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Scientists have been searching for alien life by honing in on the existence of liquid water, but we might be overlooking some types of life out there that doesn't need water at all.

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[♪ INTRO].

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 right for liquid water to exist. But as some researchers have pointed out, there's a chance that we're just being a teeny bit Earth-centric.

After all, there's an awful 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 so 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 also 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 hydrogens.

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 Celsius, 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 same 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 ones used in DNA, might be even 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. Ammonia and hydrocarbons aren't the only water alternatives, either. Scientists 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, scientists will need to learn more about biochemistry, and more about water itself. Because some researchers think we don't even understand all of 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. Thanks for watching this episode of SciShow Space!

There's lots to explore in the universe, from chemistry to astrodynamics, and we're thankful for the chance to unpack so much of it. So to our patrons on Patreon, thank you! And if you want to support the show and help us make more episodes like this, you can go to [♪ OUTRO].