Posted by: Kash Farooq | March 17, 2013

Why are astrobiologists just concentrating on exoplanets with water?

Hypothetical ocean planet with a terrestrial atmosphere and two satellites.

Hypothetical ocean planet with a terrestrial atmosphere and two satellites. Image by Luciano S. Méndez.

I recently interviewed the astrobiologist Professor Charles Cockell for The Pod Delusion.

Adam Jacobs left a comment on the Pod Delusion website:

Kash, something that I always find puzzling when I hear astrobiologists talk about the search for extraterrestrial life is that there seems to be a totally unquestioned assumption that life can only exist where there’s liquid water.

Now, that may be true on Earth. But life on Earth has evolved on a planet where there is a huge abundance of liquid water. Who’s to say that life based on some completely different sort of chemistry couldn’t exist on other planets?

I have to say (and I speak as someone with a PhD in chemistry) that I couldn’t actually imagine what sort of chemistry might evolve into life that’s completely different to what we have here on Earth, but then everything I know about the chemistry of life is conditioned by studying how it works here. So just because I couldn’t imagine a different way of doing things, it doesn’t mean that it’s impossible.

Do you think astrobiologists have good reason for focussing on liquid water in this way, or could they be missing a trick?

This is a very good question and something I should have actually asked Professor Cockell to explain.

I’ve replied on the Pod Delusion website, but I thought I could also turn that reply into a blog post.

Let’s start with the basics.

There are exoplanets everywhere!

There are various techniques that can be used to find exoplanets orbiting around stars. For example, you can find exoplanets using Doppler Spectroscopy. Another technique looks for the dips in starlight caused by an exoplanet crossing the face of a planet from our point of view – this technique, the transit method, is how the Kepler mission looks for exoplanets.

There are now hundreds of confirmed exoplanet discoveries, and hundreds more that are classed as unconfirmed (i.e. more data is needed). It is estimated that there are billions of planets out there. Planets seem to readily form around stars.

All 786 Known Planets (to scale) as of June 2012. Image by

All 786 Known Planets (to scale) as of June 2012. Image by

The habitable zone

Amongst all the discoveries and potential discoveries, astronomers and astrobiologists are looking for a so-called Earth 2.0. A planet that is Earth like. One that may be capable of sustaining life.

In particular, there is a search for planets that are in the habitable zone of a star. This is the range of distances from a star for which liquid water can exist on a planetary surface. Stars of different size and luminosity (a measure of the total amount of energy emitted) will have different sized habitable zones. There are further refinements of the habitable zone size with the Kasting Model – this attempts to determine the inner and outer edges of habitable zones taking the effects of albedo (a measure of reflectivity) and greenhouse gases into account.

But how we can possibly detect life from billions and billions of kilometres away!?

Once we have found potential target planets, how can we work out if there is any life on that planet? We could try to listen in that direction, to see if intelligent life is transmitting anything. This is something that SETI or doing.

Or, if the planet transits the star, we could look at the star’s light that is filtering through the atmosphere of the exoplanet. We can split the light into a spectrum and use a technique called spectroscopy. I’ve written a few posts about spectroscopy (I love spectroscopy!), but the post of interest is Finding Out What a Star Is Made Of With Spectroscopy. This shows how we can analyse the light and detect specific signatures in the spectrum that are created by certain elements or molecules. We can, for example, detect water or oxygen or methane.

Analysis of the spectrum of Saturn - we can see that there is methane in the atmosphere by finding the unique signature of methane.

Analysis of the spectrum of Saturn – we can see that there is methane in the atmosphere by finding the unique signature of methane.

We could detect the existence of  life on another planet by looking for gases that, as far as we know, require a biological origin. An example gas is oxygen, which on Earth is produced by life – photosynthesis in plants. There is no geological process on Earth, or indeed the rest of the Solar System, that generates oxygen in the quantities found in Earth’s atmosphere.

An even better result would be to find methane and oxygen at the same time.  These two gases readily react and become carbon dioxide and water. If they were to be found in significant quantities in the atmosphere of an exoplanet, then they must be getting continually replenished by something. On Earth, life replenishes the methane that is lost in the oxygen/methane reaction.

Why are astrobiologists always looking for an exoplanet that has water?

Finally, getting to Adam’s question. Why do we assume that water is essential to life?

Basically, liquid water appears to be an essential requirement for life. For life to get started, there needs to be medium in which molecules can bump into each other and react. Water is the perfect medium for that to happen. It has been called the universal solvent because so many different substances can dissolve in it. Water allows dissolved molecules to get close to each other and react.

And water is liquid in an ideal temperature range for chemistry – not too cold to sustain biochemical reactions, and not too hot to stop organic bonds from forming.

It has been suggested that on some exoplanet (or moon), ammonia could perform the same function as water. It is liquid on worlds much colder than Earth. The problem with this is that at such low temperatures, chemical reactions are slow and may not happen very frequently. And because the reactions aren’t happening very rapidly/frequently (or not at all), molecules would not form of sufficient complexity. Hence, life may struggle to become established under these conditions – basically, there is not much happening in the ammonia “soup” from which life could develop. [Updated: After some questions on G+ by Bob Churchill, this paragraph has been updated to (hopefully) make it clearer.]

Perhaps “life as we do not understand it” exists on another planet. However, we only know of life that is water-based. Anything else would just be speculation. There are billions of planets out there – we have plenty of targets to look at. We can afford to restrict ourselves to just look at those planets that may have liquid water; and hence we know that on these planets life could exist. [Update: This paragraph has been added following the comment conversation below.]

In summary….

We look for planets in the habitable zone, as those planets may have liquid water. And if an exoplanet has liquid water, which we think is essential for life, then life may have got going on that planet.

We can then use spectroscopy to look for the signatures of life in the atmospheres of these habitable zone planets.

Further Reading

If you want to learn more about this fascinating subject, there are are couple of books I highly recommend:

  1. Life in the Universe: A Beginner’s Guide – by Lewis Dartnell.
  2. Strange New Worlds: The Search for Alien Planets and Life Beyond Our Solar System – by Ray Jayawardhana.


  1. Thanks for your detailed answer, Kash.

    I have to say, though, I’m still not convinced. Your answers are all based on the way life works here on earth. Yes, water is a brilliant solvent for the kind of biochemical reactions that make up life on this planet. But what if life used a completely different set of biochemical reactions on a different planet? Maybe they’d work just fine at the temperature of liquid ammonia. Or liquid sodium chloride for that matter.

    Organic chemistry as we know it does a pretty bang-up job of allowing life to take place in liquid water. And maybe life everywhere is going to look pretty similar to that. Maybe life made up of organic molecules looking pretty similar to nucleic acids and proteins is really the only way it’s ever going to work.

    But can we rule out the possibility that life could have evolved somewhere else in a manner that’s just mind-blowingly different to anything we currently understand?

    Hard to see how we can be sure.

    What is for sure is that if we ever find any extraterrestrial life in our lifetimes, then neither of us would ever have lived through a more exciting scientific discovery!

    • True, we can’t rule it out.

      The other reason I’ve heard for concentrating on water is that we know life can exist where there is water. Anything else is speculation.

      With so many exoplanets discovered, and to be discovered, we may as well concentrate on the ones which may have liquid water – as we know that they are suitable for life.

      Though, perhaps the type life on Earth is actually completely at odds with everywhere else!

  2. Yes, I guess that makes sense. Perhaps, given that our best guess is that water is necessary for life, it makes sense to focus resources on those planets that seem to give the best chance of finding life.


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