Hunting for exoplanets with Arduino

Students will construct a model of a planetary system, consisting of a planet orbiting by means of a stepper motor. The system will allow them to vary the planet's speed and direction of rotation.

Part 1: Building the planetary model (duration approx. 30 minutes)

To make the planetary system, take the straw and cut a piece of about 8 - 10 centimetres from it, straddling the folding part (about 3 centimetres before the folding part and the rest after). Using a pin or toothpick, pierce the longest part of the straw and place the sphere that will serve as the planet on it. In order to keep the “L” structure made from the straw as rigid as possible, a small wire can be inserted inside to act as a skeleton. Finally, insert the short end of the straw onto the shaft of the stepper motor as in the following picture

Image 2: example of how the structure simulating the planet was made. A straw was attached to the stepper motor and connected, via a toothpick, to the sphere simulating the planet.

Then we move on to making the electrical connections to connect the planetary system to the Arduino board.

First, connect the stepper motor to its driver, or driver module (connection board supplied with the motor itself), using the appropriate connection system. The stepper motor is a DC synchronous electric motor that allows the rotation to be divided into small angles, or steps. 200 pulses are required to complete one complete revolution. A stepper motor is very precise, fast and easily controlled by means of an electronic board, called a driver. The speed of rotation depends on the waiting time between pulses within the individual rotation cycles. In order to change the speed of rotation of the planetary system, a potentiometer is used, while a button is used to change the direction of rotation.

The attached diagram shows how to make the individual connections. In general, the pins to be connected are:

Engine Driver:

Driver	Arduino Board
GND	GND
5V	5V
IN1	PIN8
IN2	PIN 9
IN3	Pin 10
IN4	Pin 11

Potentiometer connection:

Potentiometer	Arduino Board
Right Pin	5V
Left Pin	GND
Central Pin	A0

Push-button connection:

Push-button	Arduino Board
Right Pin	GND
Left Pin	Pin 4

The breadboard can also be used to “bridge” the various overlapping connections.

Image 3: the circuit of the stepper motor. The numbers on the connection cables indicate the different connections between the Arduino board and the motor driver.

Once the connections have been made, the next step is to load the software onto the Arduino board. This is done through the IDE (Arduino working environment) and can be done as follows. The procedure may change depending on the version of the IDE installed. This description refers to version 2.3.0.

First connect, with the appropriate cable, the Arduino board to the PC where the control and code upload software was previously installed. After opening the Arduino IDE, click on the drop-down menu at the top right of the three circular icons. From here, select the Arduino board (whether Arduino Uno or Arduino Mega or Nano) that you are using and the port (USB or other) to which it is connected. This way the Arduino working environment is ready and you can write and then upload the code for the planet to the board. The code can be downloaded from the attchements (File Exoplanet-code-motor.ino).

To upload the code to the Arduino board, simply click on the upload icon, the second icon from the right. Once this is done, if the connections have been made correctly, the planet should start to rotate, and clicking on the button will change the direction of motion as the potentiometer varies the speed of rotation.

Part 2: construction of the transit detector and star block (duration approx. 30 minutes)

Objective: students will construct the second block of the project consisting of the brightness variation detector, the star with the possibility of varying brightness and the apparatus for storing the data.

A photoresistor (LDR, Light Dependent Resistor) will be used to measure the brightness variation of the planetary system. This is an electronic component whose electrical resistance is inversely proportional to the intensity of the incident light: as the brightness increases, the resistance value decreases, causing more current to flow. This makes it possible to measure the variation in brightness of the “star” as it orbits the “planet” in front of it. To adjust the sensitivity of the photoresistor to ambient light, it will be connected in series to a potentiometer.

The light source (star) will be created by means of a 5 mm white LED, the brightness value of which can be varied via a further potentiometer. By varying the voltage applied via the potentiometer, the light intensity of the LED will change, thus simulating stars with different brightnesses. In this way, it will be possible to observe how the light curves change as the simulated stellar brightness varies.

Experimental data will be collected via an external memory: a microSD card connected to the Arduino via a special read/write module. All brightness variation data will be recorded on a file and can later be analysed using data processing software such as Excel, Matlab or equivalent.

The LED, representing the star, must be positioned at the vertical of the stepper motor's axis of rotation.

The light emitted by the LED must be directed towards the photoresistor, which in turn must be at the same height as the planet mounted on the straw. In this way, it will be possible to correctly detect changes in brightness during rotation.

From a structural point of view, a vertical support made of rigid cardboard can be used, on which the various components can be fixed. In particular:

Photoresistor: Make two small holes in the cardboard, pass the terminals of the photoresistor through them, and attach it to the support using hot glue or adhesive tape.

LED: Use a straw or a small tube fixed vertically to the cardboard above the axis of rotation. The LED will be inserted inside the straw so that it protrudes from the upper end. The LED terminals can be bent to correctly direct the light toward the photoresistor. The electrical connections can be routed inside the straw, keeping the installation tidy and reducing the risk of mechanical interference during movement.

The final fixing of the LED, straw, and wiring can be done with hot glue or strong adhesive tape, ensuring stability during operation.

Below is a description of how to connect the various electronic components to the second Arduino board.

Potentiometer LDR	Arduino Board
Left Pin	A0
Central Pin	GND

One of the two legs of the photoresistor should also be connected to the same A0 pin, while the second leg should be connected to the 5V pin.

Potentiometer LED	Arduino Board
Left Pin	GND
Central Pin	A1
Right Pin	5V

Likewise, the shorter leg of the LED should be connected to the GND pin of the Arduino board, while the longer leg should be connected to pin 5.

Finally, the SD card reader should be connected to the Arduino board according to the following diagram:

SD Card Reader	Arduino Board
CS	Pin 3
SCK	Pin 13
MOSI	Pin 11
MISO	Pin 12

To make the various connections, a breadboard can be used, also to bridge overlapping connections.

Image 4: circuit diagram that controls the SD card reader and the LED. The numbers on the connection cables indicate the different connections between the Arduino board and the various components of the circuit.

Once all the connections are complete, the next step is to upload the software to the Arduino board. This is done through the Arduino IDE (Integrated Development Environment) and can be carried out as follows. The procedure may vary depending on the version of the IDE installed. This description refers to version 2.3.0.

First, connect the Arduino board to the PC using the appropriate cable. Make sure the control and code upload software has already been installed on the computer. After opening the Arduino IDE, click on the dropdown menu located at the top right, to the right of the three circular icons. From here, select the Arduino board you are using (whether it's an Arduino Uno, Mega, or Nano) and the port (USB or other) to which it is connected.

At this point, the Arduino development environment is ready, and you can write and upload the code for the project to the board. The code can be downloaded from the attchements of this activity (File Exoplanet-code-detector.ino).

To upload the code to the Arduino board, simply click on the Upload icon, which is the second icon from the right. Once this is done, if all connections have been made correctly, the LED should turn on, and adjusting the potentiometer will vary the brightness of the LED.

Image 5: the simulated star-planet system

Part 3: Light Curve Analysis and Data Collection (Duration: approx. 15 minutes)

Objective:
Analyze the behavior of the transit simulator and observe, using the Arduino Serial Plotter, how light curves vary based on changes in the system’s characteristics:

• Orbital speed of the planet
• Direction of rotation
• Brightness of the star
• Size of the planet

Once the system assembly is complete, connect the second Arduino (the one connected to the photoresistor) to the computer. The first Arduino, which controls the motor, can be powered separately via a PC connection, a battery, or its dedicated power supply.
After starting the planet’s rotation around the star, open the Arduino IDE and access the Serial Plotter, which is available by clicking the specific icon (the second-to-last on the right in the toolbar).

Image 6: the experimental apparatus that simulates exoplanets transits, with all its parts.

Using the Serial Plotter, you can view in real time the light intensity detected by the photoresistor during rotation. At this point, by varying one parameter at a time using the potentiometers (i.e., the star’s brightness, the planet’s orbital speed, and the photoresistor’s sensitivity), you can observe how each affects the resulting light curves.
It is recommended to change only one parameter at a time while keeping the others constant to precisely analyze the impact of each variation.
During the experiment, it’s advised to take detailed notes for each change, which will be useful for deeper analysis later.

Image 7: example of light curves generated by Arduino

Questions to think about:

• How does the light curve change when the planet’s rotation speed varies?
• Does the intensity of the light dip or the period (frequency) of the transits change?
• If the direction of orbital motion is reversed, does the light curve change? How?
• When increasing the brightness of the star, how does the amplitude of the light curve change?
• If a larger planet is used (or simulated by reducing the distance between the LED and the photoresistor), how is the depth of the light curve affected?
• How does changing the photoresistor’s sensitivity affect the shape of the curve?
• Which real aspects of the exoplanet transit method are best simulated by this model?
• What are the main differences between this simulation and actual astronomical observations?

Part 4: Light Curve Analysis and Data Logging (Duration: approx. 30 minutes)

Objective:
Record experimental data to an external memory, import it into data analysis software (such as Excel or MATLAB), and create accurate light curve graphs to analyze the effect of system parameter variations in detail.

After observing the light curves in real time via the Arduino Serial Plotter, you can move on to systematic data collection for deeper analysis.
To do this, a microSD card connected to the Arduino that receives the signal from the photoresistor will be used. The data (i.e., the measured light intensity values over time) will be automatically saved to the card in a .csv file.

Once the recording is complete, remove the microSD card and transfer the data to a computer. The file can now be imported into Excel to construct light curve graphs.

To process the data in Excel:

1. Open Excel and create a new worksheet.
2. Go to “File” → “Open” and select the .csv file saved on the SD card.
3. Once imported, select the time column and the brightness column.
4. With the data selected, click “Insert” → “Scatter Plot” → “Smooth Lines Scatter Plot”.

The resulting graph will show the variation in brightness over time, i.e., the system’s light curve.

This graph allows for a more precise analysis of how parameter changes affect the system by enabling measurements of periods, amplitudes, minima, and maxima with greater accuracy than visual observation alone.

With more advanced software like MATLAB or Python, you can also import the .csv data and create customized graphs, add filters, highlight transits, or statistically calculate orbital periods.
As described in the previous section, when writing the data to the file, it is recommended to vary only one parameter at a time to isolate each variable’s effect on the light curve shape.

Questions to think about:

• How does the light curve shape change when the star’s brightness varies?
• When the planet’s orbital speed changes, does the transit period change? How can this be measured from the graph?
• If we increase the size of the planet (e.g., by reducing the LED-to-photoresistor distance), how does the curve’s depth change?
• By reversing the direction of rotation, are there differences in the curve’s shape or symmetry?
• Are the transits regular? Are there imperfections or noise in the signal? What could cause them?

Additional Exploration:

To make the analysis even more interesting and in-depth, you can vary not only the planet’s position and size but also its shape and physical nature. For this, you can build “planets” using different materials, such as plastic, rubber, or aluminum. An interesting variable to explore is the effect of a soft coating, like cotton or foam, to simulate a planetary atmosphere. This could affect the light’s reflection and scattering, altering the resulting light curves.
For example, a planet covered in soft material might scatter light differently than a fully reflective one made of metal or polystyrene.

The presence of materials with different optical properties (transparency, reflectivity, diffusivity) allows observation of how these affect light measurement and transit shape.
Additionally, by changing the planet’s shape — making it more spherical or irregular — you can analyze the potential effects of a non-symmetric geometry on the brightness variation pattern, simulating various types of celestial bodies, from rocky to gaseous ones.

This approach enables a much more complete and realistic analysis, better reflecting the diversity of exoplanets.

Finally, you can discuss how the data collected and analyzed in the lab relates to real astronomical observations and how accurately this model replicates the transit method used by astronomers to detect exoplanets.