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A comic-style illustration of Europa with a slice through it, showing an ocean underneath and cracks on the ice. A small Jupiter is seen in space in the upper right.

6.3. Are there environments beyond Earth that could be habitable?

A core learning question from the Astrobiology Learning Progressions

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6.2. What types of conditions can life survive in?

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7.1. What is a biosignature?

Grades K-2 or Adult Naive Learner

When people go on vacation or go camping they will pack all kinds of things. Maybe they’ll take along enough clothes or food for their trip or maybe a tent or things for cooking. It can be very difficult for people to live in the wild, especially compared to how we usually live in our homes. However, there are many living things everywhere in this world and most of them don’t need to pack a suitcase or take anything extra with them to live in the wild.

Portrait photo of A comic-style illustration of Europa with a slice through it, showing an ocean underneath and cracks on the ice. A small Jupiter is seen in space in the upper right.
There are many environments in the solar system thought to be habitable, such as Europa, one of Jupiter's moons. Europa is considered potentially habitable because beneath its icy surface lies a vast liquid water ocean, kept warm by tidal forces from Jupiter's gravity.
NASA/Aaron Gronstal

Grades 3-5 or Adult Emerging Learner

When people go on vacation or go camping they to think about where they’re going and make sure they pack all the kinds of things that they’ll need. Maybe they’ll take along enough clothes or food for their trip or maybe a tent or things for cooking. For people to live in the wild it can be pretty difficult, especially compared to how we usually live in our homes. However, there are many living things everywhere in this world and most of them don’t need to pack a suitcase or take anything extra with them to live in the wild. That’s because they’re suited to the places in which they live. But what about beyond Earth? Do you think there are things that are suited to living on worlds like Mars or Venus or in other places in our solar system? One of the goals of astrobiology is to figure this out!

Portrait photo of A comic-style illustration of Europa with a slice through it, showing an ocean underneath and cracks on the ice. A small Jupiter is seen in space in the upper right.
There are many environments in the solar system thought to be habitable, such as Europa, one of Jupiter's moons. Europa is considered potentially habitable because beneath its icy surface lies a vast liquid water ocean, kept warm by tidal forces from Jupiter's gravity.
NASA/Aaron Gronstal

Grades 6-8 or Adult Building Learner

Earth seems to be pretty special when it comes to life. Life is everywhere on the surface of our planet. It can even be found deep underground and high in the sky. As our exploration has continued around the globe, we are finding life in nearly every possible environment. Many of these environments are not very suitable for humans. In some cases, they may even be really dangerous for us. For instance, the organisms known as “extremophiles” are called that because they live in places that are too extreme for humans. These can be places like hot springs and hydrothermal vents, deep under the ocean or inside of glaciers, in deserts or even in fluids that are as acidic as battery acid. Finding life in so many places on Earth tells us that there are a lot of conditions that life can survive in. But what about other worlds? Earth is currently the only place we know of that has life, but do you think we might find some of those conditions for life in other places?

The planet Mars today is very cold, it doesn’t have a thick atmosphere, and there’s no flowing water on the surface. Long ago, Mars had rivers and lakes and maybe even an ocean of water at the surface. It had a denser atmosphere than it does today and lots of active volcanoes. Ancient Mars had the chemical ingredients for life, there was energy (solar, geothermal, and possibly chemical), and plenty of water. Many microorganisms on Earth today could have survived and likely even thrived in early Martian conditions. Maybe some things like our extremophiles on Earth were the last survivors of some Martian life that thrived there long ago. There are many people who are studying whether we could find signs of that ancient life on Mars if it ever existed there.

And there are other places where we are looking for potential signs of past or even present alien life. The moons Europa and Enceladus are really interesting to study, for instance. Europa is a moon of Jupiter and Enceladus is a moon of Saturn, and we now have evidence that both of these moons have oceans of liquid water underneath of their icy crusts. How can these moons have liquid water? Both are way too far from the Sun for it to give enough energy to melt them, and their oceans are below their icy crusts. It turns out that the way that these moons interact with their larger planets, through the action of gravity, causes them to heat up inside. This internal energy not only is enough to cause them to have liquid water oceans, but might also cause them to have hydrothermal vents on their ocean floors. Because of this possibility, many of us wonder if there could be living biospheres inside of the oceans of those icy moons.

Almost every star has planets orbiting it. Now that we know that there are many planets out there, we’re really wondering if we might one day soon find other worlds that are not only about the size of Earth and roughly the same distance from their stars, but that might also have biospheres that may be something like ours. We’re building better and better telescopes so that we can study those worlds when we find them to figure out if we can find signs of life on them. It’s a very interesting time to be alive when it comes to learning about whether or not we are alone in the universe.

Portrait photo of A comic-style illustration of Europa with a slice through it, showing an ocean underneath and cracks on the ice. A small Jupiter is seen in space in the upper right.
There are many environments in the solar system thought to be habitable, such as Europa, one of Jupiter's moons. Europa is considered potentially habitable because beneath its icy surface lies a vast liquid water ocean, kept warm by tidal forces from Jupiter's gravity.
NASA/Aaron Gronstal

Grades 9-12 or Adult Sophisticated Learner

Earth seem to be pretty special when it comes to life. Even though we have eight planets and myriad asteroids and comets in our solar system and we now have confirmed the existence of several thousands of exoplanets, Earth is the only place we know that has life so far. Life is everywhere on the surface of our planet. It can even be found deep under the ground and high in the sky. As our exploration has continued around the globe, we are finding life in nearly every possible environment. A lot of these environments are not very suitable for humans and, in some cases, are even dangerous for us. For instance, the organisms known as “extremophiles” are called that because they live in places that are too extreme for us. These can be places like hot springs and hydrothermal vents, deep under the ocean or inside of glaciers, in deserts or even in fluids that are even more acidic than battery acid. Finding life in so many places on Earth tells us that there are a lot of conditions in which life can survive. But what about other worlds? The Earth is currently the only place we know of that has life, but do you think we might find some of those conditions for life in other places?

There is a lot of evidence that early Mars was far more Earth-like than it is today. Long ago Mars had rivers and lakes and maybe even an ocean of water at the surface, it had a much denser atmosphere than it does today, and lots of active volcanoes. Ancient Mars had the chemical ingredients for life, there was energy (solar, geothermal, and possibly chemical), and plenty of water. Many microorganisms on Earth today could have survived and likely even thrived in early Martian conditions. Maybe some things like our extremophiles on Earth were the last survivors of some Martian life that thrived there long ago. Could we find such signs of past life if it’s there? Do you think there could be things living on Mars today? Well, we’re not too sure about that. The surface of Mars is really cold and the pressure is really low, but some people think there could be some limited environments on Mars, especially under the surface, where living things could be thriving today. That’s one reason for sending robots there to study the rocks and the environments around Mars.

A couple of other places that might be good to look at for possible life are the ice-shelled moons of Europa and Enceladus. Europa is a moon of Jupiter and Enceladus is a moon of Saturn, and we now have evidence that both of these moons have oceans of liquid water under their icy crusts. How can these moons have liquid water? Both are way too far from the Sun for it to give enough energy to melt them, and their oceans are below their icy crusts. It turns out that the way that these moons interact with their larger planets, through the action of gravity, causes them to heat up inside. This internal energy not only is enough to cause them to have liquid water oceans, but might also cause them to have hydrothermal vents on their ocean floors. Because of this possibility, many scientists wonder if there could be living biospheres inside of the oceans of those icy moons.

Titan, another moon of Saturn, is also a planetary body in our solar system that astrobiologists are currently researching for potentially having some form of life. Life as we know it requires energy, chemical ingredients, and a liquid medium (for the life that we have here, that liquid is water). Titan has all three requirements, but is a very different place than Earth. It is incredibly cold on the surface of Titan (way colder than where we find any living things on Earth). However, there is a thick atmosphere and lots of organic molecules. Much as we have a hydrologic cycle here on Earth, whereby water moves through the atmosphere and rain and rivers and oceans and ice, there is an active cycle on Titan made up primarily of organic molecules like methane and ethane. There are even lakes of organic goop on Titan’s surface. So even though life as we know it couldn’t survive Titan’s surface, it could be an interesting place to look for some other kind of life. Extremophiles on Earth have led scientists to consider places like Venus and Jupiter and similar exoplanets as possible abodes for life. For instance, there could be biospheres out there that reside entirely within the atmospheres of their worlds and might even live on gas giants. What would those beings be like? Could they be giant gas-filled sacs or maybe have gigantic wings to allow them to stay aloft? Almost every star has planets orbiting it. Now that we know that there are many planets out there, we’re really wondering if we might one day soon find other worlds that are not only about the size of Earth and roughly the same distance from their stars, but that might also have biospheres that may be something like ours. We’re building better and better telescopes so that we can study those worlds when we find them to figure out if we can find signs of life on them. It’s a very interesting time to be alive when it comes to learning about whether or not we are alone in the universe.

Portrait photo of A comic-style illustration of Europa with a slice through it, showing an ocean underneath and cracks on the ice. A small Jupiter is seen in space in the upper right.
There are many environments in the solar system thought to be habitable, such as Europa, one of Jupiter's moons. Europa is considered potentially habitable because beneath its icy surface lies a vast liquid water ocean, kept warm by tidal forces from Jupiter's gravity.
NASA/Aaron Gronstal