Now that we know our world is just one of countless billions of planets scattered across the universe, is it reasonable to assume ours is the only one with life?
The raw materials for life are widely available in the dust and gas clouds from which new stars and planets form, so all worlds start off with some.
There are probably millions of planets with living things on them. That does not mean they will look like us in any way, and they may well flourish under conditions that would kill us very quickly.
There could be magnetic creatures that live in superheated plasmas, or neutron creatures on the surfaces of neutron stars. Maybe there are superconducting creatures that live on cold, dark planets in the outer reaches of planetary systems. However, for the moment we are concentrating on life forms we might have a chance of recognizing or understanding: chemical life forms like those on Earth, with their life processes taking place in liquid water.
There are reasons for this other than the fact that all life on Earth is based on water chemistry. Water is a very common substance in the universe, usually in the form of ice. Moreover, in liquid form it is unique in that it supports a wide range of chemical reactions.
There are many liquids in which we can dissolve things. However, in water, many chemicals break up easily, allowing the various fragments to rearrange into new substances. Anyone who has done any chemistry knows that interesting things can happen when we dissolve things in water. That is why living things on Earth are mostly liquid water, and also why we are particularly interested in planets where liquid water is present, or is a possibility.
On the Earth's surface, water is a liquid over the temperature range zero degrees Celsius (freezing point) to one hundred degrees (boiling point). Life on Earth has shown us that as long as the water is in liquid form, things can live in it, even if it is nearly boiling. Deep in the ocean, where the pressures might be over a thousand times the pressure at the Earth's surface water boils at up to 400 C. Water this hot is coming out of hydrothermal vents, and supporting colonies of exotic creatures.
As long as the water is in liquid form, life something like ours might thrive in it. This means that on a hot world, life might still be possible, as long as water is present and the pressure is high enough for it to be a liquid.
Ideally, as on the Earth, water would be present in all three forms—solid, liquid and vapour. It evaporates from the oceans and then rains and snows on land, providing water for the ecosystems as it finds its way back to the oceans. This water cycle is critical to life and would no doubt be as important on other worlds.
Water has one other important quality. Ice floats. Ice from glaciers and ice caps float on the relatively warm surface of the ocean, slowly melting. Seasonal sea ice floats around until it finally melts. Ice being less dense than liquid water is unusual.
In almost all substances, the solid form is denser than the liquid form, and sinks. If water were like that, the icebergs and sea ice would sink to the bottom of the ocean, where temperatures are close to the freezing point, and stay there. The ice would accumulate until the oceans became just a layer of water over ice. There would be no upwelling of the nutrients that sustain life in the oceans, and indirectly life on land.
Detecting liquid water, or better still a water cycle on a world would not be a definite indicator of life, but it would certainly flag the ones we should examine carefully.
One indication of biological activity we can look for on even distant planets is the presence of reactive gases like oxygen, or cynically, evidence of atmospheric pollution.
• Venus, Mars and Mercury lie low in the dawn glow.
• Jupiter is low in the southwest after sunset.
• The Moon will reach its first quarter on Feb. 8.
This article is written by or on behalf of an outsourced columnist and does not necessarily reflect the views of Castanet.