Origins: Where are the Aliens?
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.
- copy of the "Exploring Spectra" student handout
- copy of the "Mission: The Search for Life" student handout
- copy of the "Research Journal" student handout
- copy of the "Planet Spectra" student handout
- copy of the "Mystery Planet's Spectrum" student handout
- copy of the "Research Reading: In Search of ET's Breath" student handout
- copy of the "Research Reading: Terrestrial and Jovian Planets" student handout
- copy of the "Research Reading: Chemical Fingerprints" student handout
- access to print and internet resources
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.
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
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.
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.
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
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.
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).
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
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.
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.
Have students present their reports to the class. (See Activity
Answer for more information on what to look for in student reports.)
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
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
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
Perform a simulated mission to another planet to search for evidence of life or
the conditions where life might form.
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
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
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
What gases does life (as we know it) require? What gases does life
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
What is the difference between a terrestrial planet and a Jovian
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."
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
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."
Which planets in the solar system are in (or near) the habitable
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."
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
Which gases, if any, are common to all four planet spectra?
Carbon dioxide (CO2) appears in all four planet spectra.
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.
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.
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
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.
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).
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
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.
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
Ask an Astrobiologist: Questions About Life on Earth
Offers a searchable database of questions and answers and a way to post new
Celestia Exploration Activity: Solar System Overview
Provides a brief description of terrestrial and Jovian planets and contains
information about some planetary atmospheres.
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
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
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.
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
Grady, Monica M.
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 teachspacescience.org
Far Ultraviolet Spectroscopic Explorer
Hubble Space Telescope
James Webb Space Telescope
NASA Astrobiology Institute
Spitzer Space Telescope
Stratospheric Observatory for Infrared Astronomy
The "Mission: The Search for Life" activity aligns with the following
National Science Education Standards:
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
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