Looking for life beyond Earth is an expensive business so it will be necessary to prioritise goals, although history is full of the unpredictable.
One fact we need to face up to is that SETI (search for extra-terrestrial intelligence) must confine itself to searching our own Milky Way Galaxy
(MWG) of 160 billion stars plus perhaps several times that number of attendant
planets. The searchable distances range from a few light years to tens of
thousands. Anything beyond is not detectable by any technology we can envisage.
|The Milky Way would look green from afar |
- a mixture of yellow and blue stars.
Andromeda is the only other green galaxy we know of.
This is fortuitous because ours is very likely the only galaxy that could sustain intelligent, sentient beings and the life has to be in a tightly restricted region of the galaxy because anywhere else would be hostile to life.
What is so special about our MWG?
1. The MWG is a spiral nebula, the only type for which life is possible. Only 10% of galaxies are of this type (the other 3 are elliptical, globular and irregular galaxies, all totally hostile to life.)
2. It has escaped any major merging with another galaxy. Such collisions prevent even a spiral galaxy harbouring life since the cosmic environment becomes unstable, so that all planets are bathed in hostile radiation and subject to chaotic forces. You would normally expect such collisions over the 10 billion year history of the Milky Way.
3. The Magellanic Clouds, which are appendages to our galaxy, choreograph the influx of gas from dwarf galaxies so that just the right amount enters to renew material in the spiral arms where new planets form without causing instability and structural degradation.
4. The black hole at the centre of this galaxy is exceptionally small (e.g. 1/20th the size of the corresponding one in the Andromeda galaxy) and so is less disruptive than other black holes.
5. The MWG’s black hole appears to be sending out waves of radiation periodically, orchestrating the balance of the stellar population for life in ways that are barely understood, possibly in consort with the Magellanic Clouds.
6. The MWG is a ‘green valley’ galaxy, consisting largely of yellow stars like the Sun, with a few blue stars forming each year (say 300,000 since Homo Sapiens began) as new gas is introduced by the Magellanic Clouds. The overall colour effect is green. This has allowed yellow stars to stay dominant for much longer than for any other galaxy except our neighbour the Andromeda and so the solar system has not been subject to violent stellar activity associated with blue and red stars. (In Andromeda’s case violent merger events have been the cause of the gas influxes but these prevent life sustaining cosmic environments.)
7. The MWG is more massive than would normally be expected for a galaxy of this size as seen through a telescope. This is because it contains a lot of dark material. The large mass helps it remain stable while gas is being drawn in and while its central black hole is ‘erupting’, as it does periodically.
8. The cleanly defined spiral arms of the MWG, together with its low luminosity, allow the inhabitants of any planets to probe into the depths of the universe, right back to its Big Bang beginning. Other spiral galaxies, like Andromeda, have spurs and feathers between the arms, while non-spiral nebulae have at least as bad observing conditions.
I understand that there are over 200 special features of the MWG which are crucial to life’s development. Moreover, the benevolent conditions which allow life to thrive and evolve are at least typified by the other yellow stars in the same spiral arm that houses the sun. It is this area we need to focus on to get a good idea of whether we are alone – at least as regards carbon-based biological life and this is in any case the only explorable area.
Other forms of life based on other molecules are conceivable since laboratory experiments to synthesise life forms based on alternatives to carbon , albeit suboptimal and extremely primitive, could succeed within a decade or less. What these experiments show is that life could not possibly have been started via the assembly of chemicals by blind chance – phenomenal intelligence was involved, such that we can barely begin to imitate it, although even crude, simplified imitations of microbial systems could be useful in combating disease or producing new materials with desirable properties.
It is sometimes said that life not-as-we-know-it could exist in all kinds of unlikely conditions. Yet by definition we would not know how to detect it.
If we find definite evidence of microbial life on Mars (the Curiosity rover has just landed at the time of writing) it will indicate that it could occur almost anywhere in the universe but that a suitable environment is needed to allow it to thrive and progress into sentient beings. So then it will be a matter of looking thoroughly around our corner of the MWG, the only corner we are able to examine in sufficient detail, to find such conditions.
What are these conditions?
What are these conditions?
They are not easy to define since the advanced life on our own planet actually created its own modern biosphere by working on the rocks, lava, ocean and atmosphere of the newly formed planet and turning it into the world we see today. Nevertheless, there were certain aspects of the pre-life environment which are absent from other planets, from the stars they orbit around and from the galaxies to which the stars belong. I listed a lot of these in Our Precious Planet but recent articles are showing a growing list of ways in which our planet and its place in the universe are extremely rare, if not truly unique.
(Update. This paragraph was added after listening to a podcast . I can't imagine why it had not already occurred to me.) On the other hand, if any organisms are found on Mars or elewhere in the solar system it could be they are just extremophiles, like the ones found on Earth in volcanic vents and other hostile environments. This is in fact quite likely given the abundance of life here over billions of years and the biotic debris ejected into the solar system when asteroids hit the Earth. In this case we should still search our inrerplanetary locale but with less assurance that bacteria are present throughout the Milky Way.
In short, the nearby stars in the Milky Way which we are able to observe are the very ones most likely to have life. So let’s keep looking. Whatever the findings they will have major implications for everyone.
The benevolence of black holes by Caleb Scharf (Scientific American (August 2012) p.22-7)
Why the Milky Way may be facing a mid-life crisis
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