Water molecules

Planetary puzzles

Why is Earth habitable while other planets aren’t?

In science, a sample size of one is never good. The larger the sample, the greater the reliability. Modern astronomy gives us the ability to peer into other solar systems, see what they are like, and compare and contrast them with our own. Around 2,000 exoplanets – planets in orbit around other stars – have been spotted so far.

As water plays such a crucial role as a solvent for life here on Earth, of particular interest are exoplanets that might also have water in liquid form. They are most likely to reside in the habitable zones of stars – the region where the liquid water is stable (usually where the temperature sits between 0 and 100ºC, though pressure can affect this). Estimates suggest that there could be as many as 60 billion such ‘habitable’ planets in our Milky Way galaxy.

Liquid water is not just limited to the habitable zone, however. Some of the moons of the outer solar system – like Enceladus around Saturn, and Europa around Jupiter – have liquid water too. It is kept warm by the energy released in the tidal distortion of the rocks caused by the gravitational interaction between the moons and the giant planets.

Exploration of comets and asteroids, too, has shown that both contain water ice. An outstanding mystery is how Earth came to have so much water; one theory is that it was delivered here by comets and asteroids when they collided with the early Earth. However, water found on comets is largely of a different isotopic composition – made with deuterium, an isotope of hydrogen – so asteroids are more likely.

Lead image:

Water molecules.

‘Big Picture: Space Biology’ (2015) CC BY


Questions for discussion

  • Which planetary bodies do you think have the best chance of supporting life? What type of life would you expect to thrive there?

Further reading

Downloadable resources

About this resource

This resource was first published in ‘Space Biology’ in June 2015.

Cell biology, Statistics and maths, Ecology and environment
Space Biology
Education levels:
16–19, Continuing professional development