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NOVA scienceNOW: Hunt for Alien Earths
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Viewing Ideas
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Before Watching
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Discuss what it means for a planet to be habitable.
Our solar system consists of planets, moons, asteroids, comets, gas, and dust. Other than the central star (or stars), planets are the largest objects in a solar system. Planets orbiting stars other than our sun are called "extrasolar planets" or "exoplanets," and are a focus of scientific searches for life in the universe.
Despite the fact that there are eight major planets right here in our own cosmic neighborhood, Earth stands alone as the only known "inhabited" planet. As scientists probe the skies for new worlds around other stars, astronomers seek an answer to the question "Are there other Earth-like planets in the universe?"
| Planet |
Orbital Period (yr) |
Avg. Distance from Sun (AU) |
Diameter
(Earth = 1) |
Mass (Earths/Jupiters) |
Primary Composition |
| Mercury |
0.24 |
0.39 |
0.382 |
0.06 / 0.0002 |
Rocky with no atmosphere |
| Venus |
0.62 |
0.72 |
0.949 |
0.82 / 0.0026 |
Rocky with dense atmosphere |
| Earth |
1.00 |
1.00 |
1.000 |
1.00 / 0.0031 |
Rocky with moderate atmosphere |
| Mars |
1.88 |
1.52 |
0.532 |
0.11 / 0.0003 |
Rocky with thin atmosphere |
| Jupiter |
11.86 |
5.20 |
11.209 |
317.8 / 1.0000 |
Gaseous Hydrogen, Rocky core |
| Saturn |
29.46 |
9.54 |
9.44 |
95.2 / 0.2996 |
Gaseous Hydrogen, Rocky core, Traces of ice |
| Uranus |
84.01 |
19.18 |
4.007 |
14.6 / 0.0459 |
Gaseous Hydrogen, Helium, Methane, ices, Rocky core |
| Neptune |
164.8 |
30.06 |
3.883 |
17.2 / 0.0541 |
Gaseous Hydrogen, Helium, Ammonia, Methane, Rocky core |
Discuss what it means for a planet to be habitable and what characteristics astronomers should be searching for in their quest to find other Earth-like planets. Working in small groups, have students use information in the table above, combined with what they know about Earth and the conditions required for life on our planet, to answers to the following questions:
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What makes Earth different from other planets in our solar system? Why can life survive at the surface here? (Earth is a rocky planet rather than a gaseous one. Because of its size, Earth has enough gravity to hold an atmosphere that can insulate the planet, protect it from excess harmful radiation, and produce enough pressure to maintain water at the surface. Because of its distance from the sun, Earth has a moderate average surface temperature. These features make Earth just right for supporting life as we know it.)
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As we look for planets around other stars, do you think it will be easier to find Earth-like planets or some other type of planet? Why? (Even within our own solar system, Earth is a relatively small planet, which makes finding Earth-sized planets around other stars extremely difficult. Larger planets, similar to Jupiter and Saturn, should be easier to find.
Have groups share their conclusions with the rest of the class. Introduce the "Goldilocks Principle," which is the idea that to have water and life at or near the surface, a planet can't be too hot or too cold but must be "just right." Show students this image of the solar system with its "habitable zone" highlighted (distance from the sun within which liquid water can exist).
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Check the latest exoplanet total.
As of June, 2009, 347 exoplanets have been discovered. To get students excited about the search for other worlds, have them visit the PlanetQuest Web site to check the current tally and explore the New Worlds Atlas for details about the planets that have been discovered to date. Ask students to search for answers to the following questions and discuss their findings with the rest of the class:
- What is the date of the most recent discovery?
- What is the most common size, mass, and composition of planet found so far?
- How do the masses of the discovered planets compare to Jupiter? To Earth?
- How do the orbital periods of the discovered planets compare to those in the solar system?
- What is the closest distance to our solar system at which a planet has been discovered? The farthest?
- What are the characteristics of the most Earth-like planet discovered so far (e.g., size and mass of the planet, type of star it orbits, distance from the host star's habitable zone, etc.)?
- How many star systems with multiple planets have been discovered?
- Do any planets that have been discovered seem to be similar to Earth?
Lead a discussion about what the students learned about scientists' progress in finding planets beyond our own solar system. During their exploration of the PlanetQuest site, students should have found that many of the planets discovered thus far are significantly larger than Earth and even Jupiter. Given that exoplanets are so far away and difficult to detect, this should not be a surprise. Ask students to speculate about what might have to happen before Earth-like planets are discovered (Improve precision of detection methods and longer series of observations).
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Discuss how to recognize life.
At its heart, the search for planets beyond our solar system is a search for life in the universe. Even on Earth, life is diverse and sometimes difficult to identify. This raises an important question as we search the skies for Earth-like worlds: "Would we recognize life if and when we encounter it?" Lead a discussion with students about what it means for something to be alive. (Although the specifics may vary, scientists agree that all life forms need sources of energy, nutrients, and water. You may want to give examples of organisms on Earth that live in extreme conditions.) Then ask them to come up with a list of environmental conditions under which life can exist. (If students base their answers on what they know about most life on Earth, their list will likely include conditions such as air to breathe, liquid water for drinking, moderate temperatures, nutrients, and an energy source.)
After Watching
Investigate the transit method for detecting exoplanets.
Scientists have detected planets orbiting other stars with a technique known as the transit method, by which a planet can be detected based on the effect it has on a star's apparent brightness as the planet passes in front of it (as seen from Earth). Remind students than an eclipse is also a transit event. From Earth, we can view the spectacle of a solar eclipse—where the moon passes directly between Earth and the sun, blocking out the sun's light. But the change in a star's brightness is far less dramatic during a planetary transit. This is due to the great distance between the Earth and the exoplanet. (Remember that the Moon is smaller than any exoplanet discovered to date, yet the proximity of the Moon to Earth is what causes it to be seen as a disk in the sky.)
Test how a planet passing in front of a star causes the star to temporarily appear dimmer. To do this, you will need:
- an overhead projector or slide projector
- balls/spheres of varying sizes, ranging from a pin with a round ball head to spheres a few inches across
- thread or thin string
- Set up the slide projector so it shines a beam of light onto a screen or blank wall to represent the light coming from a star. Adjust the distance between the light source and the wall so that the light disk is approximately 1-2 meters in diameter.
- Make several "planets" in a variety of sizes. Hang the balls on the thread and pass them in front of the light.
- Turn off the lights in the room. Start with the largest planet first and work your way down to the pin, passing each model planet in front of your model star about one-quarter of the distance to the wall. The ideal "orbiting" distance will vary slightly depending on your light source. Whatever distance you choose, keep it consistent throughout the demonstration. Have students observe the light shining on the screen as each planet transits the star. Are they able to detect the planet passing across the star?(The transit of smaller balls/spheres will be less noticeable that the transit of larger ones.)
- Ask students what role orbital period plays in the transit detection method? (Planets with shorter orbital periods have more-frequent transits and therefore are easier to detect. Seen from other stars, our Jupiter would only have one transit event every 11.86 years!)
- Similarly, what role does size (diameter) play? (A planet with a larger diameter creates a more noticeable transit than a smaller planet. From the transit method, a good estimate of the planet's size can be made if the size of the star is known.)
- Have students face away from the screen as you pass planets in front of the star. Are they still able to sense a drop in brightness? Point out that even with large telescopes it's not possible to "see" the transit directly, what is observed is the apparent change in the total brightness coming from the star.
- Discuss how size and distance affect our ability to detect a transiting planet. Explain that even though our eyes might not be able to perceive small planets passing in front of distant stars, astronomers have instruments sensitive enough to detect changes in brightness: below one-tenth of a percent. (Note that if we were to detect a transit of Earth across the sun at the distance of a nearby exoplanet star, it would be only be 0.08%, right at this detection limit!)
- Demonstrate that the conditions for detecting a transit rely on the condition that the orbit of the planet is at just the right angle (almost exactly in line) to produce a transit as seen from the Earth. For example, what if the planet's orbit is "pole on" from the point of view of the Earth or if its orbit is tilted so that only part of the planet is seen to eclipse the disk of the star? (Transit observations depend on the planet passing between its star and the observer. Consequently, there could be many stars with planets that can't be detected by the transit method simply because Earth is not in the right plane to see them!)
Extension: Show students some images from the 2004 transit of Venus as an example of how tiny a planet looks compared to its parent star, even at relatively close distances by astronomical standards.
Investigate how scientists use the wobble method to find planets they cannot see.
Some of the most important scientific discoveries of all time were of things we cannot directly see—like X-rays, electrons, and black holes. However, we know they exist because we can see the effect they have on objects or substances around them. Still, in a universe so vast, how can we expect to find something as tiny as a planet, which produces no light of its own, against a blinding backdrop of billions of stars? Fortunately, planets exert a small, yet detectable, influence on their parent stars. A planet and a star (or a planet and a moon) orbit about a common "center of mass"—this causes a "wobble" in the velocity of the star, and scientists have successfully used the "wobble method" to infer a planet's presence—even though we cannot see the planet directly.
Demonstrate this wobble with a simple physical model made from two one-hole rubber stoppers, a thin wooden dowel, and a piece of string. The two stoppers need to be very different in size so that the string can be slid very close to the "star" to be balanced.
- Attach one rubber stopper to each end of the wooden dowel so that you have a "barbell" configuration. In this model, the larger rubber stopper represents the star and the smaller stopper is the planet.
- Tie a piece of string (about 1 foot long) around the dowel, loosely enough that the knot is still able to slide. Move the loop as close to the larger stopper as necessary to balance the model. The system should be able to spin freely on the string.
- Give the "star" and the "planet" a gentle spin and ask students to observe how the "star" behaves as the system spins around. (They should be able to see a slight wobble in the
"star" stopper due to the mass of the "planet" stopper.)
- Ask students how they think the wobble would change if the "planet" were more or less massive. How effective do they think this method might be for discovering Earth-like planets? (Use the table above to compare the masses of discovered exoplanets and to masses of planets in our solar system.)
- Ask students:
- What role does orbital period play in the ability to detect exoplanets? (Planets that orbit closer to its parent star are easier to detect because more "wobbles" happen in a fixed period of time. For example, while Saturn has one-third the mass of Jupiter, it would take many years before it's "wobble" could be detected because of its 29-year orbital period. Precisely determining an exoplanet's period with the "wobble" method is best done over several orbits, which could mean many decades of observations!)
- What role does mass and size play? (Unlike the transit method, the diameter of the planet doesn't affect the size of the "wobble," only its mass. However, the measured mass is only an underestimate. Like the transit method, the angle of the orbit as seen from the Earth affects our ability to accurately measure the planet's mass. Also like the transit method, planets orbiting stars "pole on" as seen from Earth are not detectable with the "wobble" method, even if the planet is massive or has a short orbital period!)
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How can both methods be used together? (An exoplanet showing a transit will also show a wobble if the planet's mass is large enough. Because the orbital alignment is necessarily "edge-on" to produce a transit, the measured mass from the "wobble" method will be reasonably accurate. Because the transit gives an indication of an exoplanet's size, and the "wobble" method of a planet's mass, then the two combined give its overall density. This will confirm whether a planet is largely gaseous or rocky.)
Finding life in the universe.
Ask students if they think life can exist in more extreme environments (e.g., high or low temperature, little or no oxygen, little or no sunlight). Have students explore the information on some of the following Web sites. Then discuss their reactions to these types of organisms and their preferred environments. Ask them to think about what life looks like. Is it easy to identify? Then, challenge students to try to identify what's life and what's not. Spotting life is trickier than they might think!
Links
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Teachers' Domain — The Habitable Zone
Discusses the region around a star where environmental conditions are conducive to the existence of liquid water, and therefore to life.
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NASA/JPL — Planet Quest
Features current information about the search for other worlds, including a database of extrasolar planets.
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PBS: Seeing in the Dark — Extrasolar Planets
Describes methods of detection for exoplanets.
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NASA Astrobiology Institute — Education and Outreach
Offers astrobiology classroom activities and professional development opportunities.
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NASA Astrobiology Institute Educator Resource Guide
This guide contains five inquiry- and standards-based activities for grades 5-8 and three math extensions
spanning topics from defining life to determining the chances for extraterrestrial life.
Activity Author
Erin Bardar is a curriculum developer in Cambridge, MA. She has a bachelor's degree in Physics from Brown University and a doctorate in Astronomy from Boston University. Additional contributions by Bob Donahue, who earned his bachelor's degree in Astronomy from Villanova University and a doctorate in Astronomy from New Mexico State University.
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