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Q: Why are scientists looking for life in space by looking for water? How can they be sure that all life uses water?

The original question was: I’m reading more and more lately about the findings of the Kepler satellite and that some scientists are estimating that roughly 5% of newly discovered planets in our galaxy may have mass similar to Earth, which always leads to the second question about how far they are from their stars and whether or not they might have liquid water.

Why are we so sure that life requires (liquid) water? Seems a little Earth-centric to me (ie all life we know needs water so all other life must too). Do they also need iphones? Is there a good explanation for this grounded somewhere in physics?

Physicist: This is the central question of astrobiology.

There are two big reasons why space folk are looking for water. The first and obvious one is that, without exception, ever form of life we’ve ever found has required water. Admittedly all of those forms of life were found here on Earth, but it still holds true.

In an attempt to combat this, biologists have been scouring the planet looking for “shadow biospheres” and “extremophiles“. For example, it was once believed that nothing could survive in extreme radiation, or boiling temperatures, or without sunlight or oxygen, but each of these have been found to be false.

For example, life (sometimes even complex life!) has been found thriving deep underground, in anoxic basins (“puddles” of dense, ultra salty, oxygen-free water scattered on the bottom of the ocean), and famously around and even inside of black smokers on the bottom of the ocean.

Life is pretty sneaky (and gooey, more often than not). Recently life was found in Mono Lake that can use arsenic instead of phosphorus when it has to. There are even bacteria that have found clever ways to live inside solid ice (Technically those clever bacteria melt the ice).

Excursions have been made to places like Death Valley or the Atacama Desert to find life that doesn’t use water. Both places are extremely dry, but otherwise aren’t so bad. Unfortunately, so far everything has come back negative. Living things can survive pretty much anything except for being thirsty. So that seems to be the line in the sand (mud).

The second reason that astronomers are looking for life by looking for water is that you can’t look for something if you don’t know anything about it. If someone asks you to go into a room and “find the keys”, you can do it. But if someone sends you into a junk-shop and asks you to “find that thing”, you’d have no way of knowing if you’ve successfully found it or not.

If there’s life out there that doesn’t involve water, we have no idea what it’s like. Maybe giant crystal formations that reproduce on geological time scales? Maybe parts of the weather system are living organisms (somehow)? Maybe there’s life similar to us, but that uses something like liquid ammonia instead of water? Giant transforming robots? Who knows.

Worse than that, we have a hard time even defining what life is. It’s surprisingly difficult to come up with a definition for “living” that includes things like viruses and dormant spores, but that doesn’t include fire.

So, it’s not that we’re sure that life requires liquid water, we just don’t know what else to look for, and liquid water and life seem to always go hand in hand (here on Earth at least).

The junk shop photo is from here.

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