Planets are common. Is life?

Now 5000 exoplanets around other stars have been found, what does that tell us about the probability of life?

Our Child of the Stars assumes that another intelligent, social, technological species lives close enough that they can reach us with their faster than light drive. This is a common assumption in science fiction (which doesn’t mean the people who write it all think it could be true.)

Once we know two intelligent species exist, quite close together as galactic distance goes, the odds rise that there are plenty of others.  That raises Fermi’s famous paradox – if life is common, and technological civilisations arise, and the universe is very old, where are the older species of aliens?  Shouldn’t they already be here?  

In essence there are probably six questions

  1. Does life arise anywhere the conditions are right?
  2. How common are those right conditions and how long do they last?
  3. How often does complex life arise where the conditions remain right long enough?
  4. Will complexity given time always bring intelligent, social creatures?
  5. How often do intelligent, social creatures develop a technological civilisation?
  6. Do technological civilisations last long enough to spread between the stars – and do they want to?

Where are we on the hunt for alien life?

We have noticed no alien transmissions, but we have not searched enough of the sky, on enough frequencies, for long enough, to give up yet. Some things we need to know, we are still largely in the dark. A few we have learned a lot in the last few decades.

Thanks to the Kepler Space Telescope and some other searches, we can now be very confident the galaxy is packed with planets, and many will be in the right temperature zone, and in the right spread of sizes.

Life evolved only a few hundred million years after the Earth got cool enough to allow it. But unless a planet is within a stable range of temperatures, over a very long time, favourable conditions will end. It takes many millions of years to get from say bacteria to me.

That means, a stable orbit, and a stable star to orbit, is crucial for temperatures to remain in the right ballpark for long enough.

Astronomer Professor David Kipping says our solar system is unusual. To start with only a tenth of stars are like the sun – relatively stable output for a long time. Then, our system is tidy – one sun, and all the planets are in almost circular orbits.

Systems with multiple suns – the majority – usually won’t have planets in stable orbits. The twin suns of Tattoine? Probably not.

Jupiter’s role

Kipping argues massive planets like Jupiter are key. Current theories see Jupiter’s gravity playing a key role in developing the solar system.  Big Jupiter-like planets are common and easy to detect by radio astronomy. However, most Jupiters seem to have an eccentric orbit, swinging closer to and further away from the star.  Over time an eccentric Jupiter would pull other planets in all sorts of ways – a complex and changing set of orbits for smaller planets, including the possibility of being kicked out of the system altogether. In these solar systems, life might evolve but get shut down when its orbit changes, becoming too hot or cold.  Systems with a Jupiter close to the star tend not to have other planets near the star. Systems without any Jupiter at all could be unstable without a big gravity shepherd.

Kipping proposes we treat ‘having a Jupiter in a fairly circular orbit’ as a good way of picking solar systems that might be ‘like ours’. He thinks only one in a hundred suns like ours could have a stable earth and many of these there might be no big enough planet in the temperate zone.

There are many things which could prevent suitable planets remaining suitable. There are disasters affecting many systems at once (supernova/gamma bursts/wandering stars busting open the system, etc). A planet needs to be big enough to hold onto its atmosphere.

Nevertheless a small fraction of an enormous number is still a very big number.

A planet might have abundant life, even be intelligent, and never go spacefaring. For example, inhabitants of a world entirely covered in ocean would struggle to develop chemistry or use metals.

Life could easily be so rare, we might be alone in the Galaxy. But there is still a lot we don’t know and may not know until we run into another habitable planet.

If faster than light travel is impossible, spreading from star to star will take a lot of effort. Many cultures might not want to do so or need to. (But can we assume no culture would try?)

The real takeout is that the Earth, which we seem hell bent on destroying as a habitat, is probably rare.  Nowhere in the solar system is as favourable for us as say, the coast of Antarctica, or the Sahara desert. We need to treat our biosphere as if it is the only one we have, and possibly, the only one we will touch for a thousand years, or a million.  If Earth was hit by an asteroid, it would still be a better place to live than anywhere else we know.

In 12,000 years since we developed settled farming, we’ve got to a point of self-destruction.  There may be a simple and nasty explanation for the radio silence.  Maybe developing a technical civilisation is toxic.

Our Child of Two Worlds talks about that too.

(You may say what about life that is a wildly different chemistry and lives in the corona of stars or orbitting black holes. We don’t know how that would work or if we would recognise it if we found it.)