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Comic-style illustration showing diverse life forms and molecules in the foreground emitting what are intended to be atmospheric biosignature spectra across multiple exoplanets in space.

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

A core learning question from the Astrobiology Learning Progressions

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7.3. How do we discover worlds around other stars?

Grades K-2 or Adult Naive Learner

Have you ever noticed that when you get closer or further away from something hot that it really changes the temperature that you feel? For example, if you get too close to a fireplace, a heater, or a campfire, you can actually feel way too warm. But, if you move away, it starts to get cooler. The same thing happens with our Sun. It’s really warm for a planet that’s close to the Sun, but as we move further and further from the Sun, there’s less warmth from the Sun reaching the planets. This might be important when it comes to looking for alien life in the universe. When planets are really close to their stars, it’s actually too hot for liquid water on those planets. Their oceans would just boil away. But, if it’s really far from a star, then it’s so cold that liquid water freezes and makes ice. So planets that might have liquid water on their surface (like lakes and rivers and oceans), might only be in an area around their stars that “just right” for liquid water.

Comic-style illustration showing diverse life forms and molecules in the foreground emitting what are intended to be atmospheric biosignature spectra across multiple exoplanets in space.
Scientists study exoplanets by analyzing starlight through spectroscopy, splitting light into chemical fingerprints. Spectroscopy may reveal potential biosignatures in the form of organic compounds in the exoplanet's atmosphere or on its surface.
NASA/Aaron Gronstal

Grades 3-5 or Adult Emerging Learner

Have you ever noticed that when you get closer or further away from something hot that it really changes the temperature that you feel? For example, if you get too close to a fireplace, a heater, or a campfire, you can actually feel way too warm. But, if you move away, it starts to get cooler. The same thing happens with our Sun. It’s really warm for a planet that’s close to the Sun, but as we move further and further from the Sun, there’s less warmth from the Sun reaching the planets. This might be important when it comes to looking for alien life in the universe. When planets are really close to their stars, it’s actually too hot for liquid water on those planets. Their oceans would just boil away. But, if it’s really far from a star, then it’s so cold that liquid water freezes and makes ice. So planets that might have liquid water on their surface (like lakes and rivers and oceans), might only be in an area around their stars that “just right” for liquid water.

It’s just like the story of Goldilocks and the Three Bears. In this story, young Goldilocks goes to eat some porridge left in the kitchen by a family of three bears. Papa Bear’s porridge was too hot and Mama Bear’s porridge was too cold, but Baby Bear’s porridge was “just right” for eating. Since this is kind of like how we think about liquid water on planets, we call that place around stars where it’s “just right” for liquid water the “Goldilocks Zone.” There might be many other places where aliens could live, but planets with liquid water on the surface are an important place to look.

Comic-style illustration showing diverse life forms and molecules in the foreground emitting what are intended to be atmospheric biosignature spectra across multiple exoplanets in space.
Scientists study exoplanets by analyzing starlight through spectroscopy, splitting light into chemical fingerprints. Spectroscopy may reveal potential biosignatures in the form of organic compounds in the exoplanet's atmosphere or on its surface.
NASA/Aaron Gronstal

Grades 6-8 or Adult Building Learner

We’ve now discovered several thousand planets that orbit around other stars! What’s more, based on the number of exoplanets that we’ve detected by looking at just a small fraction of the total number of stars that are in our galaxy, we’ve estimated that there are probably way more than 100 billion planets within the galaxy. That’s a whole lot of worlds. If you were to try counting one of those planets each second, it would take over 3,000 years to count them all! With all of those worlds out there, do you think it’s possible that any of them are home to other living things? If there are any alien biospheres out there, then we need to consider how to find them and how we can learn more about them.

Our Earth is the only place we know of so far that has life, so it might be important for alien worlds to be a bit like Earth in order to have life. For instance, our Earth lies within the “Goldilocks Zone” of our solar system. The Goldilocks Zone is a region around a star where the starlight warming the surface of a planet might allow the planet to have liquid water on its surface. Just like in the story of Goldilocks and the Three Bears, where Papa Bear’s porridge was too hot and Mama Bear’s porridge was too cold, but Baby Bear’s porridge was “just right” for eating, some planets are either so close to their star that it’s too hot for liquid water on the surface and others are so far from their star that it’s too cold for liquid water on the surface, but some planets are in the area where it’s “just right” to have rivers and lakes and oceans on their surfaces. If having liquid water on a surface is important for life, then looking for worlds in the Goldilocks Zones around stars might be one good place to look.

There’s also the chance that worlds that have subsurface oceans could be important places for life. We have places like Europa and Enceladus here in our solar system. However, these are moons of larger planets. It turns out that, while finding exoplanets is hard enough, finding exomoons is even harder. And, on top of that, if there are biospheres within those subsurface oceans, we might not be able to see any evidence of them using our telescopes. What about life in atmospheres? Could you imagine a world like Jupiter or Saturn having a biosphere floating around in all of those clouds? We don’t know yet if this such worlds exist, but some people have wondered if looking for signs of life in the atmospheres of giant planets could also be one way to find alien life. There are definitely lots of worlds out there and that means that there are lots of possible places to look for life. As our telescopes and our technology get better and better, we’re even more likely to be able to find signs of life if there are alien biospheres out there!

Comic-style illustration showing diverse life forms and molecules in the foreground emitting what are intended to be atmospheric biosignature spectra across multiple exoplanets in space.
Scientists study exoplanets by analyzing starlight through spectroscopy, splitting light into chemical fingerprints. Spectroscopy may reveal potential biosignatures in the form of organic compounds in the exoplanet's atmosphere or on its surface.
NASA/Aaron Gronstal

Grades 9-12 or Adult Sophisticated Learner

We’ve now discovered several thousand planets that orbit around other stars! What’s more, based on the number of exoplanets that we’ve detected by looking at just a small fraction of the total number of stars that are in our galaxy, we’ve estimated that there are probably way more than 100 billion planets within the galaxy. That’s a whole lot of worlds. If you were to try counting one of those planets each second, it would take over 3,000 years to count them all! With all of those worlds out there, do you think it’s possible that any of them are home to other living things? If there are any alien biospheres out there, then we need to consider how to find them and how we can learn more about them. Our Earth is the only place we know of so far that has life, so it might be important for alien worlds to be a bit like Earth in order to have life. For instance, our Earth lies within the Goldilocks Zone of our solar system. The “Goldilocks Zone” is a region around a star where the starlight warming the surface of a planet might allow the planet to have liquid water on its surface. Just like in the story of Goldilocks and the Three Bears, where Papa Bear’s porridge was too hot and Mama Bear’s porridge was too cold, but Baby Bear’s porridge was “just right” for eating, some planets are either so close to their star that it’s too hot for liquid water on the surface and others are so far from their star that it’s too cold for liquid water on the surface, but some planets are in the area where it’s “just right” to have rivers and lakes and oceans on their surfaces. If having liquid water on a surface is important for life, then looking for worlds in the Goldilocks Zones around stars might be one good place to look.

When it comes to looking for Earth-like planets, another important thing to consider is the type of star (or stars) that a planet orbits. For instance, stars will change in how much radiation they emit over time, so the Goldilocks Zone changes over time as well. And not only that, but some stars might just be dangerous for life altogether. Larger stars have much wider Goldilocks Zones and therefore might have a greater potential for more planets to lie within that zone. However, large stars also burn through their hydrogen fuel quickly and have shorter lives because of it. The largest stars may only be in their main sequence phase for 10 million years before they move to supergiant and then supernova phases. That might not be enough time for life to evolve to a point where we could detect it in the atmosphere of an exoplanet. We might then say that we should look for planets around stars that live for a long time, but that could also be a problem. The smallest stars, the red dwarf stars, will be in their main sequence phase for 100s of billions to trillions of years. That should be plenty of time for life to develop. However, since those stars are a lot smaller, their Goldilocks Zones are much smaller and much closer to the star, and this can present a variety of problems. For instance, since the Goldilocks Zone is much closer to the star, some scientists have hypothesized that exoplanets around red dwarf stars are more likely to be “tidally locked”. Tidal locking is when a celestial object rotates in the same period that it revolves, such that one side of it is always facing in. Our moon is almost tidally locked, which is why we can only see one side of the Moon from Earth. A planet that is tidally locked would have one side facing its star all the time. This could be problematic for the evolution of life, but we don’t know for sure yet. Another concern for planets around red dwarf stars is that they might experience a lot of solar flares that could kill living things (red dwarf stars tend to be more variable, meaning that they go through times where they release lots of radiation and times when they release little). So maybe the biggest stars and the smallest stars aren’t the best for finding alien life.

It seems like looking for Earth-like worlds is the right way to go about looking for alien biospheres, but there are also other possible places where alien life might exist. For instance, there’s a chance that worlds that have subsurface oceans could be important places for life. We have places like Europa and Enceladus, moons with subsurface oceans, here in our solar system. However, these are moons of very large planets. It turns out that, while finding exoplanets is hard, finding exomoons around big planets is going to be even harder. And, on top of that, if there are biospheres within those subsurface oceans, we might not be able to see any evidence of them using our telescopes. So, even if these worlds might be good places for life to originate and develop, maybe they aren’t great places to look for life until we have better technology. What about life in atmospheres? Could you imagine a world like Jupiter or Saturn having a biosphere floating around in all of those clouds? We don’t know yet if such worlds exist, but some people have wondered if looking for signs of life in the atmospheres of giant planets could also be one way to find alien life. There are definitely lots of worlds out there and that means that there are lots of possible places to look for life. As our telescopes and our technology get better and better, we’re even more likely to be able to find signs of life if there are alien biospheres out there!

Comic-style illustration showing diverse life forms and molecules in the foreground emitting what are intended to be atmospheric biosignature spectra across multiple exoplanets in space.
Scientists study exoplanets by analyzing starlight through spectroscopy, splitting light into chemical fingerprints. Spectroscopy may reveal potential biosignatures in the form of organic compounds in the exoplanet's atmosphere or on its surface.
NASA/Aaron Gronstal