Extra-terrestrial life is the existence of life outside of the Earth. Are we alone in the universe? So far the only life we know about inhabits our planet – Earth – but there are planets that orbit other stars in a similar way to how the Earth orbits the Sun.

This activity could form a class discussion, and could either be done in the context of the whole planet, or at organism level. Or you could concentrate on one of the questions on the student page, such as 'How could we recognise that life on other planets was intelligent?'

If looking at the planet as a whole  - using the Gaia theory - think about the types of "signatures" that have been left on our planets due to the presence of life.

The search for life on Mars is focused on evidence for liquid water on the planet or its presence in the past. One way of doing this is by looking at the patterns of valleys and mountain-like features on the planet's surface and trying to work out if they may have been formed by flowing water. Scientists also look for certain patterns of minerals that occur in the presence of organic materials.

Earth's atmosphere is made up mostly of Nitrogen (about 78%) and Oxygen (about 21%) whereas Venus and Mars (our two closest neighbours) both have atmospheres that contain mostly carbon dioxide (about 95%). Oxygen is reactive so you would not expect it to hang around in the atmosphere for very long. The fact that it is present in the Earth's atmosphere is because there is life on Earth (plants and bacteria) that use carbon dioxide from the atmosphere and create oxygen.

If answering the question at organism level, students could discuss the molecules that organisms use in their metabolic processes.

Life needs to be able to replicate which requires energy. On Earth the energy needed for life is captured in the electron transport chain so components from this process could be used as signature molecules for the ability of energy transfer. These include porphyrins, quinines, flavins and nicotinamides (eg pigments, enzymes and cytochromes). Similar molecules in space might be associated with the presence of extra-terrestrial life.

Useful references:

In Search of the Molecules of Life, Ronald L. Crawford et al. Icarus Volume 154, Issue 2, December 2001, Pages 531-539.

Sensitive laser instrument could aid search for life on Mars Published: Wednesday, October 15, 2008 - 10:15

"Alien" Atmosphere Helped Unfreeze Early Earth, Anne Minard for National Geographic News, January 13, 2009

NASA student feature - Life on Mars?



Curriculum links

Data, evidence, theories and explanations 1b. how interpretation of data, using creative thought, provides evidence to test ideas and develop theories
1d. that there are some questions that science cannot currently answer, and some that science cannot address.
Organisms and health 5c. the ways in which organisms function are related to the genes in their cells
Environment, Earth and universe
8a. the effects of human activity on the environment can be assessed using living and non-living indicators


Scottish Curriculum

Science - Curriculum for Excellence level 4 SCN 4-06a. Planet Earth - Space: By researching developments used to observe or explore space, I can illustrate how our knowledge of the universe has evolved over time.
SCN 4-20a. Topical Science: I have researched new developments in science and can explain how their current or future applications might impact on modern life .
SCN 4-20b. Having selected scientific themes of topical interest, I can critically analyse the issues, and use relevant information to develop an informed argument
Environmental Science National 4 & 5 Potentially relevant to modules: Earth's Resources (living and non-living environment components of the Earth)
Physics National 4 & 5 Potentially relevant to modules: Space (solar system and its exploration)

How to look for alien life


EarthOne of the biggest problems in looking for alien life is how to look for it when we don't know what it looks like.

Think about life on Earth. Aliens in films are often depicted as quite human-like, but we are only one of the many millions of species on our planet - and life can exist in environments where humans cannot.

Because there is life on Earth, particular molecules in our atmosphere - such as oxygen - exist in our atmosphere. The activities of life on Earth produce a kind of signature in the atmosphere. This was a theory developed by James Lovelock in 1979, and is known as the 'Gaia Theory'. More information can be seen here, especially the box entitled "from the Royal Society's archives". The video on the same page is also a useful introduction to the topic.



What might life on other planets look like and how might we detect it?

Some things to think about, and discuss with others in your class:

  • On Earth, the organisms that survive in most extreme environments are bacteria. These have been found in some of the hottest, coldest, acidic, alkaline and high pressure environments imaginable. Find out more about life in extreme environments.
  • Life on Earth is centred on the element carbon, is there a lot carbon in the universe? Are there other elements similar to carbon that life might evolve around?
  • It is important that when searching for extraterrestrial life we must be careful not to contaminate it with molecules/life from Earth (especially bacteria).
  • This summary outlines what the European space agency plans to do and may give you some ideas.
  • In our Solar System, the places where scientists are particularly interested in searching for life are Mars, and the moons Europa, Enceladus and Titan. These objects are all much colder than Earth. Titan (one of Saturn's moons) has a dense atmosphere and lakes of liquid hydrocarbons such as methane.
  • How could we recognise that life on other planets was intelligent? And if we found 'intelligent' life, how could we communicate with it?