Original broadcast: September 29, 2004


Origins: Where Are the Aliens? 1 To help students understand that the light they see from stars is a snapshot of what happened long ago, have them consider light from PROGRAM OVERVIEW the sun. If the sun stopped shining, NOVA explores the search for how long before anyone on would see it “go out”? (150 x 106 km . ÷ 3 x 105 km/sec = 500 seconds, or 8.3 minutes) Explain to students The program: that when they look at a star at • reviews how aliens are depicted in visual media. night, they are seeing the light that • describes the Search for Extraterrestrial Intelligence (SETI), a program left the star long ago, not the light that it is currently radiating. that scans star systems for radio transmissions from advanced extraterrestrial civilizations. 2 Take a poll on how many students believe there is intelligent life • introduces the Drake equation, created by SETI founder Frank Drake, elsewhere in the Milky Way galaxy. which attempts to quantify the probability of intelligent life in the Ask students to support their Milky Way galaxy. reasoning for or against the • shows how scientists detect faraway by looking for the existence of intelligent life. characteristic wobble exhibited by a star when a Jupiter-sized is in orbit around it. • reviews the first of more than 100 discoveries of stars with AFTER WATCHING planetary wobbles. 1 In 2004, Epsilon Eridani was the • notes the importance of finding Jupiter-sized planets that could attract nearest Milky Way star believed passing asteroids and slingshot them out of the , thereby to harbor a planet. It resides 100 protecting smaller nearby planets from bombardment and allowing trillion kilometers from Earth. Have them a higher probability of forming life. students calculate how long it takes • explains why carbon and may be conducive to life. light to get to Earth from Epsilon Eridani using the fact that light • speculates that ancient oceans on the surface of indicate the travels at a speed of 3.0 x 105 planet may have once had liquid water. km/sec (1.0 x 1014 km ÷ 3.0 x 105 • investigates the intelligence of cephalopods and posits that intelligence km/sec = 3.3 x 108 sec, and then 7 may be a natural outcome in the evolution of complicated life forms. 3.3 x 108 sec ÷ 3.16 x 10 sec/yr = 10.4 years) • concludes with the question of whether humans would be able to communicate with intelligent life—that was also technologically 2 Repeat the poll about intelligent life. How do the results compare to the advanced enough to communicate—if it did exist. earlier poll? Have students explain any changes in thinking. Have Taping Rights: Can be used up to one year after the program is taped off the air. students further consider the pros and cons of finding intelligent life. Students can and register their own votes at www.pbs.org/nova/origins/aliens.html 3 Ask students what they think life might look like on another planet. Do they think it would be similar to life on Earth? Why or why not? What are the challenges involved in finding life?

1 NOVA TEACHER’S GUIDE www.pbs.org/nova/origins CLASSROOM ACTIVITY STANDARDS CONNECTIONS Objective The “Mission: The Search for Life” To learn how planetary spectra can be used to search for life on other activity aligns with the following worlds and analyze a mystery planet’s spectrum for potential signs of life. National Science Education Standards. Materials for teacher • color printout of the “Exploring Spectra” overhead GRADES 9–12 Science Standard D: Materials for each student Earth and Space Science The origin and evolution of the • copy of the “Mission: The Search for Life” student handout Earth system • copy of the “Research Journal” student handout • The sun, the Earth, and the rest • copy of the “Planet Spectra” student handout of the solar system formed from • copy of the “Mystery Planet’s Spectrum” student handout a nebular cloud of dust and 4.6 billion years ago. The early Materials for each team Earth was very different from the planet we live on today. • copy of the “Research Reading: In Search of ET’s Breath” student handout • copy of the “Research Reading: Terrestrial and Jovian Planets” • Evidence for one-celled forms of student handout life—the bacteria—extends back more than 3.5 billion years. The • copy of the “Research Reading: Chemical Fingerprints” student handout evolution of life caused dramatic • access to print and Internet resources changes in the composition of the Earth’s , which did not originally contain . Procedure 1 Currently, the search for life elsewhere in the galaxy employs ground- based telescopes that seek signals from intelligent life. In the future, Video is not required scientists hope to send telescopes into space to look at the atmo- for this activity. spheres of Earth-like planets that may be near other, larger gas planets that have already been identified. Scientists want to use

a technique that allows chemicals to be identified by their unique light Classroom Activity Author signatures—to decode the compositions of these and This activity was adapted from learn whether they might be capable of supporting either primitive or materials provided by the Hubble complex life (as it is currently understood). In this activity, students will Space Telescope formal education team and the Origins Education learn which chemicals scientists are searching for, why those chemicals Forum, in collaboration with were chosen, and the kind of spectral signature each chemical emits. scientists from the Virtual Planetary Then they will apply their knowledge to a mystery planet’s spectrum Laboratory, the NASA to determine whether the planet might be a candidate for life. Institute, and the Mars Global 2 In order to complete this activity successfully, students must under- Surveyor Thermal Emission Spectrometer team. An in-depth stand concepts about the electromagnetic spectrum and absorption inquiry-based version of this activity spectroscopy (see Prior Student Knowledge on page 3 for a complete can be obtained by contacting the list of concepts). Primer information and activities on these concepts Hubble Space Telescope’s Formal can be found at Education team by e-mail at amazing-space.stsci.edu/resources/qa/ems.php.p=Astronomy+basics [email protected] www.pbs.org/newshour/extra/teachers/lessonplans/ science/hubble.html

ORIGINS: WHERE ARE THE ALIENS? 2 NOVA TEACHER’S GUIDE www.pbs.org/nova/origins CLASSROOM ACTIVITY (CONT.) PRIOR STUDENT KNOWLEDGE 3 To begin the activity, tell students they have been hired by NASA to This activity investigates planetary determine whether a mystery planet has the potential for life. In order spectral analysis. Prior to beginning to do this, students will need to learn how scientists would like to use the activity, make sure students under- planetary spectra to determine whether other worlds may be suitable stand the following key concepts and for life. terms: 4 Review the “Mission: The Search for Life,” “Research Journal,” and • White light is composed of colors “Planet Spectra” student handouts to familiarize yourself with the that can be seen when light is dis- activity. Then distribute the handouts to students and review the persed into a spectrum. mission and activity procedure with them. • The electromagnetic (EM) spectrum 5 Prior to having students conduct their research, explain absorption consists of radio, microwave, infrared, spectra by analyzing a graphic spectra example. Using the “Exploring visible, , X-rays and gamma rays. Humans can only see visible light. Spectra” overhead, show students what stellar absorption looks like in a continuous spectrum and represented as a graph. (The • All matter is composed of elements, overhead shows hydrogen being absorbed in four specific bands of compounds, and mixtures. visible light. The two absorption lines just beyond 400 nanometers are • Chemical symbols are used to caused by in the Sun’s atmosphere.) Note to students that represent elements and compounds. this graphic represents stellar absorption spectra (in which specific • Key terms: absorption, Archean, wavelengths of starlight have been absorbed by in the sun’s atmosphere, extrasolar, extraterrestrial, lower atmosphere or Earth’s atmosphere). In this activity, students will intensity, nanometer, , spectrum be studying planetary spectra (in which specific wavelengths of starlight (spectra), wavelength (see Activity Answer on page 5 for definitions). have been absorbed by a planet’s atmosphere). Also note to students that the overhead represents spectra that are mostly in the visible part Students should also be able to read and interpret graphs. 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 on page 8 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 page 5 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 Characteristics of Life comparisons of the spectra in their “Planet Spectra” handouts and www.pbs.org/nova/origins/teachers/ activities/3112_origins.html answer the Data Analysis Questions listed on their “Research Journal” Investigate the of life on Earth. handouts. Answering the Data Analysis Questions will help students meet Project Requirement #2 (make a comparison of the data provided). Origins All team members will need to use the Research Reading handouts to www.amnh.org/education/resources/ programs/origins/aliens.php conduct research to interpret the data. Explore the question of life beyond 10 After students have analyzed the planet spectra, have them draw Earth and discover how scientists find conclusions and compile individual final reports. Encourage students extrasolar planets in this American to choose their own report format, including slide shows, skits, stories, Museum of Natural History site that computer presentations, or written accounts. Direct students to offers articles and student materials related to NOVA’s “Where Are the address the material outlined in the Final Report Requirements section Aliens?” program. of their “Research Journal” handouts when compiling their reports. 11 Have students present their reports to the class. (See page 7 of Activity Remote Communications Answer for more information on what to look for in student reports.) www.pacsci.org/origins/ Perform a simulated mission to 12 As a final assessment, provide each student with the “Mystery Planet’s another planet to search for evidence Spectrum” handout. Ask students to determine the likelihood of of life or the conditions where life finding life on this planet based on the signs of life scientists are might form. currently looking for. Allow students to use their research journals and final report to aid them. Students should support their opinions with evidence.


You may want to review the ozone (O3): A form of molecular Students can find the answer to following terms with students: oxygen containing three this question in “Research Reading: instead of the normal two. It is Chemical Fingerprints.” absorption: The process by which created by the action of ultraviolet light transfers its energy to matter. light on oxygen (O ). Earth’s ozone For example, a gas cloud can 2 layer protects the planet by absorb- 2. What is the difference between a absorb starlight that passes ing the sun’s high-energy ultraviolet terrestrial planet and a Jovian planet? through it. After the starlight radiation, which is harmful to life. Terrestrial planets include , passes through the cloud, dark , Earth, and Mars. Jovian lines called absorption lines appear spectrum (pl spectra): The result of (meaning Jupiter-like) planets in the star’s continuous spectrum spreading a beam of electromagnetic include Jupiter, , Uranus, at wavelengths corresponding to radiation so that components with and Neptune. Terrestrial and Jovian the light-absorbing elements. different wavelengths are separated. planets differ in size and structure. Archean: A geologic in wavelength: The distance between Terrestrial planets have smaller sizes Earth’s history marked by the one peak or crest of a wave and and masses, while Jovian planets emergence of life, about 3.8 the next. have much larger sizes and masses. billion to 2.5 billion years ago. In our solar system, terrestrial planets are closer to the sun than atmosphere: The layer of gases Research Questions Jovian planets, and are warmer than surrounding the surface of a planet, 1. What gases does life (as we know Jovian planets. Terrestrial planets , or star. it) require? What gases does life produce? have rocky, solid surfaces and brightness temperature: The Different types of organisms atmospheres made mostly of temperature an object must have require different gases. Plants or (except to produce the observed intensity. for Mercury, which has almost no require carbon dioxide (CO2) for atmosphere). In contrast, Jovian extrasolar: An adjective meaning photosynthesis, while animals planets do not have a solid surface “beyond the solar system.” For require oxygen (O2) for respiration. and are made mostly of gases. example, an extrasolar planet orbits However, some primitive life forms (They are also known as gas giants.) a star other than the sun. (e.g., anaerobic bacteria) require neither. As a result of their metabo- Their atmospheres are mostly extraterrestrial: An adjective that hydrogen and . lism, plants give off oxygen (O2), means “beyond the Earth.” The and animals give off carbon dioxide phrase “extraterrestrial life” refers Students can find the answer to (CO2). Some bacteria produce to possible life on other planets. this question in “Research Reading: (CH4). So, oxygen, carbon Terrestrial and Jovian Planets.” intensity: The amount, degree, or dioxide, and methane are all gases quantity of energy passing through that can be produced by life. a point per unit time. For example, However, other natural processes 3. What does it mean for a planet to the intensity of light that Earth can also produce these gases. In be in the “habitable zone”? receives from the sun is far greater order to be more confident that The planets that seem most likely than what it receives from any other they have found the potential for to harbor life are located in the star because the sun is the closest life, scientists would like to find “habitable zone;” that is, the region star to Earth. more than one of these gases in the same atmosphere. Finding both around a star where scientists can nanometer: A nanometer is one oxygen and methane in a planet’s expect to find liquid water at the -9 billionth of a meter (10 ). atmosphere would be a very good surface of a terrestrial planet. If a indication that life could exist on planet is too hot, the water that planet. ORIGINS: WHERE ARE THE ALIENS? 5 NOVA TEACHER’S GUIDE www.pbs.org/nova/origins ACTIVITY ANSWER (CONT.) becomes a gas. If a planet is too 5. Why is it important to look at 3. If ozone (O3) is present, is cold, the water freezes. Either of Archean Earth? normal oxygen (O2) also present? these conditions would make a Earth’s atmosphere has changed Does the presence of oxygen planet extremely inhospitable for over time, and early (photosynth- automatically mean life? life. The habitable zone of our solar etic) life had a significant impact Yes, if ozone is present, then system starts just beyond Venus on it. During the first billion years, normal oxygen is probably there and ends just before Mars. single-celled ancestors of modern- also. But the presence of oxygen day bacteria evolved into primitive doesn’t automatically mean life, But the habitable zone may be larg- photosynthetic organisms that because there are non-biological er than originally conceived. A released oxygen into the atmo- processes that can produce oxygen. strong gravitational pull caused by sphere. During this time, Earth’s For instance, ultraviolet sunlight large planets may produce enough Archean atmosphere contained (or starlight) can break apart water energy to sufficiently heat the methane (CH ), but not oxygen (H O) into hydrogen cores of orbiting (such 4 2 (O ). Today, Earth’s atmosphere and oxygen. The hydrogen, having as Jupiter’s moon Europa). Life 2 contains about 21 percent oxygen very low , can escape into survives in a wide variety of and .0002 percent methane. So, space while the heavier oxygen is environments on Earth. Perhaps the absence of oxygen doesn’t left behind. it could thrive in more extreme necessarily mean that no life exists. environments. 4. How does the spectrum of Students can find the answer Archean Earth compare to that to this question in “Research of present-day Earth? Why is it Students can find the answer to Reading: In Search of ET’s important to consider the atmo- this question in “Research Breath” and “Research Reading: sphere of Archean Earth when Reading: In Search of ET’s Breath.” Chemical Fingerprints.” considering how to look for life on other worlds? An infrared spectrum from the 4. Which planets in the solar Data Analysis Questions atmosphere of modern-day Earth system are in (or near) the 1. Which gases, if any, are common would show carbon dioxide, water), habitable zone? to all four planet spectra? and ozone. However, the spectrum Earth is right in the middle of the Carbon dioxide (CO ) appears in from Archean Earth would show habitable zone, while Venus and 2 all four planet spectra. carbon dioxide, water, and methane. Mars are close to, but just outside These are both suggestive of life, of, the habitable zone. 2. What does your answer to because they are gases that living question 1.) mean in terms of the organisms give off. search for life on other planets? Students can find the answer to For roughly the first billion years of Since carbon dioxide appears on this question in “Research Reading: Earth’s history, oxygen-producing, a planet even if it doesn’t have In Search of ET’s Breath.” photosynthetic life had not yet any life, carbon dioxide is not a evolved. Instead, the microorgan- good indicator for finding life. isms that dominated the planet tapped energy from gases that leaked out of Earth’s interior. Some microbes created methane as a byproduct.

ORIGINS: WHERE ARE THE ALIENS? NOVA TEACHER’S GUIDE 6 www.pbs.org/nova/origins ACTIVITY ANSWER (CONT.) Because of the methane-producing and/or methane. A planet with only mately 9,500 to 9,700 nanometers. organisms, methane was present in primitive life would likely have an Methane produces a dip in the the Archean Earth’s atmosphere. atmosphere containing carbon spectrum at a wavelength of approx- But organisms were not yet produc- dioxide, water, and methane. With imately 7,600 nanometers. One ing an abundance of oxygen, and complex life (i.e., plenty of oxygen- feature of water appears at 6,000 therefore ozone is absent in the producing organisms), a planet nanometers. spectrum of Archean Earth’s atmo- would be more likely to have a Student Final Reports spheric gases. substantial amount of oxygen in its In addition to providing information atmosphere (in the form of oxygen In more recent times, photosyn- about where and how to search for or ozone). thesis has resulted in abundant habitable planets, students’ final oxygen in the atmosphere. reports should include the follow- 6. Can the infrared portion of a Therefore, ozone is present in the ing conclusions: planetary spectrum be used to modern Earth’s atmosphere, while • finding oxygen is good, but it look for signs of life? What spectral methane is present only in trace should not be the only target. features are of interest for this? amounts. On a planet with a similar Other gases should be included— Yes, because most of the gases geology to Earth, methane levels such as and meth- produced by life create observable greater than about 100 parts per mil- ane—since life can exist even if features in the infrared part of the lion would suggest the presence of oxygen is not present. spectrum, scientists could look for life. But methane doesn’t necessarily • the composition of Earth’s ozone, methane, and water. Ozone imply life. Planets of a different atmosphere has changed as life and methane are signs of life, and geological make-up might have high has evolved, which suggests that water is an indication that the methane levels and no life. chemicals other than Earth’s planet is not so cold as to be com- present-day atmosphere may indi- pletely frozen. Water, if liquid, 5. What gases are likely to be cate the presence of life. Certain potentially provides a resource present in the atmosphere of a types of chemicals may provide for life. planet harboring life? Is the answer information about the complexity different depending on whether it’s Normal oxygen does not show up of any potential life. primitive life or advanced life? in the infrared part of the spectrum. • using only one example makes it The atmosphere of a planet harbor- So, the way to detect oxygen is to difficult to design a scientific study. ing life would likely show carbon look for one feature of ozone that In this case, having Earth as the dioxide, water, and oxygen, ozone appears at a wavelength of approxi- only place we know life exists makes it difficult to design a study

Major funding for NOVA is provided by Sprint and Google. Additional funding is to search for life on other worlds, provided by the Corporation for Public Broadcasting and public television viewers. which may or may not be similar Major funding for Origins is provided by the National Science Foundation. to life on Earth. Additional funding for Origins is provided by NASA’s Office of Space Science and the Alfred P. Sloan Foundation. 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). This material is based upon work supported by the National Science Foundation under There is clearly no ozone (O3) Grant No. 9814643. Any opinions, findings, and conclusions or recommendations expressed in this material are those of the author(s) and do not necessarily reflect the nor methane (CH4) which implies views of the National Science Foundation. that this planet probably does not harbor life.


Links NOVA Web Site—Origins Indicators of Life: Detection of Life Parker, Barry R. www.pbs.org/nova/origins/ by Remote Sensing Alien Life: The Search for Extraterrestrials In this companion Web site to the program, planetquest.jpl.nasa.gov/TPF/tpf_book/ and Beyond. find out how life could have started and Chapter_4c.pdf New York: Plenum Trade, 1998. why water is needed for life; read about the Explains why certain chemicals in Considers how life may have originated on latest discoveries in origins research; use raw the atmospheres of planets might Earth and what chemicals may be neces- data to assemble the famous Eagle Nebula be likely signatures of life. sary for produce life elsewhere. image; insert your own values into the Drake Equation; decode cosmic spectra, PlanetQuest NASA’s Origins Resources and more. planetquest.jpl.nasa.gov/ Visit the Web sites below to learn how Reviews the search for Earth-like planets individual missions in NASA’s Astro- Ask an Astrobiologist: Questions through background information, nomical Search for Origins Program About Life on Earth multimedia resources, and an atlas of are searching for the earliest stars and nai.arc.nasa.gov/astrobio/astrobio_questions. extrasolar planets. galaxies, planets around other stars, and cfm?qtype=life_earth&start=11 life elsewhere in the universe. Additional Offers a searchable database of Solar System Exploration classroom resources are available at these questions and answers and a way to sse.jpl.nasa.gov/planets/profile. sites and through NASA’s Space Science post new questions. cfm?Object=SolarSys Education Resource Directory at Includes facts about the planets in our solar teachspacescience.org Celestia Exploration Activity: system and details the status of current Solar System Overview NASA missions. Far Ultraviolet Spectroscopic Explorer learn.arc.nasa.gov/planets/main/ fuse.pha.jhu.edu/outreach/ overview.html Windows to the Universe: The Archean Provides a brief description of terrestrial and www.windows.ucar.edu/tour/link=/earth/past/ Hubble Space Telescope Jovian planets and contains information Archean.html&edu=mid amazing-space.stsci.edu/eds/ about some planetary atmospheres. Describes the changes that occurred on James Webb Space Telescope Earth during the Archean geologic period. Extreme Ecosystem jwstsite.stsci.edu/ science.nasa.gov/headlines/y2004/ Books Kepler Mission 13may_ecosystem.htm Clark, Stuart. www.lawrencehallofscience.org/kepler/ Describes the search for life in some of the Life on Other Worlds and How to Find It. Education-resources.html most inhospitable places on Earth for life London, New York: Springer-Praxis, 2000. forms: scalding heat, freezing cold, salt, Discusses what might constitute a NASA Astrobiology Institute lye, and darkness. hospitable environment for life and nai.arc.nasa.gov/teachers/teacher_topics. explores the nature of intelligence and cfm?id=8 Glossary of Planet Terms its role in evolution and survival. amazing-space.stsci.edu/glossary/ Navigator Program def.php.s=planets Darling, David. planetquest.jpl.nasa.gov/resources/resources_ Provides a glossary of astronomy Life Everywhere: The Maverick Science index.html of Astrobiology. definitions, including atmosphere, Spitzer Space Telescope New York: Basic Books, 2001. greenhouse effect, ozone layer, coolcosmos.ipac.caltech.edu/ secondary atmosphere, and more. Provides an overview of astrobiology, including a review of the conditions that Stratospheric Observatory for Infrared Hunting Planets Along the Milky Way might be necessary for supporting life, Astronomy www.spacetoday.org/DeepSpace/Stars/ what life is, and how it might evolve. sofia.arc.nasa.gov/Edu/edu.html Planets/FarawayPlanets.html Offers an in-depth look at the search Grady, Monica M. for extrasolar planets. Astrobiology. Washington, DC: Smithsonian Institution In Search of E.T.’s Breath Press, 2001. www.nasa.gov/lb/vision/universe/newworlds/ Explores the emerging field of astrobiology, ets_breath.html including the nature of extremophiles and Reviews the history of the search for planetary environments favorable to life. extrasolar planets as well as future missions designed to probe far-off worlds for the chemical signatures of alien life.

ORIGINS: WHERE ARE THE ALIENS? 8 NOVA TEACHER’S GUIDE www.pbs.org/nova/origins Origins: Where Are the Aliens? Overhead Exploring Spectra

Both of the following show the visible-light absorption spectrum for our sun. Notice that the second graph reveals more information—the amount of light absorbed is included.

The Sun’s Spectrum

Hydrogen Absorption Lines

Hydrogen Absorption Lines


Absorption Line Intensity

Wavelength in nanometers

ultraviolet visible infrared wavelengths wavelengths wavelengths Origins: Where Are the Aliens? Student Handout Mission: The Search for Life

Mission Briefing NASA is developing the technology necessary to Scientists want to build instruments that can detect search for life on other worlds. Astronomers have Earth-like planets. They want to recognize habitable discovered more than 100 planets orbiting other worlds and distinguish planets with the potential for stars in the Milky Way galaxy. They are called extra- life from those without any chance for life. Because solar planets because they reside outside our solar the solar system is the only planetary system known system. These planets are large, gaseous worlds to harbor life, it will be used as a model. like Jupiter and Saturn. Astronomers call them gas Your mission is to learn how planetary spectra may giants. Unlike Jupiter and Saturn, many of the be used as a tool to explore whether other worlds known extrasolar planets orbit very close to their are hospitable for life, and to determine whether a parent stars. mystery planet has the potential for life. Because of their composition and orbit, these planets are unlikely to harbor life. Scientists believe Procedure that Earth-like planets orbit other stars, and NASA 1 Conduct research to establish the characteristics missions are being designed to search for such a planet is believed to need to harbor life, and use worlds. Do any of these planets have the potential the information you find to answer the Research to harbor life? What factors determine a planet’s Questions listed on your “Research Journal” potential for developing life? Keep in mind that handout. astronauts cannot travel to them—the planets 2 Compare real spectra from several solar system would be orbiting stars located very far away. planets and answer the Data Analysis Questions Astronomers have found that light from a star listed on your “Research Journal” handout. carries information about its chemical composition. 3 Prepare a report that explains how planetary spectra By using spectroscopy, a technique that spreads light might be used as a tool to explore whether other into its component wavelengths (a spectrum), scien- worlds are hospitable for life. See Final Report tists can determine what chemicals make up a star. Requirements on your “Research Journal” handout (Each element or compound produces characteristic for some specific ideas to include in your report. features in the spectrum.) Scientists can also obtain 4 After you complete your report, you will be given the spectra (plural of spectrum) from planets. By analyz- spectrum of a mystery planet. Your teacher will ask ing a planet’s spectrum, astronomers can determine you to interpret the planet’s potential for harboring the composition of the planet’s atmosphere. life using the knowledge you acquired through your research and data analysis.

Earth Mars Jupiter Saturn

Seeking Other Our solar system includes both terrestrial planets, like Earth and Mars, and gas giants, like Jupiter and Saturn. Astronomers have found more than 100 gas giant planets orbiting other stars in the Milky Way galaxy; could any of them be in solar systems that have terrestrial planets suitable for life?

not to scale Courtesy of Jet Propulsion Laboratory Origins: Where Are the Aliens? Student Handout Research Journal

Use your Research Readings and relevant informa- tion from additional literature review to answer Final Report Requirements the following research and data analysis questions. Research and prepare a final report in which you Use the information in your answers to help you identify the characteristics of planets with the best write your final report. chances of harboring life, and explain how scientists hope to use planetary spectra to search for Research Questions extrasolar life.

Answering the following questions will help you Here are some specific ideas to include in your identify the qualities to look for in a planet that final report. may harbor life. 1 What gases does life (as it is currently understood) 1 Outline the qualities you would look for in a planet require? What gases does life produce? that would make it a good candidate for further 2 What is the difference between a terrestrial planet investigation in the search for life. Include the fol- and a Jovian planet? lowing in your report: 3 What does it mean for a planet to be in the • the type of planet “habitable zone”? • the location of the planet (habitable zone) 4 Which planets in our solar system are in (or near) • the composition of the planet’s atmosphere the habitable zone? • the effect of life on the planet’s atmosphere 5 How does the chemical composition of the • planets in the solar system that “fit” the terrestrial planet atmospheres differ from the description composition of the Jovian planet atmospheres? 2 Analyze the spectra—identify similarities and differ- 6 Why is it important to look at Archean Earth? ences—and explain how this solar system data can be used for conducting a search for life on extrasolar Data Analysis Questions planets. Your report should: • list the gases found in each planetary spectrum. Answering the following questions will help • identify the gases common to all spectra. you analyze the spectra. • compare similarities and differences of the two 1 Which gases, if any, are common to all four planet Earth spectra. spectra? • explain the connection between ozone (O ) 2 What does your answer to question 1.) mean in 3 and oxygen (O ) and why scientists look terms of the search for life on other planets? 2 for ozone. 3 If ozone (O ) is found, is normal oxygen (O ) 3 2 • list the gases that are good biomarkers and also present? Does the presence of oxygen those that are poor biomarkers. Defend your automatically mean life? reasoning. 4 How does the spectrum of Archean Earth compare 3 Based on your research, explain how planetary to that of present-day Earth? Why is it important to spectra might be used as a tool for exploring other consider the atmosphere of Archean Earth when worlds for life. considering how to look for life on other worlds?

5 What gases are likely to be present in the atmosphere of a planet harboring life? Is the answer different depending on whether it is primitive life or complex life? 6 Can the infrared portion of a planetary spectrum be used to look for biomarkers (signs of life)? What spectral features are of interest for this? Origins: Where Are the Aliens? Student Handout Planet Spectra

The planet spectra data shown here represent the infrared portion of the electromagnetic spectrum, where planets re-emit the energy absorbed from their parent star. The dips in the curve result when gases in the planet’s atmosphere absorb certain wavelengths of energy. Because every element and compound has a characteristic pattern of absorption, the location of these absorption bands allow scientists to identify the element or compound in the atmosphere that is absorbing the energy. Except for Archean Earth, these graphs represent real satellite data. The Archean Earth spectrum represents a scientific model of Earth’s atmosphere based on what is known about this time in Earth’s history and what is understood about modern planetary atmospheres. The major absorption bands on each planet’s spectrum have been labeled with the compound(s) responsible for the absorption.

What Is Brightness Temperature? Scientists often plot brightness temperature instead of intensity because the spectral features caused by atmospheric composition are more easily seen when plotted this way. Brightness temperature is the temperature an object must have to produce the observed intensity. Origins: Where Are the Aliens? Student Handout Research Reading: In Search of E.T.’s Breath

If “E.T.” is out there, whether in the form of If life is widespread on a planet, the planet’s intelligent beings or much simpler organisms, atmosphere should show signs of the life’s presence. scientists may soon be hot on its trail. Just as the air you exhale has more carbon dioxide and less oxygen than the air you inhale, the combined In 1995, the first planet around another sun-like star “” of all the life on a planet will change the was discovered by astronomers using Doppler of its atmosphere. If life is plentiful on the detection—a method that scientists have used to planet, these changes may be measurable. reveal Saturn-sized (or larger) planets close to their parent suns. Today, astronomers know of more than A simple premise—but what would E.T.’s breath 100 candidates for such worlds. look like? Which gases should scientists search for? Scientists know the answers for Earth, but predict- So far, all known extrasolar planets are gas giants ing how an alien biology might interact with its (also known as Jovian planets) or possibly a kind of atmosphere is no simple matter. failed star called a “brown dwarf”—both unlikely places for life as it is currently understood. Of “As astrobiologists we’ve got to be sure that we’re greater interest are Earth-size planets, which are not too Earth-centric,” says Michael Meyer, senior too small to detect with current technologies. scientist for astrobiology at NASA Headquarters in Nevertheless, many astronomers believe they exist. Washington, D.C. If all goes as planned, an important new tool for The possibility that life elsewhere has a biology that’s exploring such planets will be operating within the radically different from our own is perhaps the most next 10 to 15 years. A system of space telescopes, exciting and challenging part of astrobiology. If life collectively known as the Terrestrial Planet Finder evolves by random mutations and natural selection, (TPF), will use techniques called “coronagraphy” why should scientists expect alien life forms to be and “interferometry” to dramatically reduce the even remotely similar to Earthly life? obscuring glare from a planet’s parent star, allowing scientists to see the planet. Planets circling other stars are many light years away. (A light-year is the distance that light travels in a year—about 9.5 trillion kilometers.) Even with the TPF’s advanced optics, Earth-like worlds would appear as a single pixel of light. How, then, will it be possible to learn much about them? Amazingly, even a tiny speck of light can speak volumes about the planet from which it came. Embedded in the light from a planet are the “fingerprints” of the chemicals that have interacted Terrestrial Planer Finder with the light, including gases in the planet’s Scientists hope that this proposed design for the TPF tele- atmosphere. By splitting the light into its component scope will combine light to make Earth-size planets orbiting other stars visible in the infrared range of the electromagnetic wavelengths—through a technique known as spectros- spectrum. The mission consists of several independent copy—scientists can reveal these “fingerprints” and spacecraft orbiting in precise formation. Each spacecraft learn about the chemistry of the planet’s atmosphere. features a telescope on the shaded side of a large heat shield. NASA also plans to fly a second TPF spacecraft that could look for signs of life in visible regions of the electromagnetic spectrum. Origins: Where Are the Aliens? Student Handout Research Reading: In Search of E.T.’s Breath

“We have to be very careful about how foreign interpret the likely meaning of any methane or biology might be different from our own, especially oxygen that might be detected. Scientists hope to when you get to the bigger molecules” such as use TPF and other telescopes to measure (or at least DNA, says David Des Marais, principal investigator estimate) these other details. for the Ames Research Center team of NASA’s The best candidates for closer study would be Astrobiology Institute. located in the habitable zone, the region around a For example, people have speculated that , a system’s star where scientists expect to find liquid primary component of sand and a close cousin to water at the surface. If a planet is too hot, the water carbon, could form the basis of an extraterrestrial vaporizes and is lost from the atmosphere. If a biology. Alien life might forgo sunlight and depend planet is too cold, the water freezes. Either of these instead on the geothermal energy in hydrogen and conditions would make a planet very inhospitable compounds emitted from the planet’s interior, for life. The habitable zone for Earth’s sun starts much like the deep-sea vent ecosystems here on beyond Venus and ends before Mars. Earth. Or maybe the chemistry of alien life will be If the TPF finds a habitable planet with lots of utterly different and unimaginable. oxygen and some methane in its atmosphere, it Fortunately, the chemical constraints within which life would be a momentous discovery. But would such must function make it likely that simple molecules data really verify the existence of life? Verification such as oxygen and carbon dioxide will play the same can be difficult, especially when discussing roles in an extraterrestrial biology as they do on Earth. extraterrestrial life. Nevertheless, say astrobiologists, such evidence would be “very compelling.” “Suppose,” says Meyer, “that there is silicon-based life. [It might be] photosynthetic, and you would still end up with oxygen in the atmosphere. You could go there and the life could be completely different, but some of the chemistry could still be the same Venus Mars Jupiter Saturn [as on Earth].” Mercury Earth “The small molecules are going to be more universal,” agrees Des Marais. “Large molecules like DNA and chlorophyll represent later, highly

significant innovations of life on Earth, but also not to scale the ones that may have differed elsewhere.” habitable zone For this and other reasons, the exploration of distant Earth-like planets with TPF will focus on simple gases such as oxygen, ozone, carbon dioxide, methane, and water vapor. (For more about these chemicals, see Research Reading: Chemical Fingerprints.) In addition to their carbon dioxide and water vapor

levels, other details about these planets—such as Sources: their size, their distance from the parent star, In Search of ET’s Breath www.nasa.gov/lb/vision/universe/newworlds/ets_breath.html and how bright they appear—will help scientists Looking for Life’s Signatures planetquest.jpl.nasa.gov/science/finding_life.html Origins: Where Are the Aliens? Student Handout Research Reading: Chemical Fingerprints

Where did we come from? Are we alone in the galaxy? These questions have captured people’s imaginations for centuries. Scientists worldwide are continually seeking new ways to find answers to these questions. But how do scientists look for life on planets too far away to study by spacecraft? One way is to explore the light given off by faraway planets. Light can reveal clues about a planet’s properties. However, a planet’s visible light is dim and easily lost in the glare of its parent star. But scientists are working on technologies that will allow The same technique can be used to identify gases them to detect such planets. NASA’s Terrestrial present in a planet’s atmosphere. The surface of a Planet Finder (TPF) missions, being planned for planet emits energy in the infrared portion of the launch in the next 10 to 15 years, will include two electromagnetic spectrum (infrared light cannot spacecraft designed to examine the light being be seen with human eyes). The gases in a planet’s emitted from planets. One spacecraft will have an atmosphere absorb some of these wavelengths. instrument that will a star’s light so that the Just which wavelengths get absorbed (and end up orbiting planets will be visible. The second space- missing from the spectrum) depends on which craft will work by observing the infrared radiation atoms and molecules are present in the planet’s from the planet. This has two advantages: The first atmosphere. is that there is less glare from the star at infrared wavelengths. The second is that most molecules Because every element and all compounds produce that would be likely indicators of life emit or absorb unique patterns, scientists can identify what radiation in the infrared portion of the electromag- chemicals exist in a planet’s atmosphere by looking netic spectrum. A similar project is being designed at the patterns that are produced. for launch in 2014 by the European Space Agency. The TPF will be looking for evidence of simple Scientists hope to use spectral analysis to decode chemicals—oxygen, ozone, carbon dioxide, methane, the chemicals found in a planet’s atmosphere. and water vapor. These chemicals could be produced Spectral analysis is a method that has long been by life on the surface of a planet, and (in certain applied to deciphering the chemical composition quantities) can be absorbed by the atmosphere and of stars. If sunlight is passed through a prism, a detected by spectral analysis. Earth’s present “spectrum” of the light—a rainbow of colors—is atmosphere contains molecules of nitrogen (N2), produced. The spectrum from starlight reveals dark oxygen (O2), and trace gases that include argon (Ar), regions at certain wavelengths that are associated methane (CH4) and carbon dioxide (CO2). Water with chemical elements and compounds present vapor (H2O) occurs in varying amounts. In addition, in a star. Each element and compound produces a Earth is shielded from ultraviolet radiation by a layer unique pattern of spectral features that scientists of ozone (O3), which only forms from oxygen atoms. can use to identify a star’s composition. Origins: Where Are the Aliens? Student Handout Research Reading: Chemical Fingerprints

Analyzing an early Earth-like planet that has not Small quantities of methane and yet developed an oxygen-rich atmosphere is more might be difficult to detect with the first TPF difficult because scientists do not know much about instruments. Eventually, however, scientists hope what early life on Earth was like, or what gases were that later instruments will be better equipped to released by early life forms. However, scientists know make such measurements. This would be the best that certain types of bacteria are very ancient. These way to determine if the potential for life exists on bacteria obtain energy by converting carbon dioxide planets around other stars. and molecular hydrogen (H ) to methane and water. 2 Although certain chemicals may give clues to Both carbon dioxide and molecular hydrogen are whether a planet is habitable, they don’t reveal thought to have been relatively abundant early in the characteristics that life might take on or how Earth’s history. Early life probably evolved to take complex it could be. In addition, the chemical advantage of this. Therefore, methane would signatures scientists are searching for are based probably be an important indicator of life on an early on the understanding of what life needs to survive Earth-like planet, just as it is today. (Oxygen was on Earth. While the chemistry throughout the likely a toxic gas to early life.) Nitrous oxide, another universe is the same, no one knows all the ways byproduct of life, could also serve as an indicator it could form life. Astrobiologists are considering of possible life on another planet. what forms life may take while astronomers continue to identify extrasolar planets that may be candidates for life. Origins: Where Are the Aliens? Student Handout Research Reading: Chemical Fingerprints

The Search So, the presence of oxygen alone—while exciting The TPF will search directly or indirectly for the and significant—can’t be taken as an unambiguous following chemicals. indicator of life. Furthermore, oxygen doesn’t produce spectral lines that can be easily observed in the infrared part of the spectrum. However, another form of oxygen, ozone, made of three atoms of oxygen, does produce spectral features in the infrared. The ozone layer, which is located in Earth’s stratosphere, is important to life because it protects Earth from the Sun’s harmful ultraviolet radiation. Ultraviolet radiation can cause skin and cataracts in animals and can stunt the growth of Oxygen (O2) and Ozone (O3) many plants. Oxygen is important to life on Earth. It is mainly produced as a byproduct of photosynthesis by The presence of ozone in a planet’s atmosphere green plants and certain other organisms. is a reliable indication that normal oxygen is also present. Without oxygen, ozone could not exist. Without life, oxygen would be rare on rocky planets. Ozone is not expected to be present in significant A small amount of oxygen can be created when ultra- amounts unless oxygen is also present. violet radiation splits water vapor into hydrogen (H2) and oxygen (O). The hydrogen is very light and can The detection of ozone along with other gases, escape into space while the oxygen is left behind. But such as nitrous oxide (N2O) or methane, could be the oxygen would combine with rocks and minerals taken as convincing evidence that a planet is not on the planet’s surface in an “oxidizing” reaction to only habitable—but that it is inhabited. produce various compounds, such as rust. Gases If we do not see oxygen or ozone features in a released by volcanoes can also react with oxygen and planet’s spectrum, does that mean that life is not remove it from the atmosphere. Geological processes present? Earth’s atmosphere has contained a usually work against the accumulation of oxygen. significant amount of oxygen for only about the past Therefore, a planet with an oxygen-rich atmosphere 2 billion years. The earliest fossils suggest life existed is unusual without photosynthetic life to constantly 3.5 billion years ago. Indirect evidence for life on Earth replenish the supply. As Carl Sagan noted in a 1997 may go back even further in time. This suggests that Scientific American article, “the great concentration life was present on Earth for more than one and a of oxygen (20 percent) in Earth’s dense atmosphere half billion years before any sign of oxygen or ozone is very hard to explain by [any means other than appeared in its atmosphere. life.]” The same would likely be true of planets orbiting other stars. Where is it in the spectrum? Oxygen does not show up in the infrared part of the spectrum. However, scientists do know of non-biological But ozone produces spectral features at infrared processes that can result in an oxygen-rich wavelengths. One feature is located at about atmosphere. For example, ultraviolet light from 9,500–9,700 nanometers. the sun can break apart carbon dioxide molecules to form (CO) and oxygen. Or, as stated previously, ultraviolet sunlight (or starlight) can break apart water molecules into hydrogen and oxygen. Origins: Where Are the Aliens? Student Handout Research Reading: Chemical Fingerprints

On a planet with a similar geology to Earth, methane levels greater than about 100 parts per million would suggest a significant probability of the presence of life. But methane would be a more ambiguous discovery than oxygen, because planets Carbon Dioxide (CO2) of a different geological make-up might produce Carbon dioxide is a gas that increases a planet’s abundant methane without life. However, a spec- temperature by a process called the greenhouse trum that shows signs of both methane and oxygen effect. Carbon dioxide is also used (along with would provide a very strong indication of life. water) by plants to produce sugars used for energy. Oxygen is released into the air as a result. This Where is it in the spectrum? Methane produces process is called photosynthesis. Animals consume spectral features at infrared wavelengths. One of plants and oxygen and release carbon dioxide and these is located at about 7,600 nanometers. water through respiration. If scientists found carbon dioxide, it would mean that the planet might be able to support plant life, but it is not a strong indicator of life by itself, since planets can have carbon dioxide atmospheres without life. Venus and Mars are examples of this.

Where is it in the spectrum? Carbon dioxide produces spectral features at various infrared Water Vapor (H2O) wavelengths. A prominent feature is located at Water vapor is one of the greenhouse gases that about 15,000 nanometers. absorbs infrared radiation emitted from Earth’s surface and helps warm the planet. Water vapor is also the source of all clouds, rain, and snow. It is an important influence on weather and climate. If a scientist found water vapor in the atmosphere of another planet, it would be a good sign that there might be liquid water on its surface. Water is the most important ingredient for life on Earth, so another planet with water on it would be a Methane (CH4) promising place to look for life. Scientists suspect that for roughly the first billion years of its history, life on Earth had not yet evolved Where is it in the spectrum? Water vapor produces oxygen-producing organisms. Instead, the micro- spectral features at many infrared wavelengths. organisms that dominated the planet tapped the One feature is located at about 6,000 nanometers. energy in gases that leaked out of Earth’s interior and some microbes created methane as a by-

product. (Methane and nitrous oxide, another gas Sources: that can be associated with life, are not abundant In Search of ET’s Breath www.nasa.gov/lb/vision/universe/newworlds/ets_breath.html in present-day Earth’s atmosphere.) Looking for Life’s Signatures planetquest.jpl.nasa.gov/science/finding_life.html The Terrestrial Planet Finder, May 1999. planetquest.jpl.nasa.gov/TPF/tpf_book/index.cfm Origins: Where Are the Aliens? Student Handout Research Reading: Terrestrial and Jovian Planets

With the exception of Pluto, planets in our solar of Jovian planets. In the solar system, Jovian system are classified as either terrestrial planets are located farther from the sun than (Earth-like) or Jovian (Jupiter-like) terrestrial planets, and are therefore cooler. planets. Terrestrial planets include Scientists have found more than 100 Jovian plan- Mercury, Venus, Earth, and Mars. ets around other stars. The majority of the extraso- These planets are relatively small lar Jovian planets that have been discovered so far in size and in mass. A terrestrial are closer to their stars than the Jovian planets in planet has a solid rocky surface, the solar system are to the sun. with deep in its interior. The atmospheres of the Jovian planets in our solar In the solar system, these system are made mostly of hydrogen and helium. planets are closer to the sun and are therefore Compounds containing hydrogen, such as water, warmer than the planets located farther out in the , and methane, are also present. Differ- solar system. Future space missions are being ences in the amounts of these trace gases and designed to search remotely for terrestrial planets variations in the temperatures of these planets around other stars. contribute to the different colors seen in images The layers of gases surrounding the surface of a taken in visible light. While scientists expect the planet make up what is known as an atmo- atmospheres of Jovian planets in other solar sphere. The atmospheres of the terrestrial systems to be composed mainly of hydrogen and planets range from thin to thick. Mercury helium, they have not yet measured the properties has almost no atmosphere. A thick of their atmospheres. atmosphere made mostly of carbon Pluto, the most remote planet in our solar system, dioxide covers Venus, trapping heat and might be little more than a giant comet. Pluto raising surface temperatures. Clouds on resembles the icy, comet-like objects orbiting the Venus form from sulfuric acid. Earth’s Sun outside of Neptune’s orbit, rather than either atmosphere is 77 percent nitrogen, 21 percent the rocky terrestrial planets or the Jovian planets. oxygen, and 1 percent argon, with variable amounts Factors that distinguish Pluto from the terrestrial of water vapor, and trace amounts of other gases. and Jovian planets include its composition (ice, White clouds of water vapor hide much of Earth’s , and frozen gases), changing atmosphere, surface in views of Earth from space. Mars has a small size, comparatively large moon, and its very thin atmosphere containing mostly carbon elliptical orbit around the Sun. dioxide, with nitrogen, argon, and trace amounts of oxygen and water vapor. The atmosphere also contains thin water and carbon dioxide clouds, and is frequently affected by dust storms. Jovian planets include Jupiter, Saturn, Uranus, and Neptune. These planets have larger sizes and masses. Jovian planets do not have solid surfaces. They are sometimes called gas giants because they are large and made mostly of gases. Small amounts of rocky materials are only found deep in the cores Origins: Where Are the Aliens? Student Handout Mystery Planet’s Spectrum

You have now learned what chemicals scientists are looking for to indicate the possibility of life on another planet. Take a look at the chemical spectrum of the atmosphere on this mystery planet. What do you think? Can it support life? Defend your reasoning.