Alien Worlds Introduction & Transit Science Brainstorm Alien Worlds Eclipsing Expanding Your Horizons Page 1! of !4 Cornell University Do you know what an eclipse is? Have you ever seen one in Distant Suns 30 April 2016 person? Draw a quick diagram of a solar and lunar eclipse. Student’s Name: Activity 2: Building a mini-Kepler and ob- serving a transit! ! Part 1: Building the mini-Kepler! Eclipses do not have to involve the Earth-Moon-Sun system. Other planetsKepler in the is Solar a space telescope that constantly observes the same patch of the System can eclipse the Sun and moons can eclipse their host planets. Astronomerssky to look call for dips in the brightness of stars. We will use the littleBits circuit these “transits.” If you wanted to discover an alien planet, how might you usecomponents eclipses to to create a system that records brightness changes with time. find them? You need: 1 Arduino circuit board, 1 “wire”, 1 light sensor, 1 power supply, 1 battery, and 1 mini-USB cable to hook up the mini-Kepler to a computer. Astronomers use transits to discover alien worlds (exoplanets) all the time! Make a list of things you and other scientists might want to know about an alien world (example: radius, planet mass, etc.). What might you be able to measure from a transit to determine those properties of your exoplanet? Picture of the Kepler telescope while they were still building it in the lab. LittleBits snap together with magnets. Connect the power supply → light sensor → wire → Arduino. The wire should be connected to the Arduino at the input labelled “a0.” Make sure all the Arduino switches are set to “ana- log.” Check that the trigger switch on the light sensor is set to “light” and that the sensitivity control is turned all the way to the left (using a tiny screw driver it should turn very easily). Plug in the power supply to a bat- tery and turn the switch to “on.” Plug the Arduino in to the computer using the mini-USB cable. Ta-dah! You’ve made a mini-Kepler! Picture of a completed mini-Kepler! Building a Mini-Exoplanet System Materials • A lightbulb or lamp connected to a power source • Either of the following: o 2-3 balls of various sizes (example: tennis ball, cotton ball, marble) o Black paper & scissors • Optional: Download the free Arduino SJ app on a smartphone or tablet Constructing the Mini-Exoplanet System • Set up the lightbulb or lamp on a table: this is the star in our exoplanet system! If you are using a lamp, try to use one that leaves the bulb partially exposed so that you can see it. Try to use a lightbulb that isn’t very bright so that you don’t hurt your eyes looking at it. • If you are using balls, skip to the next step. If not, take the black paper and cut it into a few circles of various sizes – some smaller and some bigger than the lightbulb. Whether you’re using balls or paper, these are our exoplanets! • If you have access to the Arduino SJ app: o Open it up and start a new experiment. You should see a sensor box pop up and start actively measuring data. Make sure the sensor is set to measure brightness, the little sun symbol (see image to the right). o The light sensor uses the front camera of your phone (the one you use to take selfies), so make sure that you point the front camera at your lamp for the next activity. Transit Depth & Radius • Take one of your balls or paper circles and, standing or sitting in the same spot the whole time, move the ball/circle around the lamp – this is the exoplanet completing one orbit around its star. Notice what happens to the light from the lamp as the exoplanet moves around it. Does it get dimmer when the exoplanet is in front of it, and brighter when the exoplanet is behind it? Repeat this with your other balls/circles, and optionally record the data using Arduino SJ. How are the transits different for the bigger and smaller exoplanets? An example of transit data recorded with Arduino SJ is shown to the right. • The change in the star’s brightness as the exoplanet moves around it is called a “light curve.” We can use the light curve to measure some physical properties of the exoplanet. The dip in brightness as the exoplanet transits in front of the star is directly related to how big the exoplanet is – the larger the planet, the larger the drop in brightness. The fraction of the star’s light blocked by the planet is: � − � � = !"# !$% �!$% where f is the fraction of the light blocked, Bmax is the maximum brightness of the star and Bmin is the minimum brightness of the star during the transit. This fraction is also equal to the ratio of the area of your planet and its star: π�& � = π�& where r is the radius of the planet and R is the radius of the star. • Draw some examples of what the light curves look like for your different exoplanets. Make sure to include axes on your graphs (the y-axis should be the star’s brightness, and the x-axis should be time). Transit Period & Kepler’s 3rd Law • Orbit your exoplanets around the star again, but this time compare what happens to the brightness when you move the exoplanets quickly or slowly. How will the light curves look for planets in fast orbits vs. slow orbits? The amount of time it takes the planet to move around the star in a complete orbit is called the orbital period (for Earth, the orbital period is 1 year). • Draw a light curve for one of your exoplanets again, this time making sure to include multiple transits (multiple dips in the brightness). Use lines or arrows to indicate what features on the graph can be used to measure the planet’s orbital period. • The orbital period can be used to find out how far away the planet is from the star – this is called the orbital radius. We can measure the orbital radius using Kepler’s 3rd law: �' star �& = � (mass M) � a Here P is the orbital period, a is the orbital radius, M is the mass of the star, and C is a constant. This equation shows that as the distance from the star increases, the planet orbital period increases – this is why the orbital period of Mercury is only 88 days, while the orbital period of Neptune is 165 years. Act out Kepler’s 3rd law with your exoplanet system by having the planets closer to the lamp orbit faster than the planets farther from the lamp. The Habitable Zone & Alien Life • Compare how warm your exoplanet feels when it’s closer to the lamp vs. when it’s farther away from the lamp. How far from the lamp does it need to be to feel just right? In astrobiology, we call this “Goldilocks zone” the habitable zone, the region around a star where conditions are just right for life to evolve and flourish. • Whether a planet is habitable for life doesn’t just depend on its temperature. List a few other conditions that you think are necessary for life to exist on a planet (you can use Earth as an example). Become a Citizen Scientist! Now that you’ve learned how transits work, you can practice identifying transits with real data at planethunters.org, a citizen science effort to discover exoplanets in data from the TESS telescope (shown to the left). There are many other citizen science projects that you can get involved in at zooniverse.org, not only in astronomy but in many other subjects as well! The tutorials are free and easy, and you can participate in groundbreaking scientific discoveries from anywhere with internet access. You can find out more about the exoplanets we’ve discovered at exoplanets.nasa.gov. .