can exist even if oxygen is not present.

  • the composition of Earth's atmosphere has changed as life has evolved, which suggests that chemicals other than Earth's present-day atmosphere may indicate the presence of life. Certain types of chemicals may provide information about the complexity of any potential life.

  • using only one example makes it difficult to design a scientific study. In this case, having Earth as the only place we know life exists makes it difficult to design a study to search for life on other worlds, which may or may not be similar to life on Earth.

Mystery Planet's Spectrum

Students should conclude that the planet's atmosphere contains carbon dioxide (CO2) and some students may determine that there is a trace amount of water (H2O). There is clearly no ozone (O3) nor methane (CH4) which implies that this planet probably does not harbor life.

Links and Books

Web Sites

NOVA Web Site—Origins
In this companion Web site to the program, find out how life could have started and why water is needed for life; read about the latest discoveries in origins research; use raw data to assemble the famous Eagle Nebula image; insert your own values into the Drake Equation; decode cosmic spectra; and more.

Ask an Astrobiologist: Questions About Life on Earth
Offers a searchable database of questions and answers and a way to post new questions.

Celestia Exploration Activity: Solar System Overview
Provides a brief description of terrestrial and Jovian planets and contains information about some planetary atmospheres.

Extreme Ecosystem
Describes the search for life in some of the most inhospitable places on Earth for life forms: scalding heat, freezing cold, salt, lye, and darkness.

Glossary of Planet Terms
Provides a glossary of astronomy definitions, including atmosphere, greenhouse effect, ozone layer, secondary atmosphere, and more.

Hunting Planets Along the Milky Way
Offers an in-depth look at the search for extrasolar planets.

In Search of E.T.'s Breath
Reviews the history of the search for extrasolar planets as well as future missions designed to probe far-off worlds for the chemical signatures of alien life.

Indicators of Life: Detection of Life by Remote Sensing
Explains why certain chemicals in the atmospheres of planets might be likely signatures of life.

Reviews the search for Earth-like planets through background information, multimedia resources, and an atlas of extrasolar planets.

Solar System Exploration
Includes facts about the planets in our solar system and details the status of current NASA missions.

Windows to the Universe: The Archean
Describes the changes that occurred on Earth during the Archean geologic period.


Clark, Stuart. Life on Other Worlds and How to Find It. London, New York: Springer-Praxis, 2000.
Discusses what might constitute a hospitable environment for life and explores the nature of intelligence and its role in evolution and survival.

Darling, David. Life Everywhere: The Maverick Science of Astrobiology. New York: Basic Books, 2001.
Provides an overview of astrobiology, including a review of the conditions that might be necessary for supporting life, what life is, and how it might evolve.

Grady, Monica M. Astrobiology. Washington, DC: Smithsonian Institution Press, 2001.
Explores the emerging field of astrobiology, including the nature of extremophiles and planetary environments favorable to life.

Parker, Barry R. Alien Life: The Search for Extraterrestrials and Beyond. New York: Plenum Trade, 1998.
Considers how life may have originated on Earth and what chemicals may be necessary for produce life elsewhere.

NASA's Origins Resources

Visit the Web sites below to learn how individual missions in NASA's Astronomical Search for Origins Program are searching for the earliest stars and galaxies, planets around other stars, and life elsewhere in the universe. Additional classroom resources are available at these sites and through NASA's Space Science Education Resource Directory at

Far Ultraviolet Spectroscopic Explorer

Hubble Space Telescope

James Webb Space Telescope

Kepler Mission

NASA Astrobiology Institute

Navigator Program

Spitzer Space Telescope

Stratospheric Observatory for Infrared Astronomy


The "Mission: The Search for Life" activity aligns with the following National Science Education Standards:

Grades 9-12

Earth and Space Science

Science Standard D:
Earth and Space Science

The origin and evolution of the Earth system:

  • The sun, the Earth, and the rest of the solar system formed from a nebular cloud of dust and gas 4.6 billion years ago. The early Earth was very different from the planet we live on today.

  • Evidence for one-celled forms of life—the bacteria—extends back more than 3.5 billion years. The evolution of life caused dramatic changes in the composition of the Earth's atmosphere, which did not originally contain oxygen.

Classroom Activity Author

This activity was adapted from materials provided by the Hubble Space Telescope formal education team and the Origins Education Forum, in collaboration with scientists from the Virtual Planetary Laboratory, the NASA Astrobiology Institute, and the Mars Global Surveyor Thermal Emission Spectrometer team. An in-depth inquiry-based version of this activity can be obtained by contacting the Hubble Space Telescope's Formal Education team by e-mail at

Teacher's Guide
Origins: Where are the Aliens?

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