Ken Tapping, September 30, 2009

We have been wondering about the possibility of life on other worlds for as long as we have known there are other worlds. In our Solar System there are planets rather like ours, such as Mars, and planets that are very different, such as Venus. Because Mars is reasonably like our world, we conclude it is a good place for life, while Venus is hostile to it. Now that we have discovered almost 400 planets orbiting other stars, we are trying to get a better idea of the types of planet where we might expect to find living things.

Until the late 20th Century the quest looked simple. We would seek planets like ours. These orbit stars that remain stable long enough for life to appear and evolve without getting frozen, fried or irradiated, and lie at the right distance from their stars for liquid water to exist on their surfaces. Today, we can determine whether there are planets orbiting other stars without actually seeing them, and moreover, we can get an idea of what gases are present in their atmospheres. Oxygen is a very reactive gas, and will disappear very quickly if not continually topped up. This is done on Earth by plants, through photosynthesis. The presence of oxygen or some other very reactive gas, such as chlorine, would suggest the presence of life. This line of logic is attractive because it can be tested by observations we can make with available instruments. However, this was too simplistic.

The first sign of this came from finding life forms that live in ice, or in nearly boiling water, and some thriving in the total absence of oxygen. Then, more exotic life forms turned up at the bottom of the deep sea. Around the places on the sea bottom where new seabed is appearing, there are "black smokers", hydrothermal vents where hot, mineral-laden water, containing large amounts of hydrogen sulphide, are ejected into the sea. In the total darkness, under high pressure and in an environment utterly hostile to us, these vents support colonies of exotic animals needing no sunlight or solar heat at all. This raises a new candidate for bearing life: Europa, one of the Moons of Jupiter. Continuous kneading by Jupiter's gravity liberates heat, so that under a permanently frozen surface, there is a deep, dark ocean, possibly with hydrothermal vents at the bottom, supporting exotic life forms. There are plans to send a robot submarine to have a look.

Our ideas of the sheer adaptability of life received another jolt when living things were discovered in the rocks, deep underground. It seems that there is more living material underground, in the rocks, than there is on the Earth's surface, or in the oceans. It seems that life is present on every part of the Earth we have so far managed to access. Life is flexible, adaptable, and aggressively colonizes everywhere in reach. We need to think about this some more.

Down at the basics, life needs raw materials it can assimilate, and a source of energy. This may come from sunlight, heat or from chemical reactions driven by reactive gases. Living creatures must also be able to make more of themselves to sustain their populations. These are very broad rules, and show how challenging it is to seek extraterrestrial life.