A short while ago scientists operating the Kepler orbiting observatory announced the discovery of a planet very much like ours.
The telescope was watching many stars looking for the minute dimming that occurs when a planet moves between us and its star. Careful analysis of these dimmings can tell us a surprising amount about the planets causing them, such as how big they are, how far they are from their stars and so on. Of course the big question that comes up whenever a new planet is discovered is whether there are living creatures out there, and what they are like.
It seems as though every year somebody finds something living here on Earth under conditions that we thought would make life impossible. These discoveries make us continually revise our ideas about conditions needed for living creatures to thrive. On one hand it means that more planets in the universe might be homes to living creatures, but it also makes it more problematic about what to search for. It is a good idea to go right back to basics. What do living creatures need?
Firstly they need an environment that is stable enough for creatures to develop, and change sufficiently slowly for them to adapt. Short term variations must not be so severe that too many creatures are wiped out for the population to recover. Suitable raw materials - food in our case - of some kind have to be available in sufficient quantities for the creatures to grow and reproduce. A supply of energy is needed to drive the processes of life. Plants use sunlight; we use the action of oxygen in the atmosphere upon the chemicals making up our food. This covers a wide range of possible environments. It is likely there are some really bizarre life forms out there, but to help us in our search we need to focus it a bit. During their lives stars turn hydrogen gas into other elements, which they release into the clouds of gas and dust between the stars when they die.
After a few generations of stars these clouds become rich in all the elements. Then, over time these elements react to form chemical compounds. We have identified countless different compounds, including ammonia, water, carbon monoxide, methane and other hydrocarbons, hydrogen cyanide, and even simple aminoacids ñ the building blocks of proteins. When we take a mixture of these chemicals, as might form the atmosphere of a young planet, and pass an electric discharge through it, emulating lightning storms, after a while we end up with a black goop loaded with aminoacids. It looks as though cosmic chemistry might favour living creatures based upon chemistries something like ours. Since liquid water plays a key role in the chemistry of our forms of life, we are particularly interested in worlds that have, or are likely to have liquid water.
That does not rule out living creatures that swim in liquid nitrogen or are blobs of plasma in a star or planet's magnetic field, but choosing a chemistry like ours gives us something to look for. For example, we should be able to identify planets with life forms sharing our chemistry by looking for oxygen in their atmospheres. The oxygen in our atmosphere, which is essential to us, is topped up continually by plants. If we see oxygen in the atmosphere of a distant alien planet, we can assume similar things are going on there.
Similar chemistries do not mean alien creatures need look anything like us. All the living things on Earth are the results of billions of years of evolutionary choices and accidents, such as environmental catastrophes. It is unlikely other worlds would have identical histories, so their life forms will be unique to each world. The chance of an alien creature looking anything like us with minor modifications, as per Star Trek, Star Wars or Doctor Who aliens, is very very low.
Venus and Jupiter are lost in the sunset glow. Saturn lies in the south. Mars is low in the dawn glow. The Moon will reach Last Quarter on the 6th.
Ken Tapping is an astronomer with the National Research Council's Dominion Radio Astrophysical Observatory, Penticton.