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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 patreon.com/scishow.

  • [♪ OUTRO]

[♪ INTRO]

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