Searching for Planets Suitable for Life

Article excerpt

Radio astronomers first detected planets beyond our solar system in 1991 (NASA 2012). Now, armed with state-of-the-art telescopes and other high-tech tools, astronomers spot new planets at an astonishing rate. Indeed, it seems NASA announces the discovery of new planets orbiting distant stars every few months. A total of 843 such planets (in 665 planetary systems) have been identified as of October 2012 (Schneider 2012).

Astronomers can infer and mea-sure interesting properties of these planets, such as their size and mass, distance from the stars they orbit, and types of atmosphere they might have. With each new planet, astronomers close in on their ultimate goal: finding a place in space that other living things might call home. The existence of life beyond Earth presents many unanswered questions, making this topic exciting to scientists and students alike.

This article describes the High-Adventure Science curriculum unit "Is There Life in Space?" (see "On the web"). This free online investigation, developed by The Concord Consortium, helps students see how scientists use modern tools to locate planets around distant stars and explore the probability of finding extraterrestrial life. This innovative curriculum incorporates dynamic computer models using the NetLogo and Molecular Workbench modeling environments (see "On the web"), real-world data, and a video about planet hunters. It is designed for five 50-minute sessions, which can be done in class, at home, or both.

Doing the "wobble"--finding planets by detecting star motion

The space investigation begins by asking students, "If scientists want to find planets, why don't they just look for them?" There are billions and billions of stars in the universe. Many have planets orbiting them, so finding exoplanets should be simple, right? Not really. The dim light reflecting off of a planet's surface gets lost in the glare of the sun it orbits. That is why scientists need other ways to detect them.

Planets' gravitational pull causes stars they orbit to "wobble." Using computer simulations, students can change the size, density, and location of an orbiting planet and observe how the planet's mass affects the gravitational force exerted on the star. They can see the star shift, or wobble. Scientists look for star motion to detect evidence of a planet. Since this star shift is very subtle, scientists rely on measuring the Doppler effect, a change in light that results from the star moving toward Earth, then away from it. As the star moves toward Earth, the wavelengths of light it emits are compressed (shortened); as it travels away, the wavelengths grow longer (the "red shift" observed for receding stars). By measuring a star's spectrum over time, scientists can detect Doppler shifts caused by a planet or planets moving the star toward and away from Earth (Freudenrich, Kiger, and Gerbis 2012).

Students then consider how the angle of a planet's orbit (as seen from Earth) affects our ability to detect it. Finally, students can change both the mass and orbital angle of a planet and examine how these factors affect the shape of a velocity graph created by the shifting wavelengths of light (Figure 1).

Students also learn about some of the real challenges faced by scientists, including technological limitations, interference from Earth's atmosphere, and distortions introduced by space dust and gases that can blur the "vision" of telescopes. All these can cause noise (fluctuations) in the data. The model introduces a "telescope precision" slider, a variable students use to think about how breakthroughs in engineering increase scientific knowledge as the noise-to-data ratio changes.

Star crossed--the transit method

When a planet crosses in front of a star as viewed from Earth, the event is called a transit. For example, when Venus passed between Earth and the Sun on June 5-6, 2012, people could see the shape of Venus in front of the Sun. …

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