Friday, 30 December 2011

Earth-like planets: is the universe teeming with life?

Astronomers have for a long time been aware of an earth-like planet. It is virtually identical in size to the Earth, similar in mass, in a reasonably habitable zone and orbits a star identical to ours. In fact it orbits the same star, the sun, and the planet is called Venus. Recently, traces of oxygen were found in its atmosphere.

Unfortunately it is either completely arid or has a form of bacterial or plant life so microscopic that it defies current detection. Its barrenness is not surprising. It has a surface temperature hot enough to melt lead and an atmosphere of sulphuric acid & carbon dioxide, no water we can detect and a crater ridden surface where the atmospheric pressure is 93x that on Earth. 
Mars is another planet that would qualify as earth-like if detected around another star but despite having water and even traces of oxygen no sign of life has yet been discovered.

Both these planets are in what is optimistically referred to as the ‘Goldilocks’ zone, where the temperature should be ‘just right for life’. It is also called the ‘habitable zone’. These terms are misleading, almost meaningless. Even the moon is in the habitable zone yet seems unable to support life: it has no atmosphere and its surface temperature ranges from minus 153 to plus 107 deg C, depending on whether you are in shade or sunlight. One of the most likely candidates for microscopic ele ental life forms  in our solar system is Europa, a satellite of Jupiter, which is well outside the habitable zone but has a lot of frozen water.

If planets like Mars or Venus had been detected in another solar system they would have been proudly presented to the media as earth-like planets circling a sun-like star in the Goldilocks zone, conjuring up images of jungles and oceans and intelligent beings.

Yet the Earth, equally hostile to life when it was formed together with the other planets, houses an abundance of life. It is the only place in our solar system’s habitable zone which supports a living ecosystem. Why? Because life has evolved to convert its surface over billions of years from a very unpleasant place (hot rock, poisonous gases, volcanoes, cometary bombardment etc.) into a paradise just before homo sapiens appeared. Even today, despite our bad stewardship, the biosphere is benign and requires no protective clothing or spacesuits for survival.

So how did it start here, when the surface was as hostile as anywhere else in our solar system and how was it able to advance the way it has? The second part is easier to at least partly answer: the conditions are benign. It has a protective magnetosphere to guard against harmful radiation from space, a circular orbit to keep the climate stable, a steady rotation about its axis thanks to spin stabilisation by the moon, tides in its oceans to encourage evolution of sea creatures to land animals, a crust with plate tectonics and chemicals that life could use, a rare position in the galaxy free of danger and offering an unobscured view of the universe.  See also  Our precious planet parts 1,2 and 3.

 Life has advanced from primitive anaerobic forms to oxygen-breathing forms by altering the composition of the atmosphere, i.e. by plants breathing oxygen out into it.  It even evolved its own protective layer – the ozonosphere – to absorb harmful UV rays from the sun. The clear skies, the view of the regularly rotating heavens and the equality of the apparent size in the sky of the moon to that of the sun, stimulated human curiosity.

To flourish it had to start. Was it by the chance juggling of chemicals? The probability of this is infinitesimal. The late Sir Fred Hoyle put it at far less than 1 in 10 to the power of 40 (1 part in 10 with 40,000 zeros after it). There are other estimates but all mathematicians come up with a vanishingly small probability, with numbers far greater than the number of atoms in the known universe. If it was by chance then life must be so rare that no other star in our galaxy or any other will have planets with life on them. SETI would be a complete waste of time.

Or is there some property of the natural order that makes life appear wherever the conditions are right (whatever they may be)? If this is the case it has two implications:
  • 1 Life could be common throughout the universe. (Incidentally, since we don’t really know what life is, in its broadest sense, we cannot say what a suitable environment is and so can’t calculate even a rough probability.)
  • 2 The universe at least looks purpose-built for life.
Backing up the 2nd implication, a number of physical constants are fine tuned for life in the universe. Sir Martin Rees (the Astronomer Royal in the UK) lists them as follows:

  •  ratio of the strengths of gravity to that of electromagnetism;
  •  strength of the force binding together the nucleus of an atom
  •  relative importance of gravity and expansion energy in the universe;
  • cosmological constant (governing the accelerating rate at which the universe is expanding);
  • ratio of the gravitational energy required to pull a large galaxy apart to the energy equivalent of its mass; 
  • number of spatial dimensions in spacetime.

    A small difference in any one of them and life would not be able to exist. I don’t see how we can escape the conclusion that the cosmos is tailored for life without resorting to unprovable, atheistic metaphysical speculation verging on magic. The simplest, most powerful and natural theory is that it is indeed designed for life. The Creator built the production of evolving life and consciousness into it when the concept of the universe was evolving in the Creator’s ‘mind’. (I am not a young earth creationist but believe the universe started from nothing 13.5 billion years ago, until any contrary evidence emerges. I do suspect that conscious beings with free will and a sense of good and evil, probably did not start until very recently, perhaps even within the biblical time scale - tens of thousands of years.)

    As the latest observations suggest that myriad millions of planets exist I am beginning to see a happy convergence. Those wanting us to discover other life forms in the universe through SETI (Search for Extraterrestrial Intelligence) etc., and those wishing to celebrate, rather than lament or deny, the design of it as part of their faith, have a common interest in acknowledging that the universe is designed to promote life and consciousness.

    If  life started by chance at just this terrestrial micro-segment of space-time, i.e billions of years ago on Earth, we can conclude that it is a either a fluke or that its Creator imposes life-friendly order behind the perceived randomness, as the Creator does with other random events (e.g. the digits in π, pi, the ratio of the circumference of a circle to it diameter [see Is there order behind random events?] form a random series but π is not meaningless).  And, of course, it still would not be possible without the extraordinary fine tuning of physical constants and prior cosmic events that led up to life’s genesis, so in a sense it would not be a fluke even in this case.

    Plenty to think about as we move into 2012 and as the astrophysical and astrobiological observations stream into the media; but treat with caution scientists' hype about 'earth-like' planets.

    Author, 2077 AD