Discovering Other Planets
Go outside on a clear, cold night and look up at the sky. Make sure you are far away from nearby lights so that it is as dark as possible. What do you see? Thousands of points of light—stars that shine like our Sun, but they are so far away they are dots in the sky. That is what you can see using only your eyes.
Astronomers, using sophisticated telescopes and other instruments, say there are about 250 billion (250,000,000,000) stars in our galaxy, the Milky Way. And they estimate that there are about 2 trillion (2,000,000,000,000) other galaxies in the observable universe. If you multiply those two numbers together, you can see that there may be an incredible amount of stars in the universe! And what else is out there? Earth is one of eight planets that orbit the Sun. Have you thought that there may be planets orbiting other stars in the universe? Do you think they have life on them?
In the 1800s, Bahá’u’lláh, the Founder of the Bahá’í Faith, wrote, “Know thou that every fixed star hath its own planets, and every planet its own creatures, whose number no man can compute.” This is a claim that the scientists of His time could not prove or disprove, but much progress has been made since then.
Up until the last century, scientists knew only about planets in our solar system. They have even sent spacecraft to observe them up close. But other stars are so far away that we can’t send spacecraft to visit them. So would astronomers ever be able to discover whether there really are planets orbiting other stars?
Yes! In 1995, over 100 years after Baha’u’llah’s quote above, the first planet orbiting a star outside our solar system was discovered. Such planets are called exoplanets—short for extrasolar planets. As of December 2019, over 4,000 exoplanets have been confirmed!
NASA and other space agencies are discovering new exoplanets nearly every day. They estimate, on average, that at least one planet orbits every star in the galaxy. But we still have much to learn about these planets and whether they can support life like we have on Earth.
How do we find exoplanets?
The first few exoplanets were found by astronomers using ground telescopes. Later, when observatories were launched into space, such as the Hubble Space Telescope and the Kepler Space Telescope, they found many more exoplanets. But because the light from a star is so bright, even with our best telescopes, astronomers cannot actually see the planets. They detect planets orbiting distant stars by measuring changes in the starlight. Astronomers have two methods for doing this.
Method 1: Color-Changing Stars (Radial Velocity Method)
One method detects planets by finding small changes in the colors of the light from the star. The colors making up the light are called its spectrum. When a star is moving toward us, the light shifts toward the color blue, and when it is moving away from us, it shifts toward the color red. A star moves slightly when it’s orbited by a planet. So when scientists see these repeated changes in a star’s color, they assert that it’s being orbited by a planet. This is called the Radial Velocity Method. Only a very large planet—like Jupiter in our solar system—can affect a star’s movement enough to be detected this way.
Photo by European Southern Observatory (ESO)
Method 2: Dimming Stars (Transit Method)
When an orbiting planet crosses in front of the star (called a transit), the light from the star dims slightly. When the dimming follows a regular pattern, it’s likely that a planet is orbiting the star. This is called the Transit Method. Most exoplanets are currently found this way. The size of the planet can be found using this method too. And the spectrum of the light passing through the planet’s atmosphere can tell us which chemicals are present on the planet. Water, carbon dioxide, and methane have been detected on some exoplanets by comparing the colors missing in the measured spectrum with the colors we know are absorbed by these chemicals.
The simulation below shows an exoplanet orbiting a star. A telescope will measure the intensity of the light coming from the star, and when the exoplanet passes between the star and the telescope, there is a drop in the intensity. The time between the drops is the time it takes for the planet to orbit the star.
Future Method: See the Exoplanets (Direct Imaging)
The Radial Velocity Method and the Transit Method only work for finding some exoplanets. The intensity of the light will only drop if the planet passes in front of its star. So the next phase in the search for exoplanets will focus on Direct Imaging. Here telescopes will actually see the exoplanets, and not just how they affect the light from their star. And since the exoplanet could be in almost any orbit, more exoplanets could be discovered this way than from the other methods. Direct imaging could also determine whether the planet’s atmosphere contains water and other chemicals needed to support life.
Of course, direct imaging will be more difficult. It relies on light the planet reflects from its star. Since that light will be much less intense than the star’s light, larger telescopes are needed, because they gather more light. New ground-based telescopes, such as the Extremely Large Telescope and the Giant Magellan Telescope being built in Chile, and the Thirty Meter Telescope proposed for Hawaii, should be searching for new exoplanets within 10 years. Future space observatories such as the James Webb Space Telescope and Wide Field Infrared Survey Telescope (WFIRST) will also join the search.
Another problem with direct imaging is that the star is much brighter than its orbiting exoplanet. You may have experienced this problem if you’ve looked up to watch a bird flying on a sunny day—the glare from the sun blinds you. You can solve this problem by holding up your hand to shade your eyes from the sun, so you can watch the bird. Scientists propose doing something similar. NASA is developing large starshades to be sent into space to block light from a star, making it easier to capture images of exoplanets.
Starshade for a Space Telescope
A starshade must be smaller than 5 meters (about 16 ft) wide in order to fit on a launch vehicle. But in space it must expand to 26 meters (about 85 ft) in diameter to work with a telescope like WFIRST. One way to fit a starshade into a small size is to use the art of origami to fold it up compactly on Earth, and then unfold it in space. This video shows how that could be done.
NASA is still developing the origami starshade design and improving the accuracy with which it is unfolded and controlled in space, so that it better blocks starlight. The goal is to launch an origami starshade with the WFIRST telescope in the late 2020s.
You can make your own origami starshade model by following the steps below.
Fold Your Own Origami Starshade
Step 1: Download and print the starshade template at this link. Then cut it out along its thick black outline.
Step 2: (A) Create creases by folding along all the blue lines so they dip down like valleys. Use a straight edge, such as a flat ruler, to guide the paper’s folds. (B) Next make creases by folding along all the red lines (with help of a straight edge) so they extend up like peaks. Make sure the creases follow the lines from the outer edge to its inner circle.
Step 3: Use your fingers to pinch along each peak and valley to reinforce creases, making them crisp. With a rolled-up piece of tape, secure the bottom of the circle that says “Star Shade” to the table. Press a thick marker (with its cap on) in the center of the starshade, for support. Wind the starshade around itself (and the marker) counterclockwise to form a tube.
The trick is to make sure that the peaks and valleys that you folded don’t get reversed, especially near the center. Fix any peaks and valleys that get reversed as you wrap the starshade. You may want to ask a parent or a friend to help. The animated image below shows how it folds up into a tube.
Step 4: Remove the marker and tape. Use your fingers to gently squeeze tube’s creases in place and to smooth out any wrinkles inside the tube. When you’re done, the black lines inside the tube will line up, and you’ll see the words “Star Shade” at the bottom.
Step 5: Pinch a valley at each end of the tube and gently pull in opposite directions to open the starshade. Without letting go, push these valleys toward each other to wind and close it.
Have fun showing friends your model of this cool new space technology!
Dr. Steve Scotti is Brilliant Star’s STEM Education Advisor and a Distinguished Research Associate at NASA Langley Research Center in Virginia, U.S. His background is in developing lighter, stronger materials and structures for aircraft and spacecraft. Watching the first astronaut launched into space inspired his interest in space exploration, and he enjoys sharing his enthusiasm about science and space with kids.
Thanks to Evan Hilgemann of NASA–Jet Propulsion Laboratory for information on the origami starshade.
Kepler-186f by NASA Ames/JPL-Caltech/T. Pyle
STEM261 DrScotti51 Science183 Space61 Sun18 Planets18 Exoplanets4 Origami13 Crafts201 Arts and Crafts203 Creativity245 Astronomy24 STEM Station30 STEAM43 STEAMS69