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The Conditions for Life

For life to begin and to continue existing, certain physical conditions must be met. This lecture examines these conditions, and looks at the circumstances under which they might arise.

  1. The Role of Water
  2. The Habitable Zone
  3. The Continuous Habitable Zone
  4. The Most Suitable Stars

The Role of Water

Of all the conditions important to terrestrial life, liquid water is the most important. Not only is it a solvent in which organic chemicals can be transported and can react with one another, it is a vital ingredient in photosynthesis, the process which converts sunlight into sugars and thus provides the energy source for the majority of life.

The ambient temperature and pressure on the surface of a planet determine whether water can remain in liquid form there. At a pressure of 1 atmosphere (1000 millibars), water is liquid only across the temperature range 273K (0 Celsius) to 373K (100 Celsius). At higher pressures, water can remain liquid over a larger range of temperatures. However, at lower pressures the temperature range for liquid water is smaller, and below a pressure of 0.006 atmospheres, no liquid water can exist; it is all either solid (ice) or gaseous (water vapour).

The ambient temperature on the surface of a planet is determined by the balance between the amount of radiation absorbed from the parent star (which contributes to warming), and the amount emitted by the planet (which contributes to cooling); when these two are equal, the planet is in thermal equilibrium. Of the radiation received from the star, a percentage is reflected without being absorbed; this percentage is known as the albedo. In the case of the Earth, the albedo of 0.39, meaning that 39% of the Sun's radiation is reflected, and the remaining 61% is absorbed.

The greenhouse effect is an important factor in controlling how much radiation is emitted by a planet. Some atmospheric gasses, including water, carbon dioxide and sulphur dioxide, are opaque to infrared radiation, and their presence reduces the cooling of a planet by emission (see Universe, Section 8.1). Without the insulating effect provided by these greenhouse gasses, the temperature on Earth would be well below 273K, and all water would be frozen into ice.

The Habitable Zone

The habitable zone of a solar system is defined as the range of distances from the central star over which liquid water can exist on the surface of a planet. If the planet is too close to the star, then a runaway greenhouse effect will occur, where the blanketing effect of greenhouse gasses is so effective that temperatures rise well above the upper limit for any liquid surface water to exist.

This was the fate on Venus, whose atmosphere is 96.5 carbon dioxide. Most of this carbon dioxide will have come from volcanic eruptions (see Universe, Section 11.5); with little or no liquid water for it to dissolve in, the concentrations of carbon dioxide in Venus's atmosphere will have steadily increased with time, further accelerating the runaway.

Surprisingly, there is no water on Venus, even in gaseous form. Over time, solar ultraviolet radiation striking water vapour in the Venusian atmosphere will have led to photodissociation, where the water is split into its constituent hydrogen and oxygen. Since the gravity of terrestrial planets is too low to keep hydrogen in the atmosphere, the hydrogen will have escaped into space.

At the other boundary of the HZ, if a planet is too far from its parent star then carbon dioxide in the atmosphere will sublime (i.e., go from gas to solid), to form clouds of dry ice. Not only does this remove the valuable insulation provided by carbon dioxide gas (via the greenhouse effect), the dry ice clouds also reflect incoming sunlight, increasing the planet's albedo and leading to further cooling of the planet. Eventually, all liquid water will freeze.

The boundaries of the HZ

The boundaries of the HZ

The Continuous Habitable Zone

After the formation of a solar system, changes in the star's its interior means that it becomes brighter and hotter. Therefore, both the inner and outer boundaries of the HZ move outwards with time. The continuous habitable zone (CHZ) is defined as the overlap between habitable zones at two different (widely-separated) times, and represents the region where water can remain liquid over timescales long enough for life to form and evolve.

The HZ and CHZ

The HZ and CHZ

The Most Suitable Stars

Higher-mass stars tend to be larger and luminous than their lower-mass counterparts. Therefore, their habitable zones are situated further out. In addition, however, their HZs are much broader. As an illustration,

  • a 0.2 solar-mass star's HZ extends from 0.1 to 0.2 AU
  • a 1.0 solar-mass star's HZ extends from 1 to 2 AU
  • a 40 solar-mass star's HZ extends from 350 to 600 AU

On these grounds, it would seem that high-mass starts are the best candidates for finding planets within a habitable zone. However, these stars emit most of their radiation in the far ultraviolet (FUV), which can be highly damaging to life, and also contributes to photodissociation and the loss of water. Furthermore, the lifetimes of these stars is so short (around 10 million years) that there is not enough time for life to begin.

Very low mass stars have the longest lifetimes of all, but their HZs are very close in and very narrow. Therefore, the chances of a planet being formed within the HZ are small. Additionally, even if a planet did form within the HZ, it would become tidally locked, so that the same hemisphere always faced the star. Even though liquid water might exist on such a planet, the climactic conditions would probably be too severe to permit life.

The HZ for different types of star, showing the Solar System for the Sun


In between the high- and low-mass stars lie those like our own Sun, which make up about 15% percent of the stars in the galaxy. These have reasonably-broad HZs, do not suffer from FUV irradiation, and have lifetimes of the order of 10 billion years. Therefore, they are the best candidates for harbouring planets where life might be able to begin.

Updated 2009-10-13 13:22:01