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A comic-style illustration of a space telescope in the foreground of the milky way galaxy, seen pouring from it many round, colorful balls, intended to be exoplanets, which fill the foreground of the image.

7.3. How do we discover worlds around other stars?

A core learning question from the Astrobiology Learning Progressions

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7.2. How do we explore within our own Solar System for signs of life?

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7.4. How can we identify worlds around other stars that could have life?

Grades K-2 or Adult Naive Learner

As we look into the night sky and see so many stars it’s also amazing to think about how many of those stars have planets. Maybe some of those planets are similar to the planets in our own solar system. Maybe some are even similar to our Earth.

A planet that orbits a star other than our Sun is called an exoplanet. Telescopes can see stars very well, but seeing planets around other stars is actually pretty hard. However, there are ways for us to find out about something without seeing it directly. Sometimes, an exoplanet will move in front of its star. When this happens, it blocks a really small amount of the light from the star. And we can actually see that and it lets us know that there is an exoplanet there. Also, when planets orbit around stars, they tug and pull at the star and make it move around a little bit. We can also measure this and use it to find exoplanets!

We’ve now discovered thousands of exoplanets orbiting other stars, and we keep finding more and more all of the time. Most of them that we’ve found are really big planets, like Jupiter and Saturn, but we’re also finding planets that are smaller and closer to the size of Earth. Some of us even wonder if we might soon find an exoplanet that also has signs of life on it. Wouldn’t that be amazing?!

A comic-style illustration of a space telescope in the foreground of the milky way galaxy, seen pouring from it many round, colorful balls, intended to be exoplanets, which fill the foreground of the image.
A vast number of exoplanets with a wide range of properties have now been discovered around distant stars.
NASA/Aaron Gronstal

Grades 3-5 or Adult Emerging Learner

As we look into the night sky and see so many stars it’s also amazing to think about how many of those stars have planets. Maybe some of those planets are similar to the planets in our own solar system. Maybe some are even similar to our Earth.

A planet that orbits a star other than our Sun is called an exoplanet. Telescopes can see stars very well, but seeing planets around other stars is actually pretty hard. However, there are ways for us to find out about something without seeing it directly. Sometimes, an exoplanet will move in front of its star. When this happens, it blocks a really small amount of the light from the star. And we can actually see that and it lets us know that there is an exoplanet there. Also, when planets orbit around stars, they tug and pull at the star and make it move around a little bit. We can also measure this and use it to find exoplanets!

Most of the thousands of exoplanets that have been detected so far were found by the Kepler mission. Kepler was a space telescope that looked at just a very small region of the sky (if you hold your hand out with a straight arm at night and point it to the sky, the region covered by your hand is roughly the size of the area that Kepler was looking at!).

We’ve now discovered thousands of exoplanets orbiting other stars, and we keep finding more and more all of the time. Most of them that we’ve found are really big planets, like Jupiter and Saturn, but we’re also finding planets that are smaller and closer to the size of Earth. Some of us even wonder if we might soon find an exoplanet that also has signs of life on it. Wouldn’t that be amazing?!

A comic-style illustration of a space telescope in the foreground of the milky way galaxy, seen pouring from it many round, colorful balls, intended to be exoplanets, which fill the foreground of the image.
A vast number of exoplanets with a wide range of properties have now been discovered around distant stars.
NASA/Aaron Gronstal

Grades 6-8 or Adult Building Learner

As we look into the night sky and see so many stars, it’s also amazing to think about how many of those stars have planets. Maybe some of those planets are similar to the planets in our own solar system. Maybe some are even similar to our Earth. A planet that orbits another star is called an exoplanet. Telescopes can see stars very well, but seeing planets around other stars is actually pretty hard. The light that comes from a planet is mostly reflected starlight and that little bit is overwhelmed by the amount of light coming from the star. But we can observe some exoplanets directly, especially younger planets that are a little further from their stars. That’s because younger planets emit more infrared light for us to see and if they’re a little further away from their star then the starlight isn’t so overwhelming. Still, the list of planets we’ve observed directly is only a small fraction of the thousands of exoplanets that we’ve found.

How do we find most exoplanets without seeing them directly? Sometimes an exoplanet will move in front of its star. When this happens, it blocks a very small amount of the light from the star. And we can actually see that and it lets us know that there is an exoplanet there. This is the main way that we’ve found exoplanets so far. However, we can also look for exoplanets by seeing how they tug and pull at their stars. It turns out that stars appear to wobble a little bit due to the way that gravity causes them to interact with their planets (our Earth makes our own Sun wobble, but just a very little bit). We can measure this interaction and use it to infer the presence of exoplanets.

Most of the thousands of exoplanets that have been detected so far were found by the Kepler mission. Kepler was a space telescope that looked at just a very small region of the sky (if you hold your hand out with a straight arm at night and point it to the sky, the region covered by your hand is roughly the size of the area that Kepler was looking at!). By measuring the light from around 100,000 stars all at once for a long time, scientists were able to use transit photometry to find a large number of exoplanets.

We’ve now discovered thousands of exoplanets orbiting other stars, and we keep finding more and more all of the time. Most of them that we’ve found are really big planets, like Jupiter and Saturn, but we’re also finding planets that are smaller and closer to the size of Earth. Some of us even wonder if we might soon find an exoplanet that also has signs of life on it. Wouldn’t that be amazing?!

A comic-style illustration of a space telescope in the foreground of the milky way galaxy, seen pouring from it many round, colorful balls, intended to be exoplanets, which fill the foreground of the image.
A vast number of exoplanets with a wide range of properties have now been discovered around distant stars.
NASA/Aaron Gronstal

Grades 9-12 or Adult Sophisticated Learner

As we look into the night sky and see so many stars, it’s also amazing to think about how many of those stars have planets. Maybe some of those planets are similar to the planets in our own solar system. Maybe some are even similar to our Earth. A planet that orbits another star is called an exoplanet. Telescopes can see stars very well, but seeing planets around other stars is actually pretty hard. The light that comes from a planet is mostly reflected starlight and that little bit is overwhelmed by the amount of light coming from the star. But we can observe some exoplanets directly, especially younger planets that are a little further from their stars. That’s because younger planets emit more infrared light for us to see and if they’re a little further away from their star then the starlight isn’t so overwhelming. Still, the list of planets we’ve observed directly is only a small fraction of the thousands of exoplanets that we’ve found.

How do we find most exoplanets without seeing them directly? There are several ways to find exoplanets without seeing them directly, but the two that we’ve used to find the most planets so far are called “transit photometry” and “Doppler spectroscopy.” Transit photometry is used to measure how a planet blocks a star’s light. We call the passage of a planetary body (or even a spacecraft or satellite) in front of a star a “transit”, and the word “photometry” implies that we’re measuring light (light exists as discrete units of energy called “photons”). Transit photometry is a tool that we use to measure how a transiting exoplanet changes the measured light from its star. We’ve actually gotten quite good at measuring the little dip in the amount of light coming from a star when a planet passes in front of it. The other main method we’ve used for finding planets so far is Doppler spectroscopy (also called the “radial velocity” method). If you’ve ever heard a firetruck coming down the street, did you notice how the sound appeared to change from when the firetruck was coming toward you to when it was going away from you? The sound is a little higher in pitch as it comes toward you and a little bit lower as it travels away. The same thing happens with the cars at a racetrack. This is called the Doppler effect. And it turns out that it works for waves of light just as it does for sound. When something that emits light is moving toward you, it causes the light to be a little bit high pitched (to have a little bit more energy). If something emitting light is moving away from you, the light appears to have a little bit less energy. Blue visible light has the most energy of the colors of the rainbow and red visible light has the lowest. Because of this, we call the light that appears with a little bit more energy “blue-shifted” and the light with apparently a little less energy “red-shifted.” These shifts in the apparent energies of light allow us to measure the motions of stars and galaxies. We’ve used this “shifting” of light to determine that our universe is expanding andt to detect the presence of exoplanets. In Doppler spectroscopy, we measure small changes in the energy of light coming from a star that’s being tugged on by its planets. It turns out that stars appear to wobble a little bit due to the way that gravity causes them to interact with their planets. The bigger the planet, the more it causes the star to appear to wobble. In our solar system, Earth makes our own Sun wobble, but not nearly as much as Jupiter. We can detect the blue-shifting and red-shifting from planets causing their stars to wobble and we can even measure how big the planets must be based on how much the star wobbles. It’s really rather incredible!

Most of the thousands of exoplanets that have been detected so far were found by the Kepler mission. Kepler was a space telescope that looked at just a very small region of the sky (if you hold your hand out with a straight arm at night and point it to the sky, the region covered by your hand is roughly the size of the area that Kepler was looking at!). By measuring the light from around 100,000 stars all at once for a long time, scientists were able to use transit photometry to find a large number of exoplanets. However, there’s also an issue with this method that we call “observational bias.” Observational bias is when your method of measurement causes you to only be looking at a smaller number of the possible things you could be looking at. For instance, if you wanted to measure the number of people who visit a museum in one day, but your method is to count the number of adult tickets that were sold, you’d have an observational bias toward only the adult visitors and you’d be missing out on children, senior citizens, and other special kinds of tickets that museums sell or give away. With the Kepler mission, we had an observational bias toward exoplanets that were very big and that were very close to their stars. Many of the planets found early on have been about the size of Jupiter (and even bigger) and are very close to their stars (where there’s more radiation) so we’ve called them ‘Hot Jupiters’. The reason for this bias is that planets that are close to their star cause the dips in the light to happen more often, so we have more data to use to make a detection. The bigger the world, the more light that it blocks, making it easier for us to tell that there’s a planet from the data. Even with the biases that we’ve had so far, we’re starting to make more and more detections of smaller exoplanets and exoplanets that are further from their stars. We’ve discovered rocky planets that are bigger than Earth (known as super-earths), and we’re starting to discover more Earth-sized exoplanets. These detections are making many of us wonder if we might soon find an exoplanet that also has signs of life on it. Wouldn’t that be amazing?!

A comic-style illustration of a space telescope in the foreground of the milky way galaxy, seen pouring from it many round, colorful balls, intended to be exoplanets, which fill the foreground of the image.
A vast number of exoplanets with a wide range of properties have now been discovered around distant stars.
NASA/Aaron Gronstal