Caitlin: Most of the time, the search for extraterrestrial life centers on planets kind of like ours.  We look for Earth-sized planets and habitable zones around stars that look a lot like our Sun.  In fact, our way of deciding how livable a planet or other body is usually boils down to "how much is it like Earth?"

But there are good reasons to keep an open mind when it comes to the search for life.  For one, planets like Earth aren't always great places to live, I mean, just look at Venus.  It's got a good star, a decent location, it's similar to Earth in size, and it also happens to be a toxic, boiling wasteland where not even robots can survive.  Don't go there. 

On the other hand, places we may think of as unlivable might not actually be so bad.  We've talked about some of these strange places a lot over the last few years, and here are some of our favorite episodes about them. 

First, when it comes to searching for life, we traditionally look for water.  But could life survive somewhere that didn't have any?  I'll turn it over to Reid for more on that. 

Reid: There are a lot of ways to search for life on other planets, and over the years we've talked about plenty of them, from looking for certain gases, to unusual types of light.   But for the most part, all those methods are governed by one guiding principle: follow the water!  That's because on Earth, we don't know of any species that can survive without it. 

Water is so important that, by definition, the habitable zone around a star is the area where the conditions are ripe for liquid water to exist.  But, as some researchers have pointed out, there's a chance we're just being a teeny bit Earth-centric.  After all, there's a lot of chemistry out there.  So maybe there's some type of life that uses an alternative to water. 

It sounds a little ridiculous, but researchers have begun to look into it, and they've found that it might not be impossible.  At first, the idea of life without water sounds silly because water is straight up amazing.  I mean, look at its structure! 

Water is a simple molecule of just two hydrogen atoms and an oxygen.  But, because of the way that those atoms are arranged, one side of that molecule has a slightly positive charge, and the other is slightly negative.  That makes water a polar molecule.  And while there are plenty of others like it, water is special because the difference between its charges is relatively large.  Among other reasons, that makes H2O really good at dissolving things. 
  
Its charges allow water to interact with other substances so strongly that it disrupts the chemical bonds between their molecules or atoms, causing the other substance to dissolve.  And water can do that with a huge range of compounds.  It's what scientists call a universal solvent.  A powerful solvent is essential for life, since biological processes require lots of different molecules all in one place.  And solvents allow them to mingle and interact.  That isn't the only good thing about water though.  On Earth, its properties may have allowed life to evolve in the first place.

That's because some molecules can be generally attracted to or repelled by water, and certain substances have both water-loving and water-hating parts in the same package.  Phospholipids are one of those substances. 

 In an effort to get close to, and away from the water at the same time, they'll organize themselves into nice little bubbles called vesicles.  That creates tiny pockets of chemistry where reactions can happen without being disturbed, and that may have allowed life to emerge. 

As a whole, water's structure makes it a powerful medium for biochemistry, and that's not to mention all of its other qualities, like the fact that it can keep temperatures on a planet more stable.  But just because water is important here, doesn't mean it has to be important everywhere.  There are other molecules that can fill some of water's roles. 

Take ammonia, which is one nitrogen atom bonded to three hyrdrogens.  Ammonia is abundant in Jupiter's clouds and it's also been detected in the plumes emitted from Saturn's moon, Enceladus.  Like water, it has a charge difference among its atoms, making it polar. That means it can dissolve a lot of substances, including many used in biochemical reactions.  But, because its structure isn't exactly the same as water, it isn't as good at dissolving the same things.  That means life on Earth couldn't use it as a direct substitute, but scientists think it's possible that a different kind of life could have evolved to take advantage of it. 
  
There are even ammonia-loving and ammonia-hating molecules out there, which could give rise to those vesicles for chemical reactions.  So, ammonia is a promising alternative to water, but it's not perfect. Its weaker charges and interactions make chemical reactions just a bit more tough.

Plus, while water is liquid over a range of 100 degrees Celcius, ammonia's liquid range is just 44 degrees at Earth-like pressures.  That would give life less wiggle room if a planet's climate fluctuated.  It can stay liquid at higher temperatures if there's higher pressure, like in a gas giant.  But right now we just don't understand how life could get started suspended in the clouds of a gassy planet.  So, another option researchers are considering is hydrocarbons, like methane and ethane. 

Broadly speaking, these are strings of carbon and hydrogen. And on Earth, they make up natural gas.  But elsewhere in the solar system, where it's much colder, these hydrocarbons exist as liquids. We discovered the most famous example of this in 2004 when the Cassini-Huygens mission revealed that the surface of Saturn's moon, Titan, is covered with lakes and oceans of liquid hydrocarbons.

At first, things like methane might seem like a weird substitute for water because they aren't polar, so they don't have the power to dissolve things.  But it's not like hydrocarbons can't dissolve anything.  They can dissolve all kinds of oils and fats, so some reactions would be possible.  Also, some molecules, like the ones used in DNA, might even be more stable in hydrocarbons than in water.  So maybe it would be even easier for life to evolve there. 

Scientists have also suggested that something like vesicles could form in places like Titan. Although they would likely use a different molecule since there aren't really phospholipids there. So again while hydrocarbon based life might look a bit different, it's not totally out of the question. 

Amonia and hydrocarbons aren't the only water alternatives either. Scientist have also been studying everything from liquid nitrogen to sulphuric acid and each seems to have some potential. So at the end of the day, maybe water isn't necessary for life. 

Before we can say that for sure though, scientist will need to learn more about biochemistry and more about water itself because some researchers think we don't even understand all the ways that water is necessary yet. 

Either way, as we start to understand this better, scientists might have to change what they consider a habitable planet and if they do, that will literally open up a whole new world of study. 

What's an episode without a good Alladin pun? Ok, so maybe life doesn't need water but at this point it's a big maybe. So let's assume life does need some H20 and maybe life could find water in places outside of what we think of as the habitable zone. Like at least some astronomers believe life could survive on Saturn's freezing moon (?~7:15) but it could survive really close to a star? It turns out that might not be out of the question either. Here is another one from Reid: 

What do you picture when you think of an exoplanet? It's a surprisingly important question because it probably shapes how you think about other star systems. Maybe even how you think about their HABIT or how they evolve. So take a second to consider it. Off the top of your head you might imagine icy rocky worlds or scorching gas giants but what about something a little stranger like eyeball planets? These are planets that look well kind of like eyeballs with one hemisphere totally different than the other. Because of how they orbit, they would have all their ice on one side and all their deserts on the other. They could even have concentric rings with different climates, like the pupil, iris, and white of an eye.

And while they sound like something out of science fiction, astronomers think they’re probably not all that rare. They could be more common than Earth-like planets, and according to some researchers, they might even be able to support life. Despite their unusual appearance, eyeball planets form thanks to a pretty standard phenomenon: gravity.

Gravity is what pulls a star and a planet together, and it’s also mostly what keeps a planet in orbit. But strange things start to happen when a world orbits its star too closely. The closer a planet gets, the stronger the pull of its star’s gravity is.

And over time, that pull can actually slow down a planet’s rotation. Eventually, the world becomes tidally locked, meaning it rotates in exactly the same amount of time it takes to orbit its star. In other words, it’s day is as long as its year, and the planet always keeps the same face turned toward its star.

If you want an example of this, just check out the Moon. It’s tidally locked to the Earth, which is why you always see the same pattern of craters on it. The difference is, the Earth is the Earth and not a giant scorching nuclear furnace.

Which as it turns out, does make quite a big difference! Around a star, tidal locking means that one of a planet’s hemispheres would bake and experience constant day, reaching temperatures up to 100°C or more. Meanwhile, the back side would be in an icy, perpetual night, at less than -100°.

Within the last few years, scientists have suggested that worlds like this may be even more common than we thought. But that doesn’t mean you need to abandon your dream of finding life off-Earth. Because eyeball planets could potentially be habitable, at least, under some specific conditions.

For one, your planet would likely need to orbit a red dwarf star. These stars are smaller and cooler than the Sun, so even if your planet was close enough to be tidally locked, it could still be in the habitable zone. That’s the area around a star where conditions are just right for liquid water.

Admittedly, red dwarfs are also more active than the Sun, producing lots of flares. But some researchers think that if a planet had a magnetic field, it would be okay for a while. Besides a red dwarf and a magnetic field, your planet would also need to actually have liquid water, along with an atmosphere.

This is partly because we’re pretty confident lifeforms need water and air, and partly because those things would help with the temperature differences. Wind and weather could even out the temperatures on an eyeball planet to something more like we see on Earth, between 50 and -50°C. It would still be unpleasantly hot or cold on some parts of the planet, but life could enjoy more temperate conditions in the narrow ring where the two hemispheres meet.

Still, even if the eyeball planets are habitable, living there would be almost nothing like what we’re used to. In that narrow ring, the sun wouldn’t move in the sky, and life would witness a perpetual sunset. This, combined with the dimmer red dwarf star, would mean that any photosynthesizing creatures would get much less light than they would on Earth.

Plants would have to take advantage of any light they could get, so they might evolve to be totally black, since black absorbs all colors of visible light. Also, because of the temperature differences, winds would blow constantly around the planet, up to a few thousand kilometers per hour. So organisms would either have to be streamlined to withstand this gale, or could take advantage of it to move around.

Finally, there would be no day or night cycle on an eyeball world, either. That might not sound like a big deal, but pretty much every lifeform we know of on Earth has some kind of circadian rhythm, driven by the planet’s rotation. Maybe the rhythms of life would evolve differently on an eyeball world, but with no examples around here, scientists really don’t know how that would work.

So, maybe there’s an eyeball planet out there with a perfectly streamlined, black creature that’s adapted to a world of twilight. Unfortunately, it will probably be a while before our technology can figure that out. Right now, though, scientists are actively looking into the potential habitability of eyeball planets.

Because these worlds aren’t science fiction, or a distant theoretical possibility. Red dwarf stars actually make up around 70% of all the stars in our galaxy, and statistics suggest that all stars have at least one planet around them. So there could be a hundred billion or so eyeball planets out there.

Scientists have already found probable candidates, too. The TRAPPIST-1 system, which is nearly 40 light-years away, consists of seven tidally-locked planets around a small red dwarf. Whether they have conditions right for life is still unknown, but with the next generation of telescopes coming online, we’re certainly getting closer to finding out.

Caitlin: So it looks like it could at least be possible for life to arise well outside the so-called habitable zone, and on either extreme, both really far from its star, and really close. But one thing is for sure, it needs a star, right? Here's Reid again with the answer.

Reid: When we look at life on Earth, we see rich, diverse ecosystems powered by our host star, the Sun, so when we think about life elsewhere in the universe, we usually imagine something pretty similar, an Earth-like planet orbiting at an Earth-like distance from a Sun-like star, but we now know that there are billions of planets in our galaxy that look nothing like this.  

In fact, there are billions of rogue planets that aren't orbiting a star at all.  It's thought they formed inside star systems like regular planets but were then somehow ejected from their original orbit and flung into deep space.  The sheer abundance of these planets has led scientists to wonder if life could emerge without a star, and though obviously we don't have conclusive evidence, there's actually good reason to think that it could.

It's hard to imagine anything thriving with no star because our Sun is so vital to life on Earth, but it turns out that a star's light and heat might not be dealbreakers.  For instance, while the life we're most familiar with is powered by sunlight, there are plenty of things that survive without it.  In fact, for at least a little while, no life on Earth used sunlight as an energy source.  The molecular tools to perform photosynthesis arose after the first microbes and that's part of why multiple hypotheses on how life first emerged involve some pretty dark places, like today, we know the large number of micro-organisms that live deep underground, where they survive off chemical reactions in the surrounding rocks, so one hypothesis is that life first emerged in subterranean pockets of water or life could have started around hydrothermal vents, places on the sea floor where volcanic activity produces jets of steam.

A variety of organisms live around these vents, so it's not hard to imagine life beginning there before it found its way to the surface.  What all these origin stories have in common, though, is liquid water.  That's because water is vital to all life on this planet and if we assume that life, period, needs liquid water, then its existence on a rogue planet is much less likely as water can only be liquid at a very narrow range of temperatures and pressures.

Of course, it's not guaranteed that water is needed for life.  As we've said in a previous video on this topic, there's an awful lot of chemistry out there, but even if life can live without water, it probably still needs some heat.  Deep space is just too cold to envision any interesting biochemistry going on, water or no water, and without host stars to warm them, most rogue planets are probably deep space cold.  We're talking just a few degrees above absolute zero, except there are a surprising number of ways they could be heated up just enough to support life.  

They might warm themselves from the inside, for instance.  That's something we see with a lot of planets, including Earth. In our case, about 10% of the core's heat is left over from the collisions that formed Earth, while the rest is from radioactive decay and it's been suggested that similar processes could produce enough heat inside a rogue planet to warm a subsurface ocean of water for billions of years, plenty of time for life to emerge and evolve.  

Even with this kind of core heat, though, a world like this would probably need a surface layer of ice several kilometers thick to act as insulation, much like, I don't know, we see on Jupiter's moon, Titan, or there's another potential way to insulate a rogue planet: a super thick atmosphere.  A hydrogen-rich one, 10-100 times thicker than ours would do the trick, and it turns out rogue planets may be better suited to retaining these atmospheres than ones in so-called habitable zones around stars because stellar radiation can blast that sort of atmosphere away.  

It's also possible a rogue planet could get a temperature boost from a mechanism called tidal heating.  Essentially, gravity warms up two orbiting bodies for the same reason it causes tides.  The differences in gravity felt by different parts of the worlds makes them squash and stretch, generating large amounts of friction and again, there seems to be a somewhat local example of a sub-surface ocean that's heated this way, Saturn's moon Enceladus.

So there's no reason to think that this couldn't happen on a rogue planet with its own moon.  Now, I know what you're thinking.  If these planets are off roaming the galaxy, they're probably doing it solo, but it seems like rogue planets can have moons.  In fact, simulations suggest that close to 50% of moons could stick by their planets when they go rogue, but before we get too excited about the possibility of life on these wandering worlds, it's worth noting that it's hard to imagine anything more complex than micro-organisms on rogue planets.

That's because these heating mechanisms give nowhere near as much energy as direct starlight, like what we get from our Sun.  As far as we know, the Sun is what gave life on Earth the ability to evolve the diversity and complexity we see today. Still, it's fun to imagine what strange forms of life could be living in pure darkness in a vast, sub-terranean ocean.  Plus, it's definitely possible that there are some ambitious creatures out there, eking out a life for themselves on a rogue planet.

Caitlin: A place without sunrises and sunsets sounds kinda sad, but I guess if you're a microbe you might not mind because, you know, their lives are pretty sweet. These days when astronomers look for signs of life, they're often looking for signs of organic molecules, the building blocks of life on Earth. And organic molecules are all based on carbon. But astronomers have at least wondered whether it's possible to have life on a planet without carbon-based molecules. Here's what they think.

People talk about the search for alien life all the time: why we haven’t found any yet, and what we should be looking for. But somewhere out there in the universe, there might be creatures so different from us that we wouldn’t even recognize them as alive.

All living things that we know about have some basic things in common: they grow, reproduce, respond to stimuli in their environment, and evolve over time. They also all share the same basic biochemistry: they’re made out of long chains of carbon molecules that hang out in a water-based medium. But it’s also possible that there’s life that’s not carbon based. And it would be a lot different from what we’re used to.

Carbon’s ability to form long chains makes it the perfect base for building molecules complex enough to keep a living thing... alive. But even though carbon is great for building these big, complicated molecules, it’s not the only element that can do this. Silicon, which sits just under carbon on the periodic table, shares many of the same chemical properties. Just like carbon, the outer layer of a silicon atom has four unpaired electrons ready to form molecule-building bonds.

Silicon can also form long chains and bond to oxygen--again, just like carbon. But a lot of the bonds formed by silicon are weaker than those formed by carbon--especially the silicon-silicon bonds that it would need to make those long chains. And even when silicon-silicon bonds do form, they’re generally unstable if there’s oxygen around. Still, because carbon and silicon have so much in common, some scientists think that this silicon-based life could theoretically be possible, even though we’ve never found any.

Like carbon-based life, silicon-based life would grow, reproduce, respond to stimuli in its environment, and evolve over time. But we might not even recognize it as life at first. A silicon life-form might look more like a pile of rocks than a plant or animal, and it would probably do some pretty weird things.

When silicon reacts with oxygen, for example, it turns into quartz, so silicon-based organisms that breathed oxygen would exhale quartz! Silicon-based bonds are most stable at high temperatures, so if silicon-based life does exist, the best place to look for it would be in very hot places, like deep beneath a planet’s surface. But silicon-based life probably wouldn’t be very complex, because of its unstable bonds.

Still, science fiction writers have had a lot of fun with this idea, like, in one episode of the original Star Trek series, Kirk and Spock run into a tunnel-dwelling silicon-based creature that’s terrorizing miners on an alien planet. But organisms with such different biochemistry might be hard to even detect -- they could look more like rocks or crystals than anything we’d recognize as alive. So, some scientists have proposed the idea that there could be a so-called “shadow biosphere” of non-carbon based life living alongside us right here on Earth.

After all, silicon is the second most common element in the Earth’s crust after oxygen. But that silicon is bound up in rocks, where it would be hard for organisms to incorporate it into their biochemistry. If silicon-based life does exist on Earth, it might be in the form of silicon-based microbes living within magma deep inside the Earth’s mantle.

But no one’s ever found any actual evidence of this. Our best bet for finding silicon-based life is probably to just look somewhere else. The James Webb Telescope, launching in 2018, will look for signs of life in the atmospheres of planets outside our solar system.

Photosynthesis and respiration affect the amounts of oxygen and carbon dioxide in Earth’s air. Silicon-based organisms could leave traces of their existence in their own planets’ atmospheres, too. Meanwhile, some scientists are working on making their own version of silicon-based life -- or at least, the first steps toward it.

In March, Caltech scientists announced that they’d found a species of thermophilic bacteria-- bacteria that thrives in extreme heat--with an enzyme that in very rare cases incorporated silicon molecules into its carbon-based molecules, a sort of chemical accident.  Using artificial selection, they were able to modify the bacteria in a lab to produce the molecules with silicon in them 2000 times as often. Someday, artificially-created microbes like this may be used to produce complex silicon molecules that chemical companies can turn into glues and sealants.

And even cooler, research like this also helps scientists understand more about how life can use silicon. So the next time you are outside and and stop to look at a cool rock, you might want to look for signs of life. You never know.

Basically, even though Earth is a great place to live and seems to be perfect for life as we know it, there's no reason to rule out life on other kinds of worlds. But when we talk about the possibility of life on those worlds, we often just picture microbes surviving there. We assume most places couldn't support diverse ecosystems like Earth. Then again, some scientists have wondered if it would be possible to find a planet that's even better for life than our planet. Reid has got the scoop on that.

Reid: It’s pretty reasonable to assume that Earth is the ultimate Eden in the universe, a paradise planet that’s just perfect for life. After all, not only is it our home, but it’s also the only place we know life has evolved.

But, as it turns out, there might be even better places to live, planets even more suitable for life. Scientists call them superhabitable planets. They’re worlds that could sustain life more than five times longer than Earth, and while we haven’t found one for sure yet, there might be billions of them lurking in our galaxy alone.

The idea of superhabitable planets was introduced in 2014 by two North American researchers: René Heller and John Armstrong. And while each planet could look a little different, they would have two main features that would make them better than Earth.

First, these planets would be able to maintain liquid water much longer than ours. Earth’s cozy, life-sustaining environment is largely thanks to its position relative to the Sun, along with other factors like the atmosphere. We exist in what’s called the habitable zone, or the area around a star where liquid water can exist on a planet’s surface. But that won’t last forever. In five billion years or so, toward the end of its lifetime, our Sun will have expanded into a larger red giant. And it will actually go through some pretty major changes even before that point.

Over time, as the Sun burns through the fuel in its core, it will gradually become hotter and brighter. That means it will let out more energy, so the habitable zone will migrate outward. For us, that means that it will eventually become so hot that the water on Earth will be vaporized.

Which isn’t exactly great for anything that wants to live here. The Sun’s habitable zone is currently pushing outwards at about a meter a year. And it’s predicted that we’ll fall off the zone’s inner edge in some 1.75 billion years.

Two billion years is a really long time, but it turns out that other planets will stay in their stars’ habitable zones for much longer, because they’ll orbit stars better than our Sun. The Sun is a G-class yellow star, or one of the mid-sized, longer-lived stars in the universe. But smaller stars actually live even longer.

That’s because there’s less gravity to power the fusion reactions in the core, so they burn slower, and their fuel lasts longer. That means their habitable zones will migrate outwards more slowly, too. So a planet will be able to maintain the right temperatures for liquid water for many more years.

The smallest stars are M-class red dwarfs, which live for trillions of years. But they come with their own problems, like explosive solar flares. So instead, scientists think the ideal stars for habitable planets are the intermediate.

K-class orange stars. They’re about 50 to 80% the mass of our Sun and can live more than 30 or 40 billion years. But superhabitability doesn’t just come down to the star.

The second trick to being better than Earth involves tweaking the planet itself; its atmosphere, geology, and geography. Earth is pretty good, but we still have deserts, ice caps, and big chunks of deep ocean without as much complex life. A superhabitable planet would do away with these barren landscapes to support more species and more diversity.  And one of the ways of doing that is to be more massive than Earth. According to researchers, an Earth-like planet maybe twice as massive as ours would have a bunch of great things for life. For one, it would have more gravity, which would help to hold on to more atmospheric gases.

That would give the planet more greenhouse-style warming, so it could orbit a little farther from its star and still stay warm enough to have liquid water. That would buy it even more time before it fell off the inner edge of the habitable zone. Stronger gravity would probably also help heat the center of the planet, which might cause more active plate tectonics.

That would likely be a good thing for life. A geologically active planet would keep renewing elements like phosphorus and calcium, which life needs to thrive. All the shifting plates would keep bringing the elements to the surface, stopping them from getting trapped in the planet’s crust for too long.

And, if that wasn’t enough, higher gravity would also cause the iron core of a planet like this to be hotter. That would likely result in a stronger magnetic field, which would protect the planet’s atmosphere from particles and flares flying off its star. So, in around two billion years, when it’s time for humans to find a new home, we have our checklist of requirements.

And the good news is, there might be a pretty good chance of us finding one of these superhabitable worlds. When scientists peer into the night sky, K-class stars are actually about twice as common as Sun-like stars.

And since research suggests there are planets around most of the stars in the universe, there’s potential for more than 10 billion superhabitable planets in the Milky Way alone. Using new space telescopes like TESS, scientists soon hope to study the characteristics and atmospheres of promising candidates, like one 1200 light-years away called Kepler-62f. Then, they can see if their hypotheses about superhabitability hold up.  So, while it might come as a bit of an unwelcome surprise that Earth probably isn’t the best place in the universe, there’s no need to be too bummed out. We’re well on our way to finding a new Eden out there among the stars.

Caitlin: Overall, just looking for life on planets like Earth might be distracting us from all the other places life could possibly exist. I mean, it might not look anything like we imagined, but it might still be out there, living its best life in unexpected places.

Thanks for watching this episode of SciShow Space, and a special thanks to our patrons, who made all of these episodes possible. We can't make these without you. 

If you’d like to support what we do and help make science education free on the internet, you can find out more at patreon.com/scishow.