1. Ask:

• Why might you need to think like a detective to find life in the solar system? (If there is life beyond Earth, it is not obvious. People need to collect hints and figure out what they tell us.)

2. Tell students:

• I’m going to describe a scene. Based on what I tell you, what do you think happened? One evening, you visit a park. No one is there. A balloon is caught in a tree branch. The trash barrels are filled with cups, paper plates, and wrapping paper. A piece of a broken piñata hangs on a string from a branch. (There was a birthday party earlier in the day. People broke open a piñata, ate food, drank beverages, opened presents, and cleaned up.)

3. Demonstration A — Astrometric measurement: Planets make stars wobble
Tell students that the large clay ball represents a star, the small clay ball represents a planet, and the straw represents the gravity that keeps the balls in position relative to one another. Point out that these two objects form a system. Spin the system. When the system rotates, both objects are affected. Explain that the point of rotation is not the big “star,” but is a point along the straw where the string balances the balls. This is the center of the system.

Ask:

• Does the star stay in one place or does it move? Tell students to close one eye and hold up a finger at arm’s length between their open eye and the spinning system.
• Now that you have your finger as a reference point, can you see the star move back and forth? (All planets have gravity and pull on the stars they orbit. The pull makes the star wobble back and forth. The bigger the planet, the greater its gravity. And the greater its gravity, the bigger the star’s wobble. Telescopes like NASA’s Space Interferometry Mission look for the wobble of stars, which tells scientists that there is a planet orbiting it.)

4. Demonstration B — Transits: Planets can dim a star’s light
Introduce the idea of a planet transiting a star by passing your fist in front of a light and by showing the images of the transit of Venus in the presentation slides. Aim a flashlight or small lamp at a wall, projecting a circle of light on it. Tell students that the light is a star, and that you will pass a series of different-sized balls (i.e., planets) between the star and the wall. Their job is to figure out the smallest ball that dims the star’s light. (Variables include the distance between the light and wall, the size of a ball, and how far you hold it from the light. For best effect, pass each ball close to the light.) Start with the largest ball.

Ask:

• Did this planet dim the star? (All balls will dim the light, though most people’s eyes are not sensitive to minor dimming.)
• Why does a ball dim the light? (It blocks some of the light.)
• Which ones caused noticeable dimming?

Explain that telescopes like the Kepler space telescope are very sensitive. Scientists detect planets by finding stars that repeatedly dim and brighten as the planet orbits the star.

5. Demonstration C — Biomarkers: Our senses work like sensors on a spacecraft
Pass around the scent jars. Ask students to tell you what’s in each jar. Tell them that their noses can detect tiny bits of the food items that are present in each container. This helps them figure out what it is even though they can’t necessarily see it.

Explain that one way to find life is to look for telltale signs that organisms leave behind.

6. Demonstration D — Biomarkers: Life has a special chemistry
Show students the plastic bags of salt and sugar. Ask them to describe what’s inside. (Similar-looking white, granular powders.) Pour a teaspoon of salt onto one ceramic plate and a teaspoon of sugar onto another. Using a gas match, try to light the salt. (It won’t light.) Repeat with the sugar. (It will turn black and burn.)

Ask:

• Which powder came from something living? How do you know?

Explain that salt is a mineral that contains no carbon. Carbon is an essential molecule of life. Since carbon compounds burn, the fact that salt doesn’t burn suggests that it contains no carbon and is not a product of a living organism. Sugar comes from plants. It contains carbon, which turns black when burned. (Mention burned toast and meat.) Life uses carbon in so many ways that scientists think that all life will use carbon. As a result, spacecraft and rovers looking for chemicals associated with life look for carbon-based compounds.