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        Development of a Habitable Planet

        Students investigate the origin of the elements, the process of planet formation, the evolution of life on Earth, and the conditions necessary for life as we know it.

        Lesson Summary


        Earth is just one of innumerable objects in the universe, but it is the only object known to be able to support life. How did the planet Earth develop into the life- bearing planet that it is today? Is it possible that other habitable worlds have also developed? In this lesson, students investigate the origin of the elements, the process of planet formation, the evolution of life on Earth, and the conditions necessary for life as we know it. Students research particular events in the history of Earth that have led to its present state, synthesize their findings with the class, and contemplate the rarity of habitable planets.


        • Identify and sequence the major events that caused Earth to develop into the planet we know
        • Understand where the ingredients for Earth originated, including the conditions necessary for life
        • Consider the likelihood of other habitable worlds

        Grade Levels: 6-8 , 9-12

        Suggested Time

        Two to three class periods

        Multimedia Resources


        10 index cards

        Before the Lesson

        If possible, arrange computer access for all students to work individually or in pairs. Write each of the following events on an index card:

        • Formation of the solar nebula
        • Formation of a rocky planet circling the Sun
        • Formation of the Moon
        • Development of liquid water on the planet
        • Appearance of anaerobic life
        • Development of aerobic life
        • Significant accumulation of oxygen in the atmosphere
        • First ice age
        • Cambrian Explosion
        • Mass extinction of dinosaurs and other forms of life

        The Lesson

        Part I: Investigating the Formation of Earth

        1. Ask students to think about the origin of Earth. Working alone or in pairs, have students explore the Infrared Search for Origins Interactive, focusing on the "Star Formation" and "Planetary Systems" sections.

        2. Divide the class into 10 groups and hand each group an index card with the name of an event. Explain that each group will conduct their own research for their assigned event and that they should be prepared to discuss their findings with the class. You may want to remind them that the answers are not always clearly defined. They should be able to answer, to the best of their ability:

        1. The conditions prior to the event
        2. The event itself: What happened? What caused it? When and how did it happen?
        3. The conditions after the event and its impact on the future of Earth

        3. Before having the students disperse to do their research, show the class the following video segments to introduce them to some history of the evolution of Earth and life. These videos do not directly relate to one another and do not have to be shown in order - use as time allows. As students watch the video segments, ask them to record one thing that they learned from each video and one thing that they would like to find out more about. If there is time, have students share their thoughts with the class.

        1. The Elements: Forged in Stars Video explains the role of stars in creating the elements found on Earth and throughout the universe.
        2. The Origin of the Moon Video shows how samples of rock from the Moon led scientists to theorize that the Moon was formed from Earth materials.
        3. Jupiter: Earth's Shield Video shows how Jupiter's gravity has protected Earth and why scientists are looking for a similar planetary setup for extrasolar planets.
        4. Global Warming: The Physics of the Greenhouse Effect Video explains how the greenhouse effect works on Earth and how humans are affecting it.
        5. Ingredients for Life: Water Video demonstrates the importance of water to life and explores the possibilities for extreme forms of life.
        6. Life Before Oxygen Video explains how our once oxygen-free atmosphere changed dramatically when primitive bacteria evolved the capacity to harness solar energy through photosynthesis, which produced oxygen as a by-product. Thought-provoking questions are included in the resource and should help students grasp the enormity of this development.

        Part II: Putting It into Perspective

        4. Have students study the following interactive resources: Deep Time Interactive and The Wall of Time Image. Both resources present a detailed, interactive timeline of events. They will help the students develop an understanding of their event and how it fits in with geologic time.

        5. Have each group present its findings. Using the information they have gathered as a class, students should be able to piece together their own geologic timeline without help from the instructor. This can be done with students standing in front of the room and arranging themselves in the correct order, or by taping a representation of their event on a timeline wall in the classroom.

        Because of the complexity of these events and the interpretation of evidence, there may be several different "correct" versions of the timeline that the students create. Those who have collected the most evidence and have the most persuasive arguments may dictate the final result.

        6. Ask students to consider the seemingly unique conditions on Earth. Allow time at the computers to look at the Life's Little Essential: Liquid Water Document, the Mars Dead or Alive: Mars Up Close Interactive, and the Caves: Extreme Conditions for Life Video. Discuss the following:

        1. Why do most scientists think that water is necessary for life to exist?
        2. How did the discovery of extremophiles change views about life?
        3. Do you think that it is possible for life to exist on Mars?
        4. Do you think that there are other habitable planets in the universe? What about habitable moons?
        5. Did you find any answers to the questions that you had posed when watching the video segments earlier (in step 3)? Which of your questions did these resources answer?

        7. Lead a debate about the search for extraterrestrial life. Now that we understand that there are billions of galaxies in the universe, with hundreds of billions of stars in each, it seems quite probable that there may be other life or other planets similar to Earth. And now that we have found extreme forms of life in places on Earth that were previously thought unlivable, it seems possible that life may be thriving in other non-Earthlike worlds.

        1. Divide the class into teams for and against further research to answer the question of whether or not humans are alone in the universe.
        2. Issues to address may include the following: the timescale for life to develop, the limitations of space exploration, the conditions necessary for life, methods to search for life as we know it, methods to look for life NOT as we know it, and the search for extraterrestrial intelligence.

        Check for Understanding

        Have students discuss the following:

        1. What conditions are necessary for life as we know it?
        2. How does solar system formation affect whether or not life will develop on a planet?
        3. How does Earth's atmosphere impact life?
        4. What do you think would happen if a giant asteroid hit Earth now?

        The Digital Library for Earth System Education ( offers access to additional resources on this topic.


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