Extra-Terrestrial Life

The Ingredients of Life

Astrobiologists believe that the three most important ingredients on which life depends are water, energy and organic molecules such as carbon (C). We know there is plenty of energy and organic molecules in the Universe, but the chances of finding water in liquid form is much lower.

Water on Earth will remain liquid at sea-level pressures (1 bar) where the temperatures are between 0°C and 100°C. However, even on Earth, geographical, seasonal and daily variations can cause the temperature to fall well below freezing (as low as -70°C in Antarctica), and yet life can survive under these conditions.

At the other end of the scale, although most terrestrial (on Earth) organisms cannot endure temperatures consistently above 45°C, hyperthermophiles have been found thriving around hydrothermal vents (black smokers) at the bottom of our deepest oceans, where pressures are extreme and temperatures exceed 110°C.

In light of the extreme terrestrial conditions in which we find life, it is important not to let our experience of terrestrial life bias our ideas of what life is and where it might have taken a foothold elsewhere in the Universe.

The Habitable Zone

The 'Habitable Zone' (HZ) is a term used to describe the region around a star where an orbiting planet could possess water in liquid form, although the term 'Goldilocks Zone' is perhaps better in describing a region that is neither too hot, nor too cold.

The size and location of the HZ depends on the type of the star in question. Typically, hot, bright stars have wide HZs. For example, to enjoy terrestrial temperatures around the star Sirius, which is 26 times brighter than the Sun, an Earth-sized planet would have to orbit at about the distance of Jupiter from the Sun. By contrast, if the Sun were replaced by Epsilon Indi, which has only about one-tenth the Sun's brightness, the Earth would need to be in a Mercurian orbit to receive its present level of warmth.

Given that there are 200 million stars in our galaxy, if just one in a thousand had a small rocky planet in a stable orbit within the star's HZ, that would mean 200,000 planets where life might exist.

Life in the Solar System

We have talked about life around other stars, but there is a chance that life may exist elsewhere in the Solar System.

The discovery of primitive life, or fossils, on Mars would extend the HZ of the Sun further out. Indeed, the evidence shows that Mars enjoyed warmer, wetter conditions in the past under which primitive organisms may have evolved. Some scientists believe they have found such primitive life in meteorites that came from the Martian surface.

In fact ongoing and future Mars Rover missions will search for signs of life that may have existed millions of years ago.

Much further from the Sun are the gas giants, like Jupiter, and their large moons, which at first sight seem inhospitable to life. Yet the effect of tidal heating on these worlds, such as Europa and Callisto, may result in the existence of large amounts of sub-surface liquid water and the possibility of aquatic life.

Click here to find out how astronomers are looking for extra-terrestrial life.