How to Discover a Planet

How to Discover a Planet

Over the past score of years, scientists have discovered hundreds of what are known as exoplanets—planets that orbit stars outside of our own solar system. Different groups of scientists worldwide have used a variety of methods to detect these planets. In this lesson we will investigate a method that has been quite fruitful in finding exoplanets as a result of the Kepler Mission, launched by NASA in 2009. Another similar mission is CoRoT, led by the French Space Agency. These missions identify exoplanets by a method called transit, in which the brightness of a star decreases when a planet passes directly between the star and the observer. Many of us have observed a similar phenomenon on earth that occurs when the planets Mercury or Venus transit the Sun.

In this lesson, you will simulate the transit of a planet over its star by placing the littleBits fan in front of a bright LED and use a light sensor to measure how the brightness of the light changes as the blades of the fan pass between the bright LED and the light sensor. The fan blade corresponds to a planet and the bright LED to a star. The fan spins rapidly, but that is no problem thanks to the littleBits Arduino module! You will run a sketch (Arduino lingo for computer program) that automatically records the brightness changes as the fan blades “transit” the LED. You then transfer the data to an Excel program that prepares a graph of brightness versus time for you to analyze.

Duration: About 1 class period

Middle School (ages 11-13)
High School (ages 14-17)



fan (1)
light sensor (1)
power (1)
bright led (1)
Arduino (1)
wire (2)

Computer with the Arduino IDE installed 1
Mounting Boards (optional) 2
Velcro tape (optional) 1
Wood block (1″ x 2″ x 8″) (optional) 1
Excel spreadsheet software 1


STEP 1 : Prepare the Circuit

This project requires these littleBits modules in the order shown:

power>bright led>fan>wire>light sensor>wire>Arduino. Note that the wire must be attached to the Arduino module at input a0.

If you have three people in your lab group who can hold the bright LED, the fan, and the light sensor steady, then you can skip step 2. It is important that the bright led, an opening in the fan, and the sensor on the light sensor module are all aligned carefully.

Begin with a freshly charged battery or use USB power to power your circuit. The light sensor should be set to high sensitivity and set to sense light, not dark.

STEP 2 : Set Up the Circuit on Mounting Boards

How to Discover a Planet1

The three pictures below show how the circuit can be set up on mounting boards. The mounting boards are attached to the wood block with Velcro. The modules are arranged as shown. It is important that the bright led, an opening in the fan, and the sensor on the light sensor are all aligned carefully. The fan is attached to the piece of wood with Velcro.

STEP 3 : Ready the Arduino Module for Data Collection

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Note that the Arduino module’s micro-USB cable must be connected to your computer, and the computer must have the Arduino IDE software installed. THE ARDUINO SWITHCES SHOULD BOTH BE SET TO ANALOG.

Power up the Arduino module. Start the Arduino IDE software. Select Tools>Board>Arduino Leonardo. Then select Tools>Serial Port and select the serial port that the Arduino will use for communication. This will depend upon whether you are using a Windows, Mac, or Linux machine. Open the sketch file called FanSpeed.ino. Upload the sketch to the Arduino module. You will see the yellow rx/tx LEDs blink on the Arduino module while the sketch is uploaded. After the blinking stops, you will then have 15 seconds to start the serial monitor by selecting Tools>Serial Monitor from the IDE. After the 15 seconds have elapsed data collection will automatically begin and results will appear in the Serial Monitor.

The Arduino Sketch (program) on the Arduino module has collected numerous data on brightness as a function of time for just a small fraction of a second (0.1 sec to be precise). The Serial Monitor will look similar to that in the image below.

STEP 4 : Transfer Data to the Excel File

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When you are done collecting data, then you can transfer the data from the serial monitor to Excel. Here is how to do that. First, uncheck Autoscroll in the bottom left corner of the serial monitor. Use your mouse to drag and select all of the data beginning with the data line containing the column headers “Time Brightness”. Open the attached Excel .xlsx file called Brightness vs Time Template.xlsx, right-click on cell A1 and select Paste.

You should see the data in the Excel workbook on the far left. In addition you will see a chart displaying a graph of brightness versus time. Your chart should look similar to the chart displayed below. Note that the time axis is in microseconds (millionths of a second).

STEP 5 : Data Analysis

Let’s imagine that each valley in the brightness graph represents an exoplanet transiting its star. Therefore the time between valleys (or between transits) would represent the period of revolution of the exoplanet around its star. Let’s also assume that the time axis is in units of earth days rather than microseconds. What is the period of revolution of the exoplanet in days? in earth years?

STEP 6 : Further Analysis: Back Down to Earth for a Moment!

Actually, we were timing a fan. How many complete rotations does the fan make per minute (i.e., rpm)? Remember that the time axis is in microseconds. Also, keep in mind that the fan has a certain number of blades, and each of the blades transits the LED during one complete rotation.

STEP 7 : Study the Arduino Sketch (Optional)

With the aid of your teacher and the comments in the Arduino sketch, study the code so that you can get a better understanding of what it does. The sketch is written in a dialect of the C/C++ programming language. If you can learn some of the rules of C/C++ by studying this and other sketches, you too may soon be programming the Arduino module!